Changes in doc/rob_thesis/ctordtor.tex [12d3187:0111dc7]
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doc/rob_thesis/ctordtor.tex
r12d3187 r0111dc7 6 6 % doesn't seem possible to do this without allowing ttype on generic structs? 7 7 8 Since \CFA is a true systems language, it does not require a garbage collector. 9 As well, \CFA is not an object-oriented programming language, \ie, structures cannot have methods. 10 While structures can have function pointer members, this is different from methods, since methods have implicit access to structure members and methods cannot be reassigned. 8 Since \CFA is a true systems language, it does not provide a garbage collector. 9 As well, \CFA is not an object-oriented programming language, \ie, structures cannot have routine members. 11 10 Nevertheless, one important goal is to reduce programming complexity and increase safety. 12 11 To that end, \CFA provides support for implicit pre/post-execution of routines for objects, via constructors and destructors. … … 33 32 The key difference between assignment and initialization being that assignment occurs on a live object (\ie, an object that contains data). 34 33 It is important to note that this means @x@ could have been used uninitialized prior to being assigned, while @y@ could not be used uninitialized. 35 Use of uninitialized variables yields undefined behaviour \cite[p.~558]{C11}, which is a common source of errors in C programs.34 Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs. 36 35 37 36 Initialization of a declaration is strictly optional, permitting uninitialized variables to exist. … … 71 70 int x2 = opaque_get(x, 2); 72 71 \end{cfacode} 73 This pattern is cumbersome to use since every access becomes a function call , requiring awkward syntax and a performance cost.72 This pattern is cumbersome to use since every access becomes a function call. 74 73 While useful in some situations, this compromise is too restrictive. 75 74 Furthermore, even with this idiom it is easy to make mistakes, such as forgetting to destroy an object or destroying it multiple times. 76 75 77 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. 78 This goal is achieved through a \emph{guarantee} that a constructor is called \emph{implicitly} after every object is allocated from a type with associated constructors, as part of an object's \emph{definition}.79 Since a constructor is called on every object of a managed type, it is \emph{impossible}to forget to initialize such objects, as long as all constructors perform some sensible form of initialization.77 This 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. 78 Since 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. 80 79 81 80 In \CFA, a constructor is a function with the name @?{}@. … … 115 114 In other words, a default constructor is a constructor that takes a single argument: the @this@ parameter. 116 115 117 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! \footnote{Originally, the name @~?{}@ was chosen for destructors, to provide familiarity to \CC programmers. Unforunately, this name causes parsing conflicts with the bitwise-not operator when used with operator syntax (see section \ref{sub:syntax}.)}and it takes only one argument.116 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it takes only one argument. 118 117 A destructor for the @Array@ type can be defined as: 119 118 \begin{cfacode} … … 136 135 On line 2, @z@ is initialized with the value of @x@, while on line 3, @y@ is assigned the value of @x@. 137 136 The 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. 138 In particular, these cases cannot be handled the same way because in the former case @z@ has no array, while @y@ does. 139 A \emph{copy constructor} is used to perform initialization using another object of the same type. 137 In particular, these cases cannot be handled the same way because in the former case @z@ does not currently own an array, while @y@ does. 140 138 141 139 \begin{cfacode}[emph={other}, emphstyle=\color{red}] … … 153 151 } 154 152 \end{cfacode} 155 The two functions above handle the cases of initialization and assignment.156 The first function is called a copy constructor, because it constructs its argument by copying the values from another object of the same type.153 The two functions above handle these cases. 154 The first function is called a \emph{copy constructor}, because it constructs its argument by copying the values from another object of the same type. 157 155 The second function is the standard copy-assignment operator. 158 \CFA does not currently have the concept of reference types, so the most appropriate type for the source object in copy constructors and assignment operators is a value type.159 Appropriate care is taken in the implementation to avoid recursive calls to the copy constructor.160 156 The four functions (default constructor, destructor, copy constructor, and assignment operator) are special in that they safely control the state of most objects. 161 157 … … 220 216 A * y = malloc(); // copy construct: ?{}(&y, malloc()) 221 217 218 ?{}(&x); // explicit construct x, second construction 219 ?{}(y, x); // explit construct y from x, second construction 222 220 ^?{}(&x); // explicit destroy x, in different order 223 ?{}(&x); // explicit construct x, second construction224 221 ^?{}(y); // explicit destroy y 225 ?{}(y, x); // explit construct y from x, second construction226 222 227 223 // implicit ^?{}(&y); … … 283 279 \end{cfacode} 284 280 In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@. 285 Intuitively, the expression-resolver determines that @malloc@ returns some type @T *@, as does the constructor expression since it returns the type of its argument.286 This type flows outwards to the declaration site where the expected type is known to be @X *@, thus the first argument to the constructor must be @X *@, narrowing the search space.287 288 281 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. 289 282 \begin{cfacode} … … 307 300 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. 308 301 309 While it is possible to use operator syntax with destructors, destructors invalidate their argument, thus operator syntax with destructors is void-typed expression.302 It should be noted that while it is possible to use operator syntax with destructors, destructors invalidate their argument, thus operator syntax with destructors is a statement and does not produce a value. 310 303 311 304 \subsection{Function Generation} … … 315 308 If the translator can expect these functions to exist, then it can unconditionally attempt to resolve them. 316 309 Moreover, the existence of a standard interface allows polymorphic code to interoperate with new types seamlessly. 317 While automatic generation of assignment functions is present in previous versions of \CFA, the the implementation has been largely rewritten to accomodate constructors and destructors.318 310 319 311 To mimic the behaviour of standard C, the default constructor and destructor for all of the basic types and for all pointer types are defined to do nothing, while the copy constructor and assignment operator perform a bitwise copy of the source parameter (as in \CC). 320 This default is intended to maintain backwards compatibility and performance, by not imposing unexpected operations for a C programmer, as a zero-default behaviour would.321 However, it is possible for a user to define such constructors so that variables are safely zeroed by default, if desired.322 %%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%323 \begin{cfacode}324 void ?{}(int * i) { *i = 0; }325 forall(dtype T) void ?{}(T ** p) { *p = 0; } // any pointer type326 void f() {327 int x; // initialized to 0328 int * p; // initialized to 0329 }330 \end{cfacode}331 %%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%332 312 333 313 There are several options for user-defined types: structures, unions, and enumerations. 334 314 To aid in ease of use, the standard set of four functions is automatically generated for a user-defined type after its definition is completed. 335 315 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. 336 As well, these functions are always generated, since they may be needed by polymorphic functions.337 With that said, the generated functions are not called implicitly unless they are non-trivial, and are never exported, making it simple for the optimizer to strip them away when they are not used.338 316 339 317 The generated functions for enumerations are the simplest. … … 360 338 } 361 339 \end{cfacode} 362 In the future, \CFA will introduce strongly-typed enumerations, like those in \CC , wherein enumerations create a new type distinct from @int@ so that integral values require an explicit cast to be stored in an enumeration variable.340 In the future, \CFA will introduce strongly-typed enumerations, like those in \CC. 363 341 The existing generated routines are sufficient to express this restriction, since they are currently set up to take in values of that enumeration type. 364 342 Changes 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@. … … 514 492 In addition to freedom, \ateq provides a simple path for migrating legacy C code to \CFA, in that objects can be moved from C-style initialization to \CFA gradually and individually. 515 493 It 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. 516 It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary , such as memory-mapped devices, trigger devices, I/O controllers, etc.494 It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary. 517 495 518 496 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. … … 567 545 \end{cfacode} 568 546 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). 569 To override this rule, \ateq can be used to force the translator to trust the programmer's discretion.570 This form of \ateq is not yet implemented.571 547 \begin{cfacode} 572 548 void ?{}(A * a) { … … 580 556 } 581 557 582 void ?{}(A * a, int x) {583 // object forwarded to another constructor,584 // does not implicitly construct any members585 (&a){};586 }587 588 558 void ^?{}(A * a) { 589 559 ^(&a->x){}; // explicit destructor call 590 560 } // z, y, w implicitly destructed, in this order 591 561 \end{cfacode} 592 If at any point, the @this@ parameter is passed directly as the target of another constructor, then it is assumed the other constructor handles the initialization of all of the object's members and no implicit constructor calls are added to the current constructor. 562 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. 563 To override this rule, \ateq can be used to force the translator to trust the programmer's discretion. 564 This form of \ateq is not yet implemented. 593 565 594 566 Despite great effort, some forms of C syntax do not work well with constructors in \CFA. … … 647 619 The body of @A@ has been omitted, since only the constructor interfaces are important. 648 620 649 It should be noted that unmanaged objects , i.e. objects that have only trivial constructors,can still make use of designations and nested initializers in \CFA.621 It should be noted that unmanaged objects can still make use of designations and nested initializers in \CFA. 650 622 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. 651 %%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%652 \begin{cfacode}653 struct B { int x; };654 struct C { int y; };655 struct A { B b; C c; };656 void ?{}(A *, B);657 void ?{}(A *, C);658 659 A a = {660 (C){ 10 } // disambiguate with compound literal661 };662 \end{cfacode}663 %%%%%%%%%%%%%%%%%%%%%%%%%% line width %%%%%%%%%%%%%%%%%%%%%%%%%%664 623 665 624 \subsection{Implicit Destructors} … … 785 744 \end{cfacode} 786 745 787 While \CFA supports the GCC computed-goto extension, the behaviour of managed objects in combination with computed-goto is undefined.788 \begin{cfacode}789 void f(int val) {790 void * l = val == 0 ? &&L1 : &&L2;791 {792 A x;793 L1: ;794 goto *l; // branches differently depending on argument795 }796 L2: ;797 }798 \end{cfacode}799 Likewise, destructors are not executed at scope-exit due to a computed-goto in \CC, as of g++ version 6.2.800 801 746 \subsection{Implicit Copy Construction} 802 747 \label{s:implicit_copy_construction} … … 805 750 Exempt from these rules are intrinsic and built-in functions. 806 751 It 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. 807 That is, since the parameter is not marked as an unmanaged object using \ateq, it is 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.752 That is, since the parameter is not marked as an unmanaged object using \ateq, it is 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. 808 753 These semantics are important to bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed. 809 754 \begin{cfacode} 810 struct A { ... };755 struct A; 811 756 void ?{}(A *); 812 757 void ?{}(A *, A); … … 865 810 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. 866 811 867 Adding implicit copy construction imposes the additional runtime cost of the copy constructor for every argument and return value in a function call.868 This cost is necessary to maintain appropriate value semantics when calling a function.869 In the future, return-value-optimization (RVO) can be implemented for \CFA to elide unnecessary copy construction and destruction of temporary objects.870 This cost is not present for types with trivial copy constructors and destructors.871 872 812 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. 873 813 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. … … 968 908 \subsection{Array Initialization} 969 909 Arrays are a special case in the C type-system. 970 Type checking largely ignores size information for C arrays, making it impossible to write a standalone \CFA function that constructs or destructs an array,while maintaining the standard interface for constructors and destructors.910 C 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. 971 911 Instead, \CFA defines the initialization and destruction of an array recursively. 972 912 That is, when an array is defined, each of its elements is constructed in order from element 0 up to element $n-1$. … … 1207 1147 \end{cfacode} 1208 1148 1209 This implementation comes at the runtime cost of an additional branch for every @static@ local variable, each time the function is called.1210 Since initializers are not required to be compile-time constant expressions, they can involve global variables, function arguments, function calls, etc.1211 As a direct consequence, @static@ local variables cannot be initialized with an attribute-constructor routines like global variables can.1212 However, in the case where the variable is unmanaged and has a compile-time constant initializer, a C-compliant initializer is generated and the additional cost is not present.1213 \CC shares the same semantics for its @static@ local variables.1214 1215 1149 \subsection{Polymorphism} 1216 1150 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@. … … 1238 1172 These additions allow @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. 1239 1173 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 and prevent potential reuse. 1240 1241 These additional assertion parameters impose a runtime cost on all managed temporary objects created in polymorphic code, even those with trivial constructors and destructors.1242 This cost is necessary because polymorphic code does not know the actual type at compile-time, due to separate compilation.1243 Since trivial constructors and destructors either do not perform operations or are simply bit-wise copy operations, the imposed cost is essentially the cost of the function calls.1244 1245 \section{Summary}1246 1247 When creating a new object of a managed type, it is guaranteed that a constructor is be called to initialize the object at its definition point, and is destructed when the object's lifetime ends.1248 Destructors are called in the reverse order of construction.1249 1250 Every argument passed to a function is copy constructed into a temporary object that is passed by value to the functions and destructed at the end of the statement.1251 Function return values are copy constructed inside the function at the return statement, passed by value to the call-site, and destructed at the call-site at the end of the statement.1252 1253 Every complete object type has a default constructor, copy constructor, assignment operator, and destructor.1254 To accomplish this, these functions are generated as appropriate for new types.1255 User-defined functions shadow built-in and automatically generated functions, so it is possible to specialize the behaviour of a type.1256 Furthermore, default constructors and aggregate field constructors are hidden when \emph{any} constructor is defined.1257 1258 Objects dynamically allocated with @malloc@, \ateq objects, and objects with only trivial constructors and destructors are unmanaged.1259 Unmanaged objects are never the target of an implicit constructor or destructor call.
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