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  • doc/rob_thesis/ctordtor.tex

    rf92aa32 r93afb96  
    33%======================================================================
    44
    5 % TODO now: as an experiment, implement Andrei Alexandrescu's ScopeGuard http://www.drdobbs.com/cpp/generic-change-the-way-you-write-excepti/184403758?pgno=2
     5% TODO: discuss move semantics; they haven't been implemented, but could be. Currently looking at alternative models. (future work)
     6
     7% TODO: as an experiment, implement Andrei Alexandrescu's ScopeGuard http://www.drdobbs.com/cpp/generic-change-the-way-you-write-excepti/184403758?pgno=2
    68% doesn't seem possible to do this without allowing ttype on generic structs?
    79
     10% If a Cforall constructor is in scope, C style initialization is
     11% disabled by default.
     12% * initialization rule: if any constructor is in scope for type T, try
     13%   to find a matching constructor for the call. If there are no
     14%   constructors in scope for type T, then attempt to fall back on
     15%   C-style initialization.
     16% + if this rule was not in place, it would be easy to accidentally
     17%   use C-style initialization in certain cases, which could lead to
     18%   subtle errors [2]
     19% - this means we need special syntax if we want to allow users to force
     20%   a C-style initialization (to give users more control)
     21% - two different declarations in the same scope can be implicitly
     22%   initialized differently. That is, there may be two objects of type
     23%   T that are initialized differently because there is a constructor
     24%   definition between them. This is not technically specific to
     25%   constructors.
     26
     27% C-style initializers can be accessed with @= syntax
     28% + provides a way to get around the requirement of using a constructor
     29%   (for advanced programmers only)
     30% - can break invariants in the type => unsafe
     31% * provides a way of asserting that a variable is an instance of a
     32%   C struct (i.e. a POD struct), and so will not be implicitly
     33%   destructed (this can be useful at times, maybe mitigates the need
     34%   for move semantics?) [3]
     35% + can modernize a code base one step at a time
     36
     37% Cforall constructors can be used in expressions to initialize any
     38% piece of memory.
     39% + malloc() { ... } calls the appropriate constructor on the newly
     40%   allocated space; the argument is moved into the constructor call
     41%   without taking its address [4]
     42% - with the above form, there is still no way to ensure that
     43%   dynamically allocated objects are constructed. To resolve this,
     44%   we might want a stronger "new" function which always calls the
     45%   constructor, although how we accomplish that is currently still
     46%   unresolved (compiler magic vs. better variadic functions?)
     47% + This can be used as a placement syntax [5]
     48% - can call the constructor on an object more than once, which could
     49%   cause resource leaks and reinitialize const fields (can try to
     50%   detect and prevent this in some cases)
     51%   * compiler always tries to implicitly insert a ctor/dtor pair for
     52%     non-@= objects.
     53%     * For POD objects, this will resolve to an autogenerated or
     54%       intrinsic function.
     55%     * Intrinsic functions are not automatically called. Autogenerated
     56%       are, because they may call a non-autogenerated function.
     57%     * destructors are automatically inserted at appropriate branches
     58%       (e.g. return, break, continue, goto) and at the end of the block
     59%       in which they are declared.
     60%   * For @= objects, the compiler never tries to interfere and insert
     61%     constructor and destructor calls for that object. Copy constructor
     62%     calls do not count, because the object is not the target of the copy
     63%     constructor.
     64
     65% A constructor is declared with the name ?{}
     66% + combines the look of C initializers with the precedent of ?() being
     67%   the name for the function call operator
     68% + it is possible to easily search for all constructors in a project
     69%   and immediately know that a function is a constructor by seeing the
     70%   name "?{}"
     71
     72% A destructor is declared with the name ^?{}
     73% + name mirrors a constructor's name, with an extra symbol to
     74%   distinguish it
     75% - the symbol '~' cannot be used due to parsing conflicts with the
     76%   unary '~' (bitwise negation) operator - this conflict exists because
     77%   we want to allow users to write ^x{}; to destruct x, rather than
     78%   ^?{}(&x);
     79
     80% The first argument of a constructor must be a pointer. The constructed
     81% type is the base type of the pointer. E.g. void ?{}(T *) is a default
     82% constructor for a T.
     83% + can name the argument whatever you like, so not constrained by
     84%   language keyword "this" or "self", etc.
     85% - have to explicitly qualify all object members to initialize them
     86%   (e.g. this->x = 0, rather than just x = 0)
     87
     88% Destructors can take arguments other than just the destructed pointer
     89% * open research problem: not sure how useful this is
     90
     91% Pointer constructors
     92% + can construct separately compiled objects (opaque types) [6]
     93% + orthogonal design, follows directly from the definition of the first
     94%   argument of a constructor
     95% - may require copy constructor or move constructor (or equivalent)
     96%   for correct implementation, which may not be obvious to everyone
     97% + required feature for the prelude to specify as much behavior as possible
     98%   (similar to pointer assignment operators in this respect)
     99
     100% Designations can only be used for C-style initialization
     101% * designation for constructors is equivalent to designation for any
     102%   general function call. Since a function prototype can be redeclared
     103%   many times, with arguments named differently each time (or not at
     104%   all!), this is considered to be an undesirable feature. We could
     105%   construct some set of rules to allow this behaviour, but it is
     106%   probably more trouble than it's worth, and no matter what we choose,
     107%   it is not likely to be obvious to most people.
     108
     109% Constructing an anonymous member [7]
     110% + same as with calling any other function on an anonymous member
     111%   (implicit conversion by the compiler)
     112% - may be some cases where this is ambiguous => clarify with a cast
     113%   (try to design APIs to avoid sharing function signatures between
     114%   composed types to avoid this)
     115
     116% Default Constructors and Destructors are called implicitly
     117% + cannot forget to construct or destruct an object
     118% - requires special syntax to specify that an object is not to be
     119%   constructed (@=)
     120% * an object will not be implicitly constructed OR destructed if
     121%   explicitly initialized like a C object (@= syntax)
     122% * an object will be destructed if there are no constructors in scope
     123%   (even though it is initialized like a C object) [8]
     124
     125% An object which changes from POD type to non POD type will not change
     126% the semantics of a type containing it by composition
     127% * That is, constructors will not be regenerated at the point where
     128%   an object changes from POD type to non POD type, because this could
     129%   cause a cascade of constructors being regenerated for many other
     130%   types. Further, there is precedence for this behaviour in other
     131%   facets of Cforall's design, such as how nested functions interact.
     132% * This behaviour can be simplified in a language without declaration
     133%   before use, because a type can be classified as POD or non POD
     134%   (rather than potentially changing between the two at some point) at
     135%   at the global scope (which is likely the most common case)
     136% * [9]
     137
     138% Changes to polymorphic type classes
     139% * dtype and ftype remain the same
     140% * forall(otype T) is currently essentially the same as
     141%   forall(dtype T | { @size(T); void ?=?(T *, T); }).
     142%   The big addition is that you can declare an object of type T, rather
     143%   than just a pointer to an object of type T since you know the size,
     144%   and you can assign into a T.
     145%   * this definition is changed to add default constructor and
     146%     destructor declarations, to remain consistent with what type meant
     147%     before the introduction of constructors and destructors.
     148%     * that is, forall(type T) is now essentially the same as
     149%       forall(dtype T | { @size(T); void ?=?(T *, T);
     150%                          void ?{}(T *); void ^?{}(T *); })
     151%     + this is required to make generic types work correctly in
     152%       polymorphic functions
     153%     ? since declaring a constructor invalidates the autogenerated
     154%       routines, it is possible for a type to have constructors, but
     155%       not default constructors. That is, it might be the case that
     156%       you want to write a polymorphic function for a type which has
     157%       a size, but non-default constructors? Some options:
     158%       * declaring a constructor as a part of the assertions list for
     159%         a type declaration invalidates the default, so
     160%         forall(otype T | { void ?{}(T *, int); })
     161%         really means
     162%         forall(dtype T | { @size(T); void ?=?(T *, T);
     163%                            void ?{}(T *, int); void ^?{}(T *); })
     164%       * force users to fully declare the assertions list like the
     165%         above in this case (this seems very undesirable)
     166%       * add another type class with the current desugaring of type
     167%         (just size and assignment)
     168%       * provide some way of subtracting from an existing assertions
     169%         list (this might be useful to have in general)
     170
     171% Implementation issues:
     172% Changes to prelude/autogen or built in defaults?
     173% * pointer ctors/dtors [prelude]
     174%   * other pointer type routines are declared in the prelude, and this
     175%     doesn't seem like it should be any different
     176% * basic type ctors/dtors [prelude]
     177%   * other basic type routines are declared in the prelude, and this
     178%     doesn't seem like it should be any different
     179% ? aggregate types [undecided, but leaning towards autogenerate]
     180%   * prelude
     181%     * routines specific to aggregate types cannot be predeclared in
     182%       the prelude because we don't know the name of every
     183%       aggregate type in the entire program
     184%   * autogenerate
     185%     + default assignment operator is already autogenerated for
     186%       aggregate types
     187%       * this seems to lead us in the direction of autogenerating,
     188%         because we may have a struct which contains other objects
     189%         that require construction [10]. If we choose not to
     190%         autogenerate in this case, then objects which are part of
     191%         other objects by composition will not be constructed unless
     192%         a constructor for the outer type is explicitly defined
     193%       * in this case, we would always autogenerate the appropriate
     194%         constructor(s) for an aggregate type, but just like with
     195%         basic types, pointer types, and enum types, the constructor
     196%         call can be elided when when it is not necessary.
     197%     + constructors will have to be explicitly autogenerated
     198%       in the case where they are required for a polymorphic function,
     199%       when no user defined constructor is in scope, which may make it
     200%       easiest to always autogenerate all appropriate constructors
     201%     - n+2 constructors would have to be generated for a POD type
     202%       * one constructor for each number of valid arguments [0, n],
     203%         plus the copy constructor
     204%         * this is taking a simplified approach: in C, it is possible
     205%           to omit the enclosing braces in a declaration, which would
     206%           lead to a combinatorial explosion of generated constructors.
     207%           In the interest of keeping things tractable, Cforall may be
     208%           incompatible with C in this case. [11]
     209%       * for non-POD types, only autogenerate the default and copy
     210%         constructors
     211%       * alternative: generate only the default constructor and
     212%         special case initialization for any other constructor when
     213%         only the autogenerated one exists
     214%         - this is not very sensible, as by the previous point, these
     215%           constructors may be needed for polymorphic functions
     216%           anyway.
     217%     - must somehow distinguish in resolver between autogenerated and
     218%       user defined constructors (autogenerated should never be chosen
     219%       when a user defined option exists [check first parameter], even
     220%       if full signature differs) (this may also have applications
     221%       to other autogenerated routines?)
     222%     - this scheme does not naturally support designation (i.e. general
     223%       functions calls do not support designation), thus these cases
     224%       will have to be treated specially in either case
     225%   * defaults
     226%     * i.e. hardcode a new set of rules for some "appropriate" default
     227%       behaviour for
     228%     + when resolving an initialization expression, explicitly check to
     229%       see if any constructors are in scope. If yes, attempt to resolve
     230%       to a constructor, and produce an error message if a match is not
     231%       found. If there are no constructors in scope, resolve to
     232%       initializing each field individually (C-style)
     233%     + does not attempt to autogenerate constructors for POD types,
     234%       which can be seen as a space optimization for the program
     235%       binary
     236%     - as stated previously, a polymorphic routine may require these
     237%       autogenerated constructors, so this doesn't seem like a big win,
     238%       because this leads to more complicated logic and tracking of
     239%       which constructors have already been generated
     240%     - even though a constructor is not explicitly declared or used
     241%       polymorphically, we might still need one for all uses of a
     242%       struct (e.g. in the case of composition).
     243%   * the biggest tradeoff in autogenerating vs. defaulting appears to
     244%     be in where and how the special code to check if constructors are
     245%     present is handled. It appears that there are more reasons to
     246%     autogenerate than not.
     247
     248% --- examples
     249% [1] As an example of using constructors polymorphically, consider a
     250% slight modification on the foldl example I put on the mailing list a
     251% few months ago:
     252
     253% context iterable(type collection, type element, type iterator) {
     254%   void ?{}(iterator *, collection); // used to be makeIterator, but can
     255%                             // idiomatically use constructor
     256%   int hasNext(iterator);
     257%   iterator ++?(iterator *);
     258%   lvalue element *?(iterator);
     259% };
     260
     261
     262% forall(type collection, type element, type result, type iterator
     263%   | iterable(collection, element, iterator))
     264% result foldl(collection c, result acc,
     265%     result (*reduce)(result, element)) {
     266%   iterator it = { c };
     267%   while (hasNext(it)) {
     268%     acc = reduce(acc, *it);
     269%     ++it;
     270%   }
     271%   return acc;
     272% }
     273
     274% Now foldl makes use of the knowledge that the iterator type has a
     275% single argument constructor which takes the collection to iterate
     276% over. This pattern allows polymorphic code to look more natural
     277% (constructors are generally preferred to named initializer/creation
     278% routines, e.g. "makeIterator")
     279
     280% [2] An example of some potentially dangerous code that we don't want
     281% to let easily slip through the cracks - if this is really what you
     282% want, then use @= syntax for the second declaration to quiet the
     283% compiler.
     284
     285% struct A { int x, y, z; }
     286% ?{}(A *, int);
     287% ?{}(A *, int, int, int);
     288
     289% A a1 = { 1 };         // uses ?{}(A *, int);
     290% A a2 = { 2, 3 };      // C-style initialization -> no invariants!
     291% A a3 = { 4, 5, 6 };   // uses ?{}(A *, int, int, int);
     292
     293% [3] Since @= syntax creates a C object (essentially a POD, as far as
     294% the compiler is concerned), the object will not be destructed
     295% implicitly when it leaves scope, nor will it be copy constructed when
     296% it is returned. In this case, a memcpy should be equivalent to a move.
     297
     298% // Box.h
     299% struct Box;
     300% void ?{}(Box **, int};
     301% void ^?{}(Box **);
     302% Box * make_fortytwo();
     303
     304% // Box.cfa
     305% Box * make_fortytwo() {
     306%   Box *b @= {};
     307%   (&b){ 42 }; // construct explicitly
     308%   return b; // no destruction, essentially a move?
     309% }
     310
     311% [4] Cforall's typesafe malloc can be composed with constructor
     312% expressions. It is possible for a user to define their own functions
     313% similar to malloc and achieve the same effects (e.g. Aaron's example
     314% of an arena allocator)
     315
     316% // CFA malloc
     317% forall(type T)
     318% T * malloc() { return (T *)malloc(sizeof(T)); }
     319
     320% struct A { int x, y, z; };
     321% void ?{}(A *, int);
     322
     323% int foo(){
     324%   ...
     325%   // desugars to:
     326%   // A * a = ?{}(malloc(), 123);
     327%   A * a = malloc() { 123 };
     328%   ...
     329% }
     330
     331% [5] Aaron's example of combining function calls with constructor
     332% syntax to perform an operation similar to C++'s std::vector::emplace
     333% (i.e. to construct a new element in place, without the need to
     334% copy)
     335
     336% forall(type T)
     337% struct vector {
     338%   T * elem;
     339%   int len;
     340%   ...
     341% };
     342
     343% ...
     344% forall(type T)
     345% T * vector_new(vector(T) * v) {
     346%   // reallocate if needed
     347%   return &v->elem[len++];
     348% }
     349
     350% int main() {
     351%   vector(int) * v = ...
     352%   vector_new(v){ 42 };  // add element to the end of vector
     353% }
     354
     355% [6] Pointer Constructors. It could be useful to use the existing
     356% constructor syntax even more uniformly for ADTs. With this, ADTs can
     357% be initialized in the same manor as any other object in a polymorphic
     358% function.
     359
     360% // vector.h
     361% forall(type T) struct vector;
     362% forall(type T) void ?{}(vector(T) **);
     363% // adds an element to the end
     364% forall(type T) vector(T) * ?+?(vector(T) *, T);
     365
     366% // vector.cfa
     367% // don't want to expose the implementation to the user and/or don't
     368% // want to recompile the entire program if the struct definition
     369% // changes
     370
     371% forall(type T) struct vector {
     372%   T * elem;
     373%   int len;
     374%   int capacity;
     375% };
     376
     377% forall(type T) void resize(vector(T) ** v) { ... }
     378
     379% forall(type T) void ?{}(vector(T) ** v) {
     380%   vector(T) * vect = *v = malloc();
     381%   vect->capacity = 10;
     382%   vect->len = 0;
     383%   vect->elem = malloc(vect->capacity);
     384% }
     385
     386% forall(type T) vector(T) * ?+?(vector(T) *v, T elem) {
     387%   if (v->len == v->capacity) resize(&v);
     388%   v->elem[v->len++] = elem;
     389% }
     390
     391% // main.cfa
     392% #include "adt.h"
     393% forall(type T | { T ?+?(T, int); }
     394% T sumRange(int lower, int upper) {
     395%   T x;    // default construct
     396%   for (int i = lower; i <= upper; i++) {
     397%     x = x + i;
     398%   }
     399%   return x;
     400% }
     401
     402% int main() {
     403%   vector(int) * numbers = sumRange(1, 10);
     404%   // numbers is now a vector containing [1..10]
     405
     406%   int sum = sumRange(1, 10);
     407%   // sum is now an int containing the value 55
     408% }
     409
     410% [7] The current proposal is to use the plan 9 model of inheritance.
     411% Under this model, all of the members of an unnamed struct instance
     412% become members of the containing struct. In addition, an object
     413% can be passed as an argument to a function expecting one of its
     414% base structs.
     415
     416% struct Point {
     417%   double x;
     418%   double y;
     419% };
     420
     421% struct ColoredPoint {
     422%   Point;        // anonymous member (no identifier)
     423%                 // => a ColoredPoint has an x and y of type double
     424%   int color;
     425% };
     426
     427% ColoredPoint cp = ...;
     428% cp.x = 10.3;    // x from Point is accessed directly
     429% cp.color = 0x33aaff; // color is accessed normally
     430% foo(cp);        // cp can be used directly as a Point
     431
     432% void ?{}(Point *p, double x, double y) {
     433%   p->x = x;
     434%   p->y = y;
     435% }
     436
     437% void ?{}(ColoredPoint *cp, double x, double y, int color) {
     438%   (&cp){ x, y };  // unambiguous, no ?{}(ColoredPoint*,double,double)
     439%   cp->color = color;
     440% }
     441
     442% struct Size {
     443%   double width;
     444%   double height;
     445% };
     446
     447% void ?{}(Size *s, double w, double h) {
     448%   p->width = w;
     449%   p->height = h;
     450% }
     451
     452% struct Foo {
     453%   Point;
     454%   Size;
     455% }
     456
     457% ?{}(Foo &f, double x, double y, double w, double h) {
     458%   // (&F,x,y) is ambiguous => is it ?{}(Point*,double,double) or
     459%   // ?{}(Size*,double,double)? Solve with a cast:
     460%   ((Point*)&F){ x, y };
     461%   ((Size*)&F){ w, h };
     462% }
     463
     464% [8] Destructors will be called on objects that were not constructed.
     465
     466% struct A { ... };
     467% ^?{}(A *);
     468% {
     469%   A x;
     470%   A y @= {};
     471% } // x is destructed, even though it wasn't constructed
     472%   // y is not destructed, because it is explicitly a C object
     473
     474
     475% [9] A type's constructor is generated at declaration time using
     476% current information about an object's members. This is analogous to
     477% the treatment of other operators. For example, an object's assignment
     478% operator will not change to call the override of a member's assignment
     479% operator unless the object's assignment is also explicitly overridden.
     480% This problem can potentially be treated differently in Do, since each
     481% compilation unit is passed over at least twice (once to gather
     482% symbol information, once to generate code - this is necessary to
     483% achieve the "No declarations" goal)
     484
     485% struct A { ... };
     486% struct B { A x; };
     487% ...
     488% void ?{}(A *);  // from this point on, A objects will be constructed
     489% B b1;           // b1 and b1.x are both NOT constructed, because B
     490%                 // objects are not constructed
     491% void ?{}(B *);  // from this point on, B objects will be constructed
     492% B b2;           // b2 and b2.x are both constructed
     493
     494% struct C { A x; };
     495% // implicit definition of ?{}(C*), because C is not a POD type since
     496% // it contains a non-POD type by composition
     497% C c;            // c and c.x are both constructed
     498
     499% [10] Requiring construction by composition
     500
     501% struct A {
     502%   ...
     503% };
     504
     505% // declared ctor disables default c-style initialization of
     506% // A objects; A is no longer a POD type
     507% void ?{}(A *);
     508
     509% struct B {
     510%   A x;
     511% };
     512
     513% // B objects can not be C-style initialized, because A objects
     514% // must be constructed => B objects are transitively not POD types
     515% B b; // b.x must be constructed, but B is not constructible
     516%      // => must autogenerate ?{}(B *) after struct B definition,
     517%      // which calls ?{}(&b.x)
     518
     519% [11] Explosion in the number of generated constructors, due to strange
     520% C semantics.
     521
     522% struct A { int x, y; };
     523% struct B { A u, v, w; };
     524
     525% A a = { 0, 0 };
     526
     527% // in C, you are allowed to do this
     528% B b1 = { 1, 2, 3, 4, 5, 6 };
     529% B b2 = { 1, 2, 3 };
     530% B b3 = { a, a, a };
     531% B b4 = { a, 5, 4, a };
     532% B b5 = { 1, 2, a, 3 };
     533
     534% // we want to disallow b1, b2, b4, and b5 in Cforall.
     535% // In particular, we will autogenerate these constructors:
     536% void ?{}(A *);             // default/0 parameters
     537% void ?{}(A *, int);        // 1 parameter
     538% void ?{}(A *, int, int);   // 2 parameters
     539% void ?{}(A *, const A *);  // copy constructor
     540
     541% void ?{}(B *);             // default/0 parameters
     542% void ?{}(B *, A);          // 1 parameter
     543% void ?{}(B *, A, A);       // 2 parameters
     544% void ?{}(B *, A, A, A);    // 3 parameters
     545% void ?{}(B *, const B *);  // copy constructor
     546
     547% // we will not generate constructors for every valid combination
     548% // of members in C. For example, we will not generate
     549% void ?{}(B *, int, int, int, int, int, int);   // b1 would need this
     550% void ?{}(B *, int, int, int);                  // b2 would need this
     551% void ?{}(B *, A, int, int, A);                 // b4 would need this
     552% void ?{}(B *, int, int, A, int);               // b5 would need this
     553% // and so on
     554
     555
     556
     557% TODO: talk somewhere about compound literals?
     558
    8559Since \CFA is a true systems language, it does not provide a garbage collector.
    9 As well, \CFA is not an object-oriented programming language, i.e., structures cannot have routine members.
     560As well, \CFA is not an object-oriented programming language, i.e. structures cannot have routine members.
    10561Nevertheless, one important goal is to reduce programming complexity and increase safety.
    11562To that end, \CFA provides support for implicit pre/post-execution of routines for objects, via constructors and destructors.
    12563
     564% TODO: this is old. remove or refactor
     565% Manual resource management is difficult.
     566% Part of the difficulty results from not having any guarantees about the current state of an object.
     567% Objects can be internally composed of pointers that may reference resources which may or may not need to be manually released, and keeping track of that state for each object can be difficult for the end user.
     568
     569% Constructors and destructors provide a mechanism to bookend the lifetime of an object, allowing the designer of a type to establish invariants for objects of that type.
     570% Constructors guarantee that object initialization code is run before the object can be used, while destructors provide a mechanism that is guaranteed to be run immediately before an object's lifetime ends.
     571% Constructors and destructors can help to simplify resource management when used in a disciplined way.
     572% In particular, when all resources are acquired in a constructor, and all resources are released in a destructor, no resource leaks are possible.
     573% This pattern is a popular idiom in several languages, such as \CC, known as RAII (Resource Acquisition Is Initialization).
     574
    13575This chapter details the design of constructors and destructors in \CFA, along with their current implementation in the translator.
    14 Generated code samples have been edited for clarity and brevity.
     576Generated code samples have been edited to provide comments for clarity and to save on space.
    15577
    16578\section{Design Criteria}
    … …  
    30592Next, @x@ is assigned the value of @y@.
    31593In the last line, @z@ is implicitly initialized to 0 since it is marked @static@.
    32 The key difference between assignment and initialization being that assignment occurs on a live object (i.e., an object that contains data).
     594The key difference between assignment and initialization being that assignment occurs on a live object (i.e. an object that contains data).
    33595It is important to note that this means @x@ could have been used uninitialized prior to being assigned, while @y@ could not be used uninitialized.
    34 Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs.
    35 
    36 Initialization of a declaration is strictly optional, permitting uninitialized variables to exist.
    37 Furthermore, declaration initialization is limited to expressions, so there is no way to insert arbitrary code before a variable is live, without delaying the declaration.
    38 Many C compilers give good warnings for uninitialized variables most of the time, but they cannot in all cases.
    39 \begin{cfacode}
    40 int f(int *);  // output parameter: never reads, only writes
    41 int g(int *);  // input parameter: never writes, only reads,
    42                // so requires initialized variable
     596Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs. % TODO: *citation*
     597
     598Declaration initialization is insufficient, because it permits uninitialized variables to exist and because it does not allow for the insertion of arbitrary code before the variable is live.
     599Many C compilers give good warnings most of the time, but they cannot in all cases.
     600\begin{cfacode}
     601int f(int *);  // never reads the parameter, only writes
     602int g(int *);  // reads the parameter - expects an initialized variable
    43603
    44604int x, y;
    45605f(&x);  // okay - only writes to x
    46 g(&y);  // uses y uninitialized
    47 \end{cfacode}
    48 Other languages are able to give errors in the case of uninitialized variable use, but due to backwards compatibility concerns, this is not the case in \CFA.
    49 
    50 In C, constructors and destructors are often mimicked by providing routines that create and tear down objects, where the tear down function is typically only necessary if the type modifies the execution environment.
     606g(&y);  // will use y uninitialized
     607\end{cfacode}
     608Other languages are able to give errors in the case of uninitialized variable use, but due to backwards compatibility concerns, this cannot be the case in \CFA.
     609
     610In C, constructors and destructors are often mimicked by providing routines that create and teardown objects, where the teardown function is typically only necessary if the type modifies the execution environment.
    51611\begin{cfacode}
    52612struct array_int {
    … …  
    54614};
    55615struct array_int create_array(int sz) {
    56   return (struct array_int) { calloc(sizeof(int)*sz) };
     616  return (struct array_int) { malloc(sizeof(int)*sz) };
    57617}
    58618void destroy_rh(struct resource_holder * rh) {
    … …  
    74634Furthermore, even with this idiom it is easy to make mistakes, such as forgetting to destroy an object or destroying it multiple times.
    75635
    76 A constructor provides a way of ensuring that the necessary aspects of object initialization is performed, from setting up invariants to providing compile- and run-time checks for appropriate initialization parameters.
     636A constructor provides a way of ensuring that the necessary aspects of object initialization is performed, from setting up invariants to providing compile-time checks for appropriate initialization parameters.
    77637This goal is achieved through a guarantee that a constructor is called implicitly after every object is allocated from a type with associated constructors, as part of an object's definition.
    78638Since a constructor is called on every object of a managed type, it is impossible to forget to initialize such objects, as long as all constructors perform some sensible form of initialization.
    79639
    80640In \CFA, a constructor is a function with the name @?{}@.
    81 Like other operators in \CFA, the name represents the syntax used to call the constructor, e.g., @struct S = { ... };@.
    82641Every constructor must have a return type of @void@ and at least one parameter, the first of which is colloquially referred to as the \emph{this} parameter, as in many object-oriented programming-languages (however, a programmer can give it an arbitrary name).
    83642The @this@ parameter must have a pointer type, whose base type is the type of object that the function constructs.
    … …  
    96655
    97656In C, if the user creates an @Array@ object, the fields @data@ and @len@ are uninitialized, unless an explicit initializer list is present.
    98 It is the user's responsibility to remember to initialize both of the fields to sensible values, since there are no implicit checks for invalid values or reasonable defaults.
     657It is the user's responsibility to remember to initialize both of the fields to sensible values.
    99658In \CFA, the user can define a constructor to handle initialization of @Array@ objects.
    100659
    … …  
    112671This constructor initializes @x@ so that its @length@ field has the value 10, and its @data@ field holds a pointer to a block of memory large enough to hold 10 @int@s, and sets the value of each element of the array to 0.
    113672This particular form of constructor is called the \emph{default constructor}, because it is called on an object defined without an initializer.
    114 In other words, a default constructor is a constructor that takes a single argument: the @this@ parameter.
    115 
    116 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it take only one argument.
     673In other words, a default constructor is a constructor that takes a single argument, the @this@ parameter.
     674
     675In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}!.
    117676A destructor for the @Array@ type can be defined as such.
    118677\begin{cfacode}
    … …  
    121680}
    122681\end{cfacode}
    123 The destructor is automatically called at deallocation for all objects of type @Array@.
    124 Hence, the memory associated with an @Array@ is automatically freed when the object's lifetime ends.
     682Since the destructor is automatically called at deallocation for all objects of type @Array@, the memory associated with an @Array@ is automatically freed when the object's lifetime ends.
    125683The exact guarantees made by \CFA with respect to the calling of destructors are discussed in section \ref{sub:implicit_dtor}.
    126684
    … …  
    133691\end{cfacode}
    134692By the previous definition of the default constructor for @Array@, @x@ and @y@ are initialized to valid arrays of length 10 after their respective definitions.
    135 On line 2, @z@ is initialized with the value of @x@, while on line 3, @y@ is assigned the value of @x@.
     693On line 3, @z@ is initialized with the value of @x@, while on line @4@, @y@ is assigned the value of @x@.
    136694The key distinction between initialization and assignment is that a value to be initialized does not hold any meaningful values, whereas an object to be assigned might.
    137695In particular, these cases cannot be handled the same way because in the former case @z@ does not currently own an array, while @y@ does.
    … …  
    154712The first function is called a \emph{copy constructor}, because it constructs its argument by copying the values from another object of the same type.
    155713The second function is the standard copy-assignment operator.
    156 The four functions (default constructor, destructor, copy constructor, and assignment operator) are special in that they safely control the state of most objects.
     714These four functions are special in that they control the state of most objects.
    157715
    158716It is possible to define a constructor that takes any combination of parameters to provide additional initialization options.
    159 For example, a reasonable extension to the array type would be a constructor that allocates the array to a given initial capacity and initializes the elements of the array to a given @fill@ value.
     717For example, a reasonable extension to the array type would be a constructor that allocates the array to a given initial capacity and initializes the array to a given @fill@ value.
    160718\begin{cfacode}
    161719void ?{}(Array * arr, int capacity, int fill) {
    … …  
    171729Array x, y = { 20, 0xdeadbeef }, z = y;
    172730\end{cfacode}
    173 
    174731In \CFA, constructor calls look just like C initializers, which allows them to be inserted into legacy C code with minimal code changes, and also provides a very simple syntax that veteran C programmers are familiar with.
    175732One downside of reusing C initialization syntax is that it isn't possible to determine whether an object is constructed just by looking at its declaration, since that requires knowledge of whether the type is managed at that point.
    … …  
    191748Destructors are implicitly called in reverse declaration-order so that objects with dependencies are destructed before the objects they are dependent on.
    192749
    193 \subsection{Calling Syntax}
    194 \label{sub:syntax}
     750\subsection{Syntax}
     751\label{sub:syntax} % TODO: finish this section
    195752There are several ways to construct an object in \CFA.
    196753As previously introduced, every variable is automatically constructed at its definition, which is the most natural way to construct an object.
    … …  
    216773A * y = malloc();  // copy construct: ?{}(&y, malloc())
    217774
    218 ?{}(&x);    // explicit construct x, second construction
    219 ?{}(y, x);  // explit construct y from x, second construction
    220 ^?{}(&x);   // explicit destroy x, in different order
     775?{}(&x);    // explicit construct x
     776?{}(y, x);  // explit construct y from x
     777^?{}(&x);   // explicit destroy x
    221778^?{}(y);    // explicit destroy y
    222779
    … …  
    224781// implicit ^?{}(&x);
    225782\end{cfacode}
    226 Calling a constructor or destructor directly is a flexible feature that allows complete control over the management of storage.
     783Calling a constructor or destructor directly is a flexible feature that allows complete control over the management of a piece of storage.
    227784In particular, constructors double as a placement syntax.
    228785\begin{cfacode}
    … …  
    247804Finally, constructors and destructors support \emph{operator syntax}.
    248805Like other operators in \CFA, the function name mirrors the use-case, in that the first $N$ arguments fill in the place of the question mark.
    249 This syntactic form is similar to the new initialization syntax in \CCeleven, except that it is used in expression contexts, rather than declaration contexts.
    250806\begin{cfacode}
    251807struct A { ... };
    … …  
    266822Destructor operator syntax is actually an statement, and requires parentheses for symmetry with constructor syntax.
    267823
    268 One of these three syntactic forms should appeal to either C or \CC programmers using \CFA.
    269 
    270 \subsection{Constructor Expressions}
    271 In \CFA, it is possible to use a constructor as an expression.
    272 Like other operators, the function name @?{}@ matches its operator syntax.
    273 For example, @(&x){}@ calls the default constructor on the variable @x@, and produces @&x@ as a result.
    274 A key example for this capability is the use of constructor expressions to initialize the result of a call to standard C routine @malloc@.
    275 \begin{cfacode}
    276 struct X { ... };
    277 void ?{}(X *, double);
    278 X * x = malloc(sizeof(X)){ 1.5 };
    279 \end{cfacode}
    280 In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@.
    281 If this extension is not present, constructing dynamically allocated objects is much more cumbersome, requiring separate initialization of the pointer and initialization of the pointed-to memory.
    282 \begin{cfacode}
    283 X * x = malloc(sizeof(X));
    284 x{ 1.5 };
    285 \end{cfacode}
    286 Not only is this verbose, but it is also more error prone, since this form allows maintenance code to easily sneak in between the initialization of @x@ and the initialization of the memory that @x@ points to.
    287 This feature is implemented via a transformation producing the value of the first argument of the constructor, since constructors do not themselves have a return value.
    288 Since this transformation results in two instances of the subexpression, care is taken to allocate a temporary variable to hold the result of the subexpression in the case where the subexpression may contain side effects.
    289 The previous example generates the following code.
    290 \begin{cfacode}
    291 struct X *_tmp_ctor;
    292 struct X *x = ?{}(  // construct result of malloc
    293   _tmp_ctor=malloc(sizeof(struct X)), // store result of malloc
    294   1.5
    295 ), _tmp_ctor; // produce constructed result of malloc
    296 \end{cfacode}
    297 It should be noted that this technique is not exclusive to @malloc@, and allows a user to write a custom allocator that can be idiomatically used in much the same way as a constructed @malloc@ call.
    298 
    299 It is also possible to use operator syntax with destructors.
    300 Unlike constructors, operator syntax with destructors is a statement and thus does not produce a value, since the destructed object is invalidated by the use of a destructor.
    301 For example, \lstinline!^(&x){}! calls the destructor on the variable @x@.
    302 
    303824\subsection{Function Generation}
    304 In \CFA, every type is defined to have the core set of four special functions described previously.
     825In \CFA, every type is defined to have the core set of four functions described previously.
    305826Having these functions exist for every type greatly simplifies the semantics of the language, since most operations can simply be defined directly in terms of function calls.
    306827In addition to simplifying the definition of the language, it also simplifies the analysis that the translator must perform.
    … …  
    312833There are several options for user-defined types: structures, unions, and enumerations.
    313834To aid in ease of use, the standard set of four functions is automatically generated for a user-defined type after its definition is completed.
    314 By auto-generating these functions, it is ensured that legacy C code continues to work correctly in every context where \CFA expects these functions to exist, since they are generated for every complete type.
     835By auto-generating these functions, it is ensured that legacy C code will continue to work correctly in every context where \CFA expects these functions to exist, since they are generated for every complete type.
    315836
    316837The generated functions for enumerations are the simplest.
    317 Since enumerations in C are essentially just another integral type, the generated functions behave in the same way that the built-in functions for the basic types work.
     838Since enumerations in C are essentially just another integral type, the generated functions behave in the same way that the builtin functions for the basic types work.
     839% TODO: examples for enums
    318840For example, given the enumeration
    319841\begin{cfacode}
    … …  
    328850}
    329851void ?{}(enum Colour *_dst, enum Colour _src){
    330   *_dst=_src;  // bitwise copy
     852  (*_dst)=_src;  // bitwise copy
    331853}
    332854void ^?{}(enum Colour *_dst){
    … …  
    334856}
    335857enum Colour ?=?(enum Colour *_dst, enum Colour _src){
    336   return *_dst=_src; // bitwise copy
     858  return (*_dst)=_src; // bitwise copy
    337859}
    338860\end{cfacode}
    339861In the future, \CFA will introduce strongly-typed enumerations, like those in \CC.
    340 The existing generated routines are sufficient to express this restriction, since they are currently set up to take in values of that enumeration type.
     862The existing generated routines will be sufficient to express this restriction, since they are currently set up to take in values of that enumeration type.
    341863Changes related to this feature only need to affect the expression resolution phase, where more strict rules will be applied to prevent implicit conversions from integral types to enumeration types, but should continue to permit conversions from enumeration types to @int@.
    342 In this way, it is still possible to add an @int@ to an enumeration, but the resulting value is an @int@, meaning it cannot be reassigned to an enumeration without a cast.
     864In this way, it will still be possible to add an @int@ to an enumeration, but the resulting value will be an @int@, meaning that it won't be possible to reassign the value into an enumeration without a cast.
    343865
    344866For structures, the situation is more complicated.
    345 Given a structure @S@ with members @M$_0$@, @M$_1$@, ... @M$_{N-1}$@, each function @f@ in the standard set calls \lstinline{f(s->M$_i$, ...)} for each @$i$@.
    346 That is, a default constructor for @S@ default constructs the members of @S@, the copy constructor copy constructs them, and so on.
    347 For example, given the structure definition
     867For a structure @S@ with members @M$_0$@, @M$_1$@, ... @M$_{N-1}$@, each function @f@ in the standard set calls \lstinline{f(s->M$_i$, ...)} for each @$i$@.
     868That is, a default constructor for @S@ default constructs the members of @S@, the copy constructor with copy construct them, and so on.
     869For example given the struct definition
    348870\begin{cfacode}
    349871struct A {
    … …  
    371893}
    372894\end{cfacode}
    373 It is important to note that the destructors are called in reverse declaration order to prevent conflicts in the event there are dependencies among members.
     895It is important to note that the destructors are called in reverse declaration order to resolve conflicts in the event there are dependencies among members.
    374896
    375897In addition to the standard set, a set of \emph{field constructors} is also generated for structures.
    376 The field constructors are constructors that consume a prefix of the structure's member-list.
     898The field constructors are constructors that consume a prefix of the struct's member list.
    377899That is, $N$ constructors are built of the form @void ?{}(S *, T$_{\text{M}_0}$)@, @void ?{}(S *, T$_{\text{M}_0}$, T$_{\text{M}_1}$)@, ..., @void ?{}(S *, T$_{\text{M}_0}$, T$_{\text{M}_1}$, ..., T$_{\text{M}_{N-1}}$)@, where members are copy constructed if they have a corresponding positional argument and are default constructed otherwise.
    378 The addition of field constructors allows structures in \CFA to be used naturally in the same ways as used in C (i.e., to initialize any prefix of the structure), e.g., @A a0 = { b }, a1 = { b, c }@.
     900The addition of field constructors allows structs in \CFA to be used naturally in the same ways that they could be used in C (i.e. to initialize any prefix of the struct), e.g., @A a0 = { b }, a1 = { b, c }@.
    379901Extending the previous example, the following constructors are implicitly generated for @A@.
    380902\begin{cfacode}
    … …  
    389911\end{cfacode}
    390912
    391 For unions, the default constructor and destructor do nothing, as it is not obvious which member, if any, should be constructed.
     913For unions, the default constructor and destructor do nothing, as it is not obvious which member if any should be constructed.
    392914For copy constructor and assignment operations, a bitwise @memcpy@ is applied.
    393915In standard C, a union can also be initialized using a value of the same type as its first member, and so a corresponding field constructor is generated to perform a bitwise @memcpy@ of the object.
    394 An alternative to this design is to always construct and destruct the first member of a union, to match with the C semantics of initializing the first member of the union.
     916An alterantive to this design is to always construct and destruct the first member of a union, to match with the C semantics of initializing the first member of the union.
    395917This approach ultimately feels subtle and unsafe.
    396918Another option is to, like \CC, disallow unions from containing members that are themselves managed types.
    … …  
    425947
    426948% This feature works in the \CFA model, since constructors are simply special functions and can be called explicitly, unlike in \CC. % this sentence isn't really true => placement new
    427 In \CCeleven, unions may have managed members, with the caveat that if there are any members with a user-defined operation, then that operation is not implicitly defined, forcing the user to define the operation if necessary.
     949In \CCeleven, this restriction has been loosened to allow unions with managed members, with the caveat that any if there are any members with a user-defined operation, then that operation is not implicitly defined, forcing the user to define the operation if necessary.
    428950This restriction could easily be added into \CFA once \emph{deleted} functions are added.
    429951
    … …  
    448970Here, @&s@ and @&s2@ are cast to unqualified pointer types.
    449971This mechanism allows the same constructors and destructors to be used for qualified objects as for unqualified objects.
    450 This applies only to implicitly generated constructor calls.
    451 Hence, explicitly re-initializing qualified objects with a constructor requires an explicit cast.
    452 
    453 As discussed in Section \ref{sub:c_background}, compound literals create unnamed objects.
    454 This mechanism can continue to be used seamlessly in \CFA with managed types to create temporary objects.
    455 The object created by a compound literal is constructed using the provided brace-enclosed initializer-list, and is destructed at the end of the scope it is used in.
    456 For example,
    457 \begin{cfacode}
    458 struct A { int x; };
    459 void ?{}(A *, int, int);
    460 {
    461   int x = (A){ 10, 20 }.x;
    462 }
    463 \end{cfacode}
    464 is equivalent to
    465 \begin{cfacode}
    466 struct A { int x, y; };
    467 void ?{}(A *, int, int);
    468 {
    469   A _tmp;
    470   ?{}(&_tmp, 10, 20);
    471   int x = _tmp.x;
    472   ^?{}(&tmp);
    473 }
    474 \end{cfacode}
     972Since this applies only to implicitly generated constructor calls, the language does not allow qualified objects to be re-initialized with a constructor without an explicit cast.
    475973
    476974Unlike \CC, \CFA provides an escape hatch that allows a user to decide at an object's definition whether it should be managed or not.
    … …  
    486984A a2 @= { 0 };  // unmanaged
    487985\end{cfacode}
    488 In this example, @a1@ is a managed object, and thus is default constructed and destructed at the start/end of @a1@'s lifetime, while @a2@ is an unmanaged object and is not implicitly constructed or destructed.
    489 Instead, @a2->x@ is initialized to @0@ as if it were a C object, because of the explicit initializer.
    490 
    491 In addition to freedom, \ateq provides a simple path to migrating legacy C code to \CFA, in that objects can be moved from C-style initialization to \CFA gradually and individually.
     986In this example, @a1@ is a managed object, and thus is default constructed and destructed at the end of @a1@'s lifetime, while @a2@ is an unmanaged object and is not implicitly constructed or destructed.
     987Instead, @a2->x@ is initialized to @0@ as if it were a C object, due to the explicit initializer.
     988Existing constructors are ignored when \ateq is used, so that any valid C initializer is able to initialize the object.
     989
     990In addition to freedom, \ateq provides a simple path to migrating legacy C code to Cforall, in that objects can be moved from C-style initialization to \CFA gradually and individually.
    492991It is worth noting that the use of unmanaged objects can be tricky to get right, since there is no guarantee that the proper invariants are established on an unmanaged object.
    493992It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary.
    494993
    495 When a user declares any constructor or destructor, the corresponding intrinsic/generated function and all field constructors for that type are hidden, so that they are not found during expression resolution until the user-defined function goes out of scope.
    496 Furthermore, if the user declares any constructor, then the intrinsic/generated default constructor is also hidden, precluding default construction.
    497 These semantics closely mirror the rule for implicit declaration of constructors in \CC, wherein the default constructor is implicitly declared if there is no user-declared constructor \cite[p.~186]{ANSI98:C++}.
     994When the user declares any constructor or destructor, the corresponding intrinsic/generated function and all field constructors for that type are hidden, so that they will not be found during expression resolution unless the user-defined function goes out of scope.
     995Furthermore, if the user declares any constructor, then the intrinsic/generated default constructor is also hidden, making it so that objects of a type may not be default constructable.
     996This closely mirrors the rule for implicit declaration of constructors in \CC, wherein the default constructor is implicitly declared if there is no user-declared constructor. % TODO: cite C++98 page 186??
    498997\begin{cfacode}
    499998struct S { int x, y; };
    … …  
    5021001  S s0, s1 = { 0 }, s2 = { 0, 2 }, s3 = s2;  // okay
    5031002  {
    504     void ?{}(S * s, int i) { s->x = i*2; } // locally hide autogen constructors
     1003    void ?{}(S * s, int i) { s->x = i*2; }
    5051004    S s4;  // error
    5061005    S s5 = { 3 };  // okay
    … …  
    5151014When defining a constructor or destructor for a struct @S@, any members that are not explicitly constructed or destructed are implicitly constructed or destructed automatically.
    5161015If an explicit call is present, then that call is taken in preference to any implicitly generated call.
    517 A consequence of this rule is that it is possible, unlike \CC, to precisely control the order of construction and destruction of sub-objects on a per-constructor basis, whereas in \CC sub-object initialization and destruction is always performed based on the declaration order.
     1016A consequence of this rule is that it is possible, unlike \CC, to precisely control the order of construction and destruction of subobjects on a per-constructor basis, whereas in \CC subobject initialization and destruction is always performed based on the declaration order.
    5181017\begin{cfacode}
    5191018struct A {
    … …  
    5341033}
    5351034\end{cfacode}
    536 Finally, it is illegal for a sub-object to be explicitly constructed for the first time after it is used for the first time.
     1035Finally, it is illegal for a subobject to be explicitly constructed for the first time after it is used for the first time.
    5371036If the translator cannot be reasonably sure that an object is constructed prior to its first use, but is constructed afterward, an error is emitted.
    538 More specifically, the translator searches the body of a constructor to ensure that every sub-object is initialized.
     1037More specifically, the translator searches the body of a constructor to ensure that every subobject is initialized.
    5391038\begin{cfacode}
    5401039void ?{}(A * a, double x) {
    … …  
    5431042}
    5441043\end{cfacode}
    545 However, if the translator sees a sub-object used within the body of a constructor, but does not see a constructor call that uses the sub-object as the target of a constructor, then the translator assumes the object is to be implicitly constructed (copy constructed in a copy constructor and default constructed in any other constructor).
     1044However, if the translator sees a subobject used within the body of a constructor, but does not see a constructor call that uses the subobject as the target of a constructor, then the translator assumes the object is to be implicitly constructed (copy constructed in a copy constructor and default constructed in any other constructor).
    5461045\begin{cfacode}
    5471046void ?{}(A * a) {
    … …  
    5591058} // z, y, w implicitly destructed, in this order
    5601059\end{cfacode}
    561 If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed that constructor handles the initialization of all of the object's members and no implicit constructor calls are added.
     1060If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed that constructor handles the initialization of all of the object's members and no implicit constructor calls are added. % TODO: confirm that this is correct. It might be possible to get subtle errors if you initialize some members then call another constructor... -- in fact, this is basically always wrong. if anything, I should check that such a constructor does not initialize any members, otherwise it'll always initialize the member twice (once locally, once by the called constructor).
    5621061To override this rule, \ateq can be used to force the translator to trust the programmer's discretion.
    5631062This form of \ateq is not yet implemented.
    … …  
    5651064Despite great effort, some forms of C syntax do not work well with constructors in \CFA.
    5661065In particular, constructor calls cannot contain designations (see \ref{sub:c_background}), since this is equivalent to allowing designations on the arguments to arbitrary function calls.
     1066In C, function prototypes are permitted to have arbitrary parameter names, including no names at all, which may have no connection to the actual names used at function definition.
     1067Furthermore, a function prototype can be repeated an arbitrary number of times, each time using different names.
    5671068\begin{cfacode}
    5681069// all legal forward declarations in C
    … …  
    5751076f(b:10, a:20, c:30);  // which parameter is which?
    5761077\end{cfacode}
    577 In C, function prototypes are permitted to have arbitrary parameter names, including no names at all, which may have no connection to the actual names used at function definition.
    578 Furthermore, a function prototype can be repeated an arbitrary number of times, each time using different names.
    5791078As a result, it was decided that any attempt to resolve designated function calls with C's function prototype rules would be brittle, and thus it is not sensible to allow designations in constructor calls.
    580 
    581 In addition, constructor calls do not support unnamed nesting.
    582 \begin{cfacode}
    583 struct B { int x; };
    584 struct C { int y; };
    585 struct A { B b; C c; };
    586 void ?{}(A *, B);
    587 void ?{}(A *, C);
    588 
    589 A a = {
    590   { 10 },  // construct B? - invalid
    591 };
    592 \end{cfacode}
    593 In C, nesting initializers means that the programmer intends to initialize sub-objects with the nested initializers.
    594 The reason for this omission is to both simplify the mental model for using constructors, and to make initialization simpler for the expression resolver.
    595 If this were allowed, it would be necessary for the expression resolver to decide whether each argument to the constructor call could initialize to some argument in one of the available constructors, making the problem highly recursive and potentially much more expensive.
    596 That is, in the previous example the line marked as an error could mean construct using @?{}(A *, B)@ or with @?{}(A *, C)@, since the inner initializer @{ 10 }@ could be taken as an intermediate object of type @B@ or @C@.
    597 In practice, however, there could be many objects that can be constructed from a given @int@ (or, indeed, any arbitrary parameter list), and thus a complete solution to this problem would require fully exploring all possibilities.
    598 
    599 More precisely, constructor calls cannot have a nesting depth greater than the number of array components in the type of the initialized object, plus one.
     1079% Many other languages do allow named arguments, such as Python and Scala, but they do not allow multiple arbitrarily named forward declarations of a function.
     1080
     1081In addition, constructor calls cannot have a nesting depth greater than the number of array components in the type of the initialized object, plus one.
    6001082For example,
    6011083\begin{cfacode}
    … …  
    6141096}
    6151097\end{cfacode}
     1098% TODO: in CFA if the array dimension is empty, no object constructors are added -- need to fix this.
    6161099The body of @A@ has been omitted, since only the constructor interfaces are important.
    617 
     1100In C, having a greater nesting depth means that the programmer intends to initialize subobjects with the nested initializer.
     1101The reason for this omission is to both simplify the mental model for using constructors, and to make initialization simpler for the expression resolver.
     1102If this were allowed, it would be necessary for the expression resolver to decide whether each argument to the constructor call could initialize to some argument in one of the available constructors, making the problem highly recursive and potentially much more expensive.
     1103That is, in the previous example the line marked as an error could mean construct using @?{}(A *, A, A)@, since the inner initializer @{ 11 }@ could be taken as an intermediate object of type @A@ constructed with @?{}(A *, int)@.
     1104In practice, however, there could be many objects that can be constructed from a given @int@ (or, indeed, any arbitrary parameter list), and thus a complete solution to this problem would require fully exploring all possibilities.
    6181105It should be noted that unmanaged objects can still make use of designations and nested initializers in \CFA.
    619 It is simple to overcome this limitation for managed objects by making use of compound literals, so that the arguments to the constructor call are explicitly typed.
    6201106
    6211107\subsection{Implicit Destructors}
    … …  
    6411127    if (i == 2) return; // destruct x, y
    6421128  } // destruct y
    643 } // destruct x
    644 \end{cfacode}
     1129}
     1130\end{cfacode}
     1131
     1132%% having this feels excessive, but it's here if necessary
     1133% This procedure generates the following code.
     1134% \begin{cfacode}
     1135% void f(int i){
     1136%   struct A x;
     1137%   ?{}(&x);
     1138%   {
     1139%     struct A y;
     1140%     ?{}(&y);
     1141%     {
     1142%       struct A z;
     1143%       ?{}(&z);
     1144%       {
     1145%         if ((i==0)!=0) {
     1146%           ^?{}(&z);
     1147%           ^?{}(&y);
     1148%           ^?{}(&x);
     1149%           return;
     1150%         }
     1151%       }
     1152%       if (((i==1)!=0) {
     1153%           ^?{}(&z);
     1154%           ^?{}(&y);
     1155%           ^?{}(&x);
     1156%           return ;
     1157%       }
     1158%       ^?{}(&z);
     1159%     }
     1160
     1161%     if ((i==2)!=0) {
     1162%       ^?{}(&y);
     1163%       ^?{}(&x);
     1164%       return;
     1165%     }
     1166%     ^?{}(&y);
     1167%   }
     1168
     1169%   ^?{}(&x);
     1170% }
     1171% \end{cfacode}
    6451172
    6461173The next example illustrates the use of simple continue and break statements and the manner that they interact with implicit destructors.
    … …  
    6561183\end{cfacode}
    6571184Since a destructor call is automatically inserted at the end of the block, nothing special needs to happen to destruct @x@ in the case where control reaches the end of the loop.
    658 In the case where @i@ is @2@, the continue statement runs the loop update expression and attempts to begin the next iteration of the loop.
    659 Since continue is a C statement, which does not understand destructors, it is transformed into a @goto@ statement that branches to the end of the loop, just before the block's destructors, to ensure that @x@ is destructed.
     1185In the case where @i@ is @2@, the continue statement runs the loop update expression and attemps to begin the next iteration of the loop.
     1186Since continue is a C statement, which does not understand destructors, a destructor call is added just before the continue statement to ensure that @x@ is destructed.
    6601187When @i@ is @3@, the break statement moves control to just past the end of the loop.
    661 Unlike the previous case, the destructor for @x@ cannot be reused, so a destructor call for @x@ is inserted just before the break statement.
    662 
    663 \CFA also supports labeled break and continue statements, which allow more precise manipulation of control flow.
    664 Labeled break and continue allow the programmer to specify which control structure to target by using a label attached to a control structure.
     1188Like the previous case, a destructor call for @x@ is inserted just before the break statement.
     1189
     1190\CFA also supports labelled break and continue statements, which allow more precise manipulation of control flow.
     1191Labelled break and continue allow the programmer to specify which control structure to target by using a label attached to a control structure.
    6651192\begin{cfacode}[emph={L1,L2}, emphstyle=\color{red}]
    6661193L1: for (int i = 0; i < 10; i++) {
    6671194  A x;
    668   for (int j = 0; j < 10; j++) {
     1195  L2: for (int j = 0; j < 10; j++) {
    6691196    A y;
    670     if (i == 1) {
     1197    if (j == 0) {
     1198      continue;    // destruct y
     1199    } else if (j == 1) {
     1200      break;       // destruct y
     1201    } else if (i == 1) {
    6711202      continue L1; // destruct y
    6721203    } else if (i == 2) {
    … …  
    6771208\end{cfacode}
    6781209The statement @continue L1@ begins the next iteration of the outer for-loop.
    679 Since the semantics of continue require the loop update expression to execute, control branches to the end of the outer for loop, meaning that the block destructor for @x@ can be reused, and it is only necessary to generate the destructor for @y@.
    680 Break, on the other hand, requires jumping out of both loops, so the destructors for both @x@ and @y@ are generated and inserted before the @break L1@ statement.
     1210Since the semantics of continue require the loop update expression to execute, control branches to the \emph{end} of the outer for loop, meaning that the block destructor for @x@ can be reused, and it is only necessary to generate the destructor for @y@.
     1211Break, on the other hand, requires jumping out of the loop, so the destructors for both @x@ and @y@ are generated and inserted before the @break L1@ statement.
    6811212
    6821213Finally, an example which demonstrates goto.
    … …  
    7251256}
    7261257\end{cfacode}
    727 All break and continue statements are implemented in \CFA in terms of goto statements, so the more constrained forms are precisely governed by these rules.
     1258Labelled break and continue are implemented in \CFA in terms of goto statements, so the more constrained forms are precisely goverened by these rules.
    7281259
    7291260The next example demonstrates the error case.
    … …  
    7421273
    7431274\subsection{Implicit Copy Construction}
    744 \label{s:implicit_copy_construction}
    7451275When a function is called, the arguments supplied to the call are subject to implicit copy construction (and destruction of the generated temporary), and the return value is subject to destruction.
    7461276When a value is returned from a function, the copy constructor is called to pass the value back to the call site.
    747 Exempt from these rules are intrinsic and built-in functions.
     1277Exempt from these rules are intrinsic and builtin functions.
    7481278It should be noted that unmanaged objects are subject to copy constructor calls when passed as arguments to a function or when returned from a function, since they are not the \emph{target} of the copy constructor call.
    749 That is, since the parameter is not marked as an unmanaged object using \ateq, it will be copy constructed if it is returned by value or passed as an argument to another function, so to guarantee consistent behaviour, unmanaged objects must be copy constructed when passed as arguments.
    7501279This is an important detail to bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed.
    7511280\begin{cfacode}
    … …  
    7551284void ^?{}(A *);
    7561285
    757 A identity(A x) { // pass by value => need local copy
    758   return x;       // return by value => make call-site copy
     1286A f(A x) {
     1287  return x;
    7591288}
    7601289
    7611290A y, z @= {};
    762 identity(y);  // copy construct y into x
    763 identity(z);  // copy construct z into x
     1291identity(y);
     1292identity(z);
    7641293\end{cfacode}
    7651294Note that @z@ is copy constructed into a temporary variable to be passed as an argument, which is also destructed after the call.
     1295A special syntactic form, such as a variant of \ateq, could be implemented to specify at the call site that an argument should not be copy constructed, to regain some control for the C programmer.
    7661296
    7671297This generates the following
    7681298\begin{cfacode}
    7691299struct A f(struct A x){
    770   struct A _retval_f;    // return value
    771   ?{}((&_retval_f), x);  // copy construct return value
     1300  struct A _retval_f;
     1301  ?{}((&_retval_f), x);
    7721302  return _retval_f;
    7731303}
    7741304
    7751305struct A y;
    776 ?{}(&y);                 // default construct
    777 struct A z = { 0 };      // C default
    778 
    779 struct A _tmp_cp1;       // argument 1
    780 struct A _tmp_cp_ret0;   // return value
    781 _tmp_cp_ret0=f(
    782   (?{}(&_tmp_cp1, y) , _tmp_cp1)  // argument is a comma expression
    783 ), _tmp_cp_ret0;         // return value for cascading
    784 ^?{}(&_tmp_cp_ret0);     // destruct return value
    785 ^?{}(&_tmp_cp1);         // destruct argument 1
    786 
    787 struct A _tmp_cp2;       // argument 1
    788 struct A _tmp_cp_ret1;   // return value
    789 _tmp_cp_ret1=f(
    790   (?{}(&_tmp_cp2, z), _tmp_cp2)  // argument is a common expression
    791 ), _tmp_cp_ret1;         // return value for cascading
    792 ^?{}(&_tmp_cp_ret1);     // destruct return value
    793 ^?{}(&_tmp_cp2);         // destruct argument 1
     1306?{}(&y);
     1307struct A z = { 0 };
     1308
     1309struct A _tmp_cp1;     // argument 1
     1310struct A _tmp_cp_ret0; // return value
     1311_tmp_cp_ret0=f((?{}(&_tmp_cp1, y) , _tmp_cp1)), _tmp_cp_ret0;
     1312^?{}(&_tmp_cp_ret0);   // return value
     1313^?{}(&_tmp_cp1);       // argument 1
     1314
     1315struct A _tmp_cp2;     // argument 1
     1316struct A _tmp_cp_ret1; // return value
     1317_tmp_cp_ret1=f((?{}(&_tmp_cp2, z), _tmp_cp2)), _tmp_cp_ret1;
     1318^?{}(&_tmp_cp_ret1);   // return value
     1319^?{}(&_tmp_cp2);       // argument 1
    7941320^?{}(&y);
    7951321\end{cfacode}
    7961322
    797 A special syntactic form, such as a variant of \ateq, can be implemented to specify at the call site that an argument should not be copy constructed, to regain some control for the C programmer.
    798 \begin{cfacode}
    799 identity(z@);  // do not copy construct argument
    800                // - will copy construct/destruct return value
    801 A@ identity_nocopy(A @ x) {  // argument not copy constructed or destructed
    802   return x;  // not copy constructed
    803              // return type marked @ => not destructed
    804 }
    805 \end{cfacode}
    806 It should be noted that reference types will allow specifying that a value does not need to be copied, however reference types do not provide a means of preventing implicit copy construction from uses of the reference, so the problem is still present when passing or returning the reference by value.
    807 
    808 A known issue with this implementation is that the argument and return value temporaries are not guaranteed to have the same address for their entire lifetimes.
    809 In the previous example, since @_retval_f@ is allocated and constructed in @f@, then returned by value, the internal data is bitwise copied into the caller's stack frame.
     1323A known issue with this implementation is that the return value of a function is not guaranteed to have the same address for its entire lifetime.
     1324Specifically, since @_retval_f@ is allocated and constructed in @f@ then returned by value, the internal data is bitwise copied into the caller's stack frame.
    8101325This approach works out most of the time, because typically destructors need to only access the fields of the object and recursively destroy.
    811 It is currently the case that constructors and destructors that use the @this@ pointer as a unique identifier to store data externally do not work correctly for return value objects.
    812 Thus, it is currently not safe to rely on an object's @this@ pointer to remain constant throughout execution of the program.
     1326It is currently the case that constructors and destructors which use the @this@ pointer as a unique identifier to store data externally will not work correctly for return value objects.
     1327Thus is it not safe to rely on an object's @this@ pointer to remain constant throughout execution of the program.
    8131328\begin{cfacode}
    8141329A * external_data[32];
    … …  
    8261341  }
    8271342}
    828 
    829 A makeA() {
    830   A x;  // stores &x in external_data
    831   return x;
    832 }
    833 makeA();  // return temporary has a different address than x
    834 // equivalent to:
    835 //   A _tmp;
    836 //   _tmp = makeA(), _tmp;
    837 //   ^?{}(&_tmp);
    8381343\end{cfacode}
    8391344In the above example, a global array of pointers is used to keep track of all of the allocated @A@ objects.
    840 Due to copying on return, the current object being destructed does not exist in the array if an @A@ object is ever returned by value from a function, such as in @makeA@.
    841 
    842 This problem could be solved in the translator by changing the function signatures so that the return value is moved into the parameter list.
     1345Due to copying on return, the current object being destructed will not exist in the array if an @A@ object is ever returned by value from a function.
     1346
     1347This problem could be solved in the translator by mutating the function signatures so that the return value is moved into the parameter list.
    8431348For example, the translator could restructure the code like so
    8441349\begin{cfacode}
    … …  
    8581363\end{cfacode}
    8591364This transformation provides @f@ with the address of the return variable so that it can be constructed into directly.
    860 It is worth pointing out that this kind of signature rewriting already occurs in polymorphic functions that return by value, as discussed in \cite{Bilson03}.
     1365It is worth pointing out that this kind of signature rewriting already occurs in polymorphic functions which return by value, as discussed in \cite{Bilson03}.
    8611366A key difference in this case is that every function would need to be rewritten like this, since types can switch between managed and unmanaged at different scope levels, e.g.
    8621367\begin{cfacode}
    8631368struct A { int v; };
    864 A x; // unmanaged, since only trivial constructors are available
     1369A x; // unmanaged
    8651370{
    8661371  void ?{}(A * a) { ... }
    … …  
    8701375A z; // unmanaged
    8711376\end{cfacode}
    872 Hence there is not enough information to determine at function declaration whether a type is managed or not, and thus it is the case that all signatures have to be rewritten to account for possible copy constructor and destructor calls.
     1377Hence there is not enough information to determine at function declaration to determine whether a type is managed or not, and thus it is the case that all signatures have to be rewritten to account for possible copy constructor and destructor calls.
    8731378Even with this change, it would still be possible to declare backwards compatible function prototypes with an @extern "C"@ block, which allows for the definition of C-compatible functions within \CFA code, however this would require actual changes to the way code inside of an @extern "C"@ function is generated as compared with normal code generation.
    874 Furthermore, it is not possible to overload C functions, so using @extern "C"@ to declare functions is of limited use.
    875 
    876 It would be possible to regain some control by adding an attribute to structs that specifies whether they can be managed or not (perhaps \emph{manageable} or \emph{unmanageable}), and to emit an error in the case that a constructor or destructor is declared for an unmanageable type.
     1379Furthermore, it isn't possible to overload C functions, so using @extern "C"@ to declare functions is of limited use.
     1380
     1381It would be possible to regain some control by adding an attribute to structs which specifies whether they can be managed or not (perhaps \emph{manageable} or \emph{unmanageable}), and to emit an error in the case that a constructor or destructor is declared for an unmanageable type.
    8771382Ideally, structs should be manageable by default, since otherwise the default case becomes more verbose.
    8781383This means that in general, function signatures would have to be rewritten, and in a select few cases the signatures would not be rewritten.
    … …  
    8871392\end{cfacode}
    8881393An alternative is to instead make the attribute \emph{identifiable}, which states that objects of this type use the @this@ parameter as an identity.
    889 This strikes more closely to the visible problem, in that only types marked as identifiable would need to have the return value moved into the parameter list, and every other type could remain the same.
     1394This strikes more closely to the visibile problem, in that only types marked as identifiable would need to have the return value moved into the parameter list, and every other type could remain the same.
    8901395Furthermore, no restrictions would need to be placed on whether objects can be constructed.
    8911396\begin{cfacode}
    … …  
    8971402\end{cfacode}
    8981403
    899 Ultimately, both of these are patchwork solutions.
    900 Since a real compiler has full control over its calling conventions, it can seamlessly allow passing the return parameter without outwardly changing the signature of a routine.
    901 As such, it has been decided that this issue is not currently a priority and will be fixed when a full \CFA compiler is implemented.
     1404Ultimately, this is the type of transformation that a real compiler would make when generating assembly code.
     1405Since a compiler has full control over its calling conventions, it can seamlessly allow passing the return parameter without outwardly changing the signature of a routine.
     1406As such, it has been decided that this issue is not currently a priority.
    9021407
    9031408\section{Implementation}
    9041409\subsection{Array Initialization}
    905 Arrays are a special case in the C type-system.
     1410Arrays are a special case in the C type system.
    9061411C arrays do not carry around their size, making it impossible to write a standalone \CFA function that constructs or destructs an array while maintaining the standard interface for constructors and destructors.
    9071412Instead, \CFA defines the initialization and destruction of an array recursively.
    … …  
    10201525By default, objects within a translation unit are constructed in declaration order, and destructed in the reverse order.
    10211526The default order of construction of objects amongst translation units is unspecified.
     1527% TODO: not yet implemented, but g++ provides attribute init_priority, which allows specifying the order of global construction on a per object basis
     1528%   https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes
     1529% suggestion: implement this in CFA by picking objects with a specified priority and pulling them into their own init functions (could even group them by priority level -> map<int, list<ObjectDecl*>>) and pull init_priority forward into constructor and destructor attributes with the same priority level
    10221530It is, however, guaranteed that any global objects in the standard library are initialized prior to the initialization of any object in the user program.
    10231531
    1024 This feature is implemented in the \CFA translator by grouping every global constructor call into a function with the GCC attribute \emph{constructor}, which performs most of the heavy lifting \cite[6.31.1]{GCCExtensions}.
     1532This feature is implemented in the \CFA translator by grouping every global constructor call into a function with the GCC attribute \emph{constructor}, which performs most of the heavy lifting. % CITE: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
    10251533A similar function is generated with the \emph{destructor} attribute, which handles all global destructor calls.
    10261534At the time of writing, initialization routines in the library are specified with priority \emph{101}, which is the highest priority level that GCC allows, whereas initialization routines in the user's code are implicitly given the default priority level, which ensures they have a lower priority than any code with a specified priority level.
    1027 This mechanism allows arbitrarily complicated initialization to occur before any user code runs, making it possible for library designers to initialize their modules without requiring the user to call specific startup or tear-down routines.
     1535This mechanism allows arbitrarily complicated initialization to occur before any user code runs, making it possible for library designers to initialize their modules without requiring the user to call specific startup or teardown routines.
    10281536
    10291537For example, given the following global declarations.
    … …  
    10511559\end{cfacode}
    10521560
    1053 %   https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes
    1054 % suggestion: implement this in CFA by picking objects with a specified priority and pulling them into their own init functions (could even group them by priority level -> map<int, list<ObjectDecl*>>) and pull init_priority forward into constructor and destructor attributes with the same priority level
    1055 GCC provides an attribute @init_priority@, which allows specifying the relative priority for initialization of global objects on a per-object basis in \CC.
    1056 A similar attribute can be implemented in \CFA by pulling marked objects into global constructor/destructor-attribute functions with the specified priority.
    1057 For example,
    1058 \begin{cfacode}
    1059 struct A { ... };
    1060 void ?{}(A *, int);
    1061 void ^?{}(A *);
    1062 __attribute__((init_priority(200))) A x = { 123 };
    1063 \end{cfacode}
    1064 would generate
    1065 \begin{cfacode}
    1066 A x;
    1067 __attribute__((constructor(200))) __init_x() {
    1068   ?{}(&x, 123);  // construct x with priority 200
    1069 }
    1070 __attribute__((destructor(200))) __destroy_x() {
    1071   ?{}(&x);       // destruct x with priority 200
    1072 }
    1073 \end{cfacode}
    1074 
    10751561\subsection{Static Local Variables}
    10761562In standard C, it is possible to mark variables that are local to a function with the @static@ storage class.
    1077 Unlike normal local variables, a @static@ local variable is defined to live for the entire duration of the program, so that each call to the function has access to the same variable with the same address and value as it had in the previous call to the function.
    1078 Much like global variables, in C @static@ variables can only be initialized to a \emph{compile-time constant value} so that a compiler is able to create storage for the variable and initialize it at compile-time.
     1563Unlike normal local variables, a @static@ local variable is defined to live for the entire duration of the program, so that each call to the function has access to the same variable with the same address and value as it had in the previous call to the function. % TODO: mention dynamic loading caveat??
     1564Much like global variables, in C @static@ variables must be initialized to a \emph{compile-time constant value} so that a compiler is able to create storage for the variable and initialize it before the program begins running.
    10791565
    10801566Yet again, this rule is too restrictive for a language with constructors and destructors.
    … …  
    10871573Construction of @static@ local objects is implemented via an accompanying @static bool@ variable, which records whether the variable has already been constructed.
    10881574A conditional branch checks the value of the companion @bool@, and if the variable has not yet been constructed then the object is constructed.
    1089 The object's destructor is scheduled to be run when the program terminates using @atexit@ \footnote{When using the dynamic linker, it is possible to dynamically load and unload a shared library. Since glibc 2.2.3 \cite{atexit}, functions registered with @atexit@ within the shared library are called when unloading the shared library. As such, static local objects can be destructed using this mechanism even in shared libraries on Linux systems.}, and the companion @bool@'s value is set so that subsequent invocations of the function do not reconstruct the object.
     1575The object's destructor is scheduled to be run when the program terminates using @atexit@, and the companion @bool@'s value is set so that subsequent invocations of the function will not reconstruct the object.
    10901576Since the parameter to @atexit@ is a parameter-less function, some additional tweaking is required.
    10911577First, the @static@ variable must be hoisted up to global scope and uniquely renamed to prevent name clashes with other global objects.
    … …  
    10931579Finally, the newly generated function is registered with @atexit@, instead of registering the destructor directly.
    10941580Since @atexit@ calls functions in the reverse order in which they are registered, @static@ local variables are guaranteed to be destructed in the reverse order that they are constructed, which may differ between multiple executions of the same program.
     1581
    10951582Extending the previous example
    10961583\begin{cfacode}
    … …  
    11431630\end{cfacode}
    11441631
    1145 \subsection{Polymorphism}
    1146 As mentioned in section \ref{sub:polymorphism}, \CFA currently has 3 type-classes that are used to designate polymorphic data types: @otype@, @dtype@, and @ftype@.
    1147 In previous versions of \CFA, @otype@ was syntactic sugar for @dtype@ with known size/alignment information and an assignment function.
    1148 That is,
    1149 \begin{cfacode}
    1150 forall(otype T)
    1151 void f(T);
    1152 \end{cfacode}
    1153 was equivalent to
    1154 \begin{cfacode}
    1155 forall(dtype T | sized(T) | { T ?=?(T *, T); })
    1156 void f(T);
    1157 \end{cfacode}
    1158 This allows easily specifying constraints that are common to all complete object types very simply.
    1159 
    1160 Now that \CFA has constructors and destructors, more of a complete object's behaviour can be specified by than was previously possible.
    1161 As such, @otype@ has been augmented to include assertions for a default constructor, copy constructor, and destructor.
    1162 That is, the previous example is now equivalent to
    1163 \begin{cfacode}
    1164 forall(dtype T | sized(T) | { T ?=?(T *, T); void ?{}(T *); void ?{}(T *, T); void ^?{}(T *); })
    1165 void f(T);
    1166 \end{cfacode}
    1167 This allows @f@'s body to create and destroy objects of type @T@, and pass objects of type @T@ as arguments to other functions, following the normal \CFA rules.
    1168 A point of note here is that objects can be missing default constructors (and eventually other functions through deleted functions), so it is important for \CFA programmers to think carefully about the operations needed by their function, as to not over-constrain the acceptable parameter types.
     1632\subsection{Constructor Expressions}
     1633In \CFA, it is possible to use a constructor as an expression.
     1634Like other operators, the function name @?{}@ matches its operator syntax.
     1635For example, @(&x){}@ calls the default constructor on the variable @x@, and produces @&x@ as a result.
     1636The significance of constructors as expressions rather than as statements is that the result of a constructor expression can be used as part of a larger expression.
     1637A key example is the use of constructor expressions to initialize the result of a call to standard C routine @malloc@.
     1638\begin{cfacode}
     1639struct X { ... };
     1640void ?{}(X *, double);
     1641X * x = malloc(sizeof(X)){ 1.5 };
     1642\end{cfacode}
     1643In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@.
     1644If this extension is not present, constructing dynamically allocated objects is much more cumbersome, requiring separate initialization of the pointer and initialization of the pointed-to memory.
     1645\begin{cfacode}
     1646X * x = malloc(sizeof(X));
     1647x{ 1.5 };
     1648\end{cfacode}
     1649Not only is this verbose, but it is also more error prone, since this form allows maintenance code to easily sneak in between the initialization of @x@ and the initialization of the memory that @x@ points to.
     1650This feature is implemented via a transformation produceing the value of the first argument of the constructor, since constructors do not themslves have a return value.
     1651Since this transformation results in two instances of the subexpression, care is taken to allocate a temporary variable to hold the result of the subexpression in the case where the subexpression may contain side effects.
     1652The previous example generates the following code.
     1653\begin{cfacode}
     1654struct X *_tmp_ctor;
     1655struct X *x = ?{}((_tmp_ctor=((_tmp_cp_ret0=
     1656  malloc(sizeof(struct X))), _tmp_cp_ret0))), 1.5), _tmp_ctor);
     1657\end{cfacode}
     1658It should be noted that this technique is not exclusive to @malloc@, and allows a user to write a custom allocator that can be idiomatically used in much the same way as a constructed @malloc@ call.
     1659
     1660It is also possible to use operator syntax with destructors.
     1661Unlike constructors, operator syntax with destructors is a statement and thus does not produce a value, since the destructed object is invalidated by the use of a destructor.
     1662For example, \lstinline!^(&x){}! calls the destructor on the variable @x@.
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