# Changeset a722c7a

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Timestamp:
Feb 9, 2018, 3:33:55 PM (5 years ago)
Branches:
aaron-thesis, arm-eh, cleanup-dtors, deferred_resn, demangler, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, new-env, no_list, persistent-indexer, pthread-emulation, qualifiedEnum, resolv-new, with_gc
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92f8e18, bede27b
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565a3d6f
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doc/papers/general
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2 edited

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 r565a3d6f FIGURES = ${addsuffix .tex, \ Cdecl \ } • ## doc/papers/general/Paper.tex  r565a3d6f \makeatother \newenvironment{cquote}{% \list{}{\lstset{resetmargins=true,aboveskip=0pt,belowskip=0pt}\topsep=4pt\parsep=0pt\leftmargin=\parindent\rightmargin\leftmargin}% \item\relax }{% \endlist }% cquote % CFA programming language, based on ANSI C (with some gcc additions) \lstdefinelanguage{CFA}[ANSI]{C}{ int forty_two = identity( 42 );$\C{// T is bound to int, forty\_two == 42}\end{lstlisting} The @identity@ function above can be applied to any complete \emph{object type} (or @otype@). The @identity@ function above can be applied to any complete \newterm{object type} (or @otype@). The type variable @T@ is transformed into a set of additional implicit parameters encoding sufficient information about @T@ to create and return a variable of that type. The \CFA implementation passes the size and alignment of the type represented by an @otype@ parameter, as well as an assignment operator, constructor, copy constructor and destructor. If this extra information is not needed, \eg for a pointer, the type parameter can be declared as a \emph{data type} (or @dtype@). If this extra information is not needed, \eg for a pointer, the type parameter can be declared as a \newterm{data type} (or @dtype@). In \CFA, the polymorphism runtime-cost is spread over each polymorphic call, due to passing more arguments to polymorphic functions; A design advantage is that, unlike \CC template-functions, \CFA polymorphic-functions are compatible with C \emph{separate compilation}, preventing compilation and code bloat. Since bare polymorphic-types provide a restricted set of available operations, \CFA provides a \emph{type assertion}~\cite[pp.~37-44]{Alphard} mechanism to provide further type information, where type assertions may be variable or function declarations that depend on a polymorphic type-variable. Since bare polymorphic-types provide a restricted set of available operations, \CFA provides a \newterm{type assertion}~\cite[pp.~37-44]{Alphard} mechanism to provide further type information, where type assertions may be variable or function declarations that depend on a polymorphic type-variable. For example, the function @twice@ can be defined using the \CFA syntax for operator overloading: \begin{lstlisting} \subsection{Traits} \CFA provides \emph{traits} to name a group of type assertions, where the trait name allows specifying the same set of assertions in multiple locations, preventing repetition mistakes at each function declaration: \CFA provides \newterm{traits} to name a group of type assertions, where the trait name allows specifying the same set of assertions in multiple locations, preventing repetition mistakes at each function declaration: \begin{lstlisting} trait summable( otype T ) { Given the information provided for an @otype@, variables of polymorphic type can be treated as if they were a complete type: stack-allocatable, default or copy-initialized, assigned, and deleted. In summation, the \CFA type-system uses \emph{nominal typing} for concrete types, matching with the C type-system, and \emph{structural typing} for polymorphic types. In summation, the \CFA type-system uses \newterm{nominal typing} for concrete types, matching with the C type-system, and \newterm{structural typing} for polymorphic types. Hence, trait names play no part in type equivalence; the names are simply macros for a list of polymorphic assertions, which are expanded at usage sites. Furthermore, writing and using preprocessor macros can be unnatural and inflexible. \CC, Java, and other languages use \emph{generic types} to produce type-safe abstract data-types. \CC, Java, and other languages use \newterm{generic types} to produce type-safe abstract data-types. \CFA also implements generic types that integrate efficiently and naturally with the existing polymorphic functions, while retaining backwards compatibility with C and providing separate compilation. However, for known concrete parameters, the generic-type definition can be inlined, like \CC templates. \end{lstlisting} \CFA classifies generic types as either \emph{concrete} or \emph{dynamic}. \CFA classifies generic types as either \newterm{concrete} or \newterm{dynamic}. Concrete types have a fixed memory layout regardless of type parameters, while dynamic types vary in memory layout depending on their type parameters. A type may have polymorphic parameters but still be concrete, called \emph{dtype-static}. A type may have polymorphic parameters but still be concrete, called \newterm{dtype-static}. Polymorphic pointers are an example of dtype-static types, \eg @forall(dtype T) T *@ is a polymorphic type, but for any @T@, @T *@ is a fixed-sized pointer, and therefore, can be represented by a @void *@ in code generation. Though \CFA implements concrete generic-types efficiently, it also has a fully general system for dynamic generic types. As mentioned in Section~\ref{sec:poly-fns}, @otype@ function parameters (in fact all @sized@ polymorphic parameters) come with implicit size and alignment parameters provided by the caller. Dynamic generic-types also have an \emph{offset array} containing structure-member offsets. Dynamic generic-types also have an \newterm{offset array} containing structure-member offsets. A dynamic generic-union needs no such offset array, as all members are at offset 0, but size and alignment are still necessary. Access to members of a dynamic structure is provided at runtime via base-displacement addressing with the structure pointer and the member offset (similar to the @offsetof@ macro), moving a compile-time offset calculation to runtime. For instance, modularity is generally provided in C by including an opaque forward-declaration of a structure and associated accessor and mutator functions in a header file, with the actual implementations in a separately-compiled @.c@ file. \CFA supports this pattern for generic types, but the caller does not know the actual layout or size of the dynamic generic-type, and only holds it by a pointer. The \CFA translator automatically generates \emph{layout functions} for cases where the size, alignment, and offset array of a generic struct cannot be passed into a function from that function's caller. The \CFA translator automatically generates \newterm{layout functions} for cases where the size, alignment, and offset array of a generic struct cannot be passed into a function from that function's caller. These layout functions take as arguments pointers to size and alignment variables and a caller-allocated array of member offsets, as well as the size and alignment of all @sized@ parameters to the generic structure (un@sized@ parameters are forbidden from being used in a context that affects layout). Results of these layout functions are cached so that they are only computed once per type per function. %, as in the example below for @pair@. Since @pair(T *, T * )@ is a concrete type, there are no implicit parameters passed to @lexcmp@, so the generated code is identical to a function written in standard C using @void *@, yet the \CFA version is type-checked to ensure the fields of both pairs and the arguments to the comparison function match in type. Another useful pattern enabled by reused dtype-static type instantiations is zero-cost \emph{tag-structures}. Another useful pattern enabled by reused dtype-static type instantiations is zero-cost \newterm{tag-structures}. Sometimes information is only used for type-checking and can be omitted at runtime, \eg: \begin{lstlisting} The addition of multiple-return-value functions (MRVF) are useless without a syntax for accepting multiple values at the call-site. The simplest mechanism for capturing the return values is variable assignment, allowing the values to be retrieved directly. As such, \CFA allows assigning multiple values from a function into multiple variables, using a square-bracketed list of lvalue expressions (as above), called a \emph{tuple}. However, functions also use \emph{composition} (nested calls), with the direct consequence that MRVFs must also support composition to be orthogonal with single-returning-value functions (SRVF), \eg: As such, \CFA allows assigning multiple values from a function into multiple variables, using a square-bracketed list of lvalue expressions (as above), called a \newterm{tuple}. However, functions also use \newterm{composition} (nested calls), with the direct consequence that MRVFs must also support composition to be orthogonal with single-returning-value functions (SRVF), \eg: \begin{lstlisting} printf( "%d %d\n", div( 13, 5 ) );\C{// return values seperated into arguments}$printf( "%d %d\n", qr ); \end{lstlisting} \CFA also supports \emph{tuple indexing} to access single components of a tuple expression: \CFA also supports \newterm{tuple indexing} to access single components of a tuple expression: \begin{lstlisting} [int, int] * p = &qr;$\C{// tuple pointer}$\subsection{Tuple Assignment} An assignment where the left side is a tuple type is called \emph{tuple assignment}. There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a tuple type or a non-tuple type, called \emph{multiple} and \emph{mass assignment}, respectively. An assignment where the left side is a tuple type is called \newterm{tuple assignment}. There are two kinds of tuple assignment depending on whether the right side of the assignment operator has a tuple type or a non-tuple type, called \newterm{multiple} and \newterm{mass assignment}, respectively. %\lstDeleteShortInline@% %\par\smallskip \subsection{Member Access} It is also possible to access multiple fields from a single expression using a \emph{member-access}. It is also possible to access multiple fields from a single expression using a \newterm{member-access}. The result is a single tuple-valued expression whose type is the tuple of the types of the members, \eg: \begin{lstlisting} Matching against a @ttype@ parameter consumes all remaining argument components and packages them into a tuple, binding to the resulting tuple of types. In a given parameter list, there must be at most one @ttype@ parameter that occurs last, which matches normal variadic semantics, with a strong feeling of similarity to \CCeleven variadic templates. As such, @ttype@ variables are also called \emph{argument packs}. As such, @ttype@ variables are also called \newterm{argument packs}. Like variadic templates, the main way to manipulate @ttype@ polymorphic functions is via recursion. \subsection{Implementation} Tuples are implemented in the \CFA translator via a transformation into \emph{generic types}. Tuples are implemented in the \CFA translator via a transformation into \newterm{generic types}. For each$N$, the first time an$N$-tuple is seen in a scope a generic type with$N$type parameters is generated, \eg: \begin{lstlisting} Similarly, tuple member expressions are recursively expanded into a list of member access expressions. Expressions that may contain side effects are made into \emph{unique expressions} before being expanded by the flattening conversion. Expressions that may contain side effects are made into \newterm{unique expressions} before being expanded by the flattening conversion. Each unique expression is assigned an identifier and is guaranteed to be executed exactly once: \begin{lstlisting} % In object-oriented programming, there is an implicit first parameter, often names @self@ or @this@, which is elided. % In any programming language, some functions have a naturally close relationship with a particular data type. % Object-oriented programming allows this close relationship to be codified in the language by making such functions \emph{class methods} of their related data type. % Object-oriented programming allows this close relationship to be codified in the language by making such functions \newterm{class methods} of their related data type. % Class methods have certain privileges with respect to their associated data type, notably un-prefixed access to the fields of that data type. % When writing C functions in an object-oriented style, this un-prefixed access is swiftly missed, as access to fields of a @Foo* f@ requires an extra three characters @f->@ every time, which disrupts coding flow and clutters the produced code. C declaration syntax is notoriously confusing and error prone. For example, many C programmers are confused by a declaration as simple as: \begin{flushleft} \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}ll@{}} \end{tabular} \lstMakeShortInline@% \end{flushleft} \end{cquote} Is this an array of 5 pointers to integers or a pointer to an array of 5 integers? The fact this declaration is unclear to many C programmers means there are productivity and safety issues even for basic programs. If there is any doubt, it implies productivity and safety issues even for basic programs. Another example of confusion results from the fact that a routine name and its parameters are embedded within the return type, mimicking the way the return value is used at the routine's call site. For example, a routine returning a pointer to an array of integers is defined and used in the following way: In the following example, \R{red} is the base type and \B{blue} is qualifiers. The \CFA declarations move the qualifiers to the left of the base type, \ie move the blue to the left of the red, while the qualifiers have the same meaning but are ordered left to right to specify a variable's type. \begin{quote} \begin{cquote} \lstDeleteShortInline@% \lstset{moredelim=**[is][\color{blue}]{+}{+}} \end{tabular} \lstMakeShortInline@% \end{quote} \end{cquote} The only exception is bit field specification, which always appear to the right of the base type. % Specifically, the character ©*© is used to indicate a pointer, square brackets ©[©\,©]© are used to represent an array or function return value, and parentheses ©()© are used to indicate a routine parameter. % Specifically, the character @*@ is used to indicate a pointer, square brackets @[@\,@]@ are used to represent an array or function return value, and parentheses @()@ are used to indicate a routine parameter. However, unlike C, \CFA type declaration tokens are distributed across all variables in the declaration list. For instance, variables ©x© and ©y© of type pointer to integer are defined in \CFA as follows: \begin{quote} For instance, variables @x@ and @y@ of type pointer to integer are defined in \CFA as follows: \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}l@{\hspace{3em}}l@{}} \end{tabular} \lstMakeShortInline@% \end{quote} \end{cquote} The downside of this semantics is the need to separate regular and pointer declarations: \begin{quote} \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}l@{\hspace{3em}}l@{}} \end{tabular} \lstMakeShortInline@% \end{quote} \end{cquote} which is prescribing a safety benefit. Other examples are: \begin{quote} \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}l@{\hspace{3em}}l@{\hspace{2em}}l@{}} \end{tabular} \lstMakeShortInline@% \end{quote} All type qualifiers, \eg ©const©, ©volatile©, etc., are used in the normal way with the new declarations and also appear left to right, \eg: \begin{quote} \end{cquote} All type qualifiers, \eg @const@, @volatile@, etc., are used in the normal way with the new declarations and also appear left to right, \eg: \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}l@{\hspace{1em}}l@{\hspace{1em}}l@{}} \end{tabular} \lstMakeShortInline@% \end{quote} All declaration qualifiers, \eg ©extern©, ©static©, etc., are used in the normal way with the new declarations but can only appear at the start of a \CFA routine declaration,\footnote{\label{StorageClassSpecifier} \end{cquote} All declaration qualifiers, \eg @extern@, @static@, etc., are used in the normal way with the new declarations but can only appear at the start of a \CFA routine declaration,\footnote{\label{StorageClassSpecifier} The placement of a storage-class specifier other than at the beginning of the declaration specifiers in a declaration is an obsolescent feature.~\cite[\S~6.11.5(1)]{C11}} \eg: \begin{quote} \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}l@{\hspace{3em}}l@{\hspace{2em}}l@{}} \end{tabular} \lstMakeShortInline@% \end{quote} The new declaration syntax can be used in other contexts where types are required, \eg casts and the pseudo-routine ©sizeof©: \begin{quote} \end{cquote} The new declaration syntax can be used in other contexts where types are required, \eg casts and the pseudo-routine @sizeof@: \begin{cquote} \lstDeleteShortInline@% \begin{tabular}{@{}l@{\hspace{3em}}l@{}} \multicolumn{1}{c@{\hspace{3em}}}{\textbf{\CFA}} & \multicolumn{1}{c}{\textbf{C}} \\ \begin{cfa} y = (* int)x; i = sizeof([ 5 ] * int); y = (* int)x; i = sizeof([ 5 ] * int); \end{cfa} & \begin{cfa} y = (int *)x; i = sizeof(int * [ 5 ]); y = (int *)x; i = sizeof(int * [ 5 ]); \end{cfa} \end{tabular} \lstMakeShortInline@% \end{quote} \end{cquote} Finally, new \CFA declarations may appear together with C declarations in the same program block, but cannot be mixed within a specific declaration. Therefore, a programmer has the option of either continuing to use traditional C declarations or take advantage of the new style. Clearly, both styles need to be supported for some time due to existing C-style header-files, particularly for UNIX systems. Clearly, both styles need to be supported for some time due to existing C-style header-files, particularly for UNIX-like systems. \subsection{References} All variables in C have an \emph{address}, a \emph{value}, and a \emph{type}; at the position in the program's memory denoted by the address, there exists a sequence of bits (the value), with the length and semantic meaning of this bit sequence defined by the type. The C type system does not always track the relationship between a value and its address; a value that does not have a corresponding address is called a \emph{rvalue} (for right-hand value''), while a value that does have an address is called a \emph{lvalue} (for left-hand value''); in @int x; x = 42;@ the variable expression @x@ on the left-hand-side of the assignment is a lvalue, while the constant expression @42@ on the right-hand-side of the assignment is a rvalue. Which address a value is located at is sometimes significant; the imperative programming paradigm of C relies on the mutation of values at specific addresses. Within a lexical scope, lvalue exressions can be used in either their \emph{address interpretation} to determine where a mutated value should be stored or in their \emph{value interpretation} to refer to their stored value; in @x = y;@ in @{ int x, y = 7; x = y; }@, @x@ is used in its address interpretation, while y is used in its value interpretation. Though this duality of interpretation is useful, C lacks a direct mechanism to pass lvalues between contexts, instead relying on \emph{pointer types} to serve a similar purpose. In C, for any type @T@ there is a pointer type @T*@, the value of which is the address of a value of type @T@; a pointer rvalue can be explicitly \emph{dereferenced} to the pointed-to lvalue with the dereference operator @*?@, while the rvalue representing the address of a lvalue can be obtained with the address-of operator @&?@. All variables in C have an \newterm{address}, a \newterm{value}, and a \newterm{type}; at the position in the program's memory denoted by the address, there exists a sequence of bits (the value), with the length and semantic meaning of this bit sequence defined by the type. The C type-system does not always track the relationship between a value and its address; a value that does not have a corresponding address is called a \newterm{rvalue} (for right-hand value''), while a value that does have an address is called a \newterm{lvalue} (for left-hand value''). For example, in @int x; x = 42;@ the variable expression @x@ on the left-hand-side of the assignment is a lvalue, while the constant expression @42@ on the right-hand-side of the assignment is a rvalue. In imperative programming, the address of a value is used for both reading and writing (mutating) a value. Within a lexical scope, lvalue expressions have an \newterm{address interpretation} for writing a value or a \newterm{value interpretation} to read a value. For example, in @x = y@, @x@ has an address interpretation, while @y@ has a value interpretation. Though this duality of interpretation is useful, C lacks a direct mechanism to pass lvalues between contexts, instead relying on \newterm{pointer types} to serve a similar purpose. In C, for any type @T@ there is a pointer type @T *@, the value of which is the address of a value of type @T@. A pointer rvalue can be explicitly \newterm{dereferenced} to the pointed-to lvalue with the dereference operator @*?@, while the rvalue representing the address of a lvalue can be obtained with the address-of operator @&?@. \begin{cfa} int x = 1, y = 2, * p1, * p2, ** p3; p1 = &x;$\C{// p1 points to x}$p2 = &y;$\C{// p2 points to y}$p3 = &p1;$\C{// p3 points to p1}$p1 = &x;$\C{// p1 points to x}$p2 = &y;$\C{// p2 points to y}$p3 = &p1;$\C{// p3 points to p1}$*p2 = ((*p1 + *p2) * (**p3 - *p1)) / (**p3 - 15); \end{cfa} Unfortunately, the dereference and address-of operators introduce a great deal of syntactic noise when dealing with pointed-to values rather than pointers, as well as the potential for subtle bugs. For both brevity and clarity, it would be desirable to have the compiler figure out how to elide the dereference operators in a complex expression such as the assignment to @*p2@ above. However, since C defines a number of forms of \emph{pointer arithmetic}, two similar expressions involving pointers to arithmetic types (\eg @*p1 + x@ and @p1 + x@) may each have well-defined but distinct semantics, introducing the possibility that a user programmer may write one when they mean the other, and precluding any simple algorithm for elision of dereference operators. However, since C defines a number of forms of \newterm{pointer arithmetic}, two similar expressions involving pointers to arithmetic types (\eg @*p1 + x@ and @p1 + x@) may each have well-defined but distinct semantics, introducing the possibility that a user programmer may write one when they mean the other, and precluding any simple algorithm for elision of dereference operators. To solve these problems, \CFA introduces reference types @T&@; a @T&@ has exactly the same value as a @T*@, but where the @T*@ takes the address interpretation by default, a @T&@ takes the value interpretation by default, as below: \begin{cfa} inx x = 1, y = 2, & r1, & r2, && r3; int x = 1, y = 2, & r1, & r2, && r3; &r1 = &x;$\C{// r1 points to x}$&r2 = &y;$\C{// r2 points to y}$This allows \CFA references to be default-initialized (\eg to a null pointer), and also to point to different addresses throughout their lifetime. This rebinding is accomplished without adding any new syntax to \CFA, but simply by extending the existing semantics of the address-of operator in C. In C, the address of a lvalue is always a rvalue, as in general that address is not stored anywhere in memory, and does not itself have an address. In \CFA, the address of a @T&@ is a lvalue @T*@, as the address of the underlying @T@ is stored in the reference, and can thus be mutated there. if @L@ is an lvalue of type {@T &@$_1 \cdots$@ &@$_l$} where$l \ge 0$references (@&@ symbols) then @&L@ has type {@T *&@$_{\color{red}1} \cdots$@ &@$_{\color{red}l}$}, \\ \ie @T@ pointer with$l\$ references (@&@ symbols). \end{itemize} Since pointers and references share the same internal representation, code using either is equally performant; in fact the \CFA compiler converts references to pointers internally, and the choice between them in user code can be made based solely on convenience. By analogy to pointers, \CFA references also allow cv-qualifiers: More generally, this initialization of references from lvalues rather than pointers is an instance of a lvalue-to-reference'' conversion rather than an elision of the address-of operator; this conversion can actually be used in any context in \CFA an implicit conversion would be allowed. Similarly, use of a the value pointed to by a reference in an rvalue context can be thought of as a reference-to-rvalue'' conversion, and \CFA also includes a qualifier-adding reference-to-reference'' conversion, analagous to the @T *@ to @const T *@ conversion in standard C. Similarly, use of a the value pointed to by a reference in an rvalue context can be thought of as a reference-to-rvalue'' conversion, and \CFA also includes a qualifier-adding reference-to-reference'' conversion, analogous to the @T *@ to @const T *@ conversion in standard C. The final reference conversion included in \CFA is rvalue-to-reference'' conversion, implemented by means of an implicit temporary. When an rvalue is used to initialize a reference, it is instead used to initialize a hidden temporary value with the same lexical scope as the reference, and the reference is initialized to the address of this temporary. This allows complex values to be succinctly and efficiently passed to functions, without the syntactic overhead of explicit definition of a temporary variable or the runtime cost of pass-by-value. \CC allows a similar binding, but only for @const@ references; the more general semantics of \CFA are an attempt to avoid the \emph{const hell} problem, in which addition of a @const@ qualifier to one reference requires a cascading chain of added qualifiers. \CC allows a similar binding, but only for @const@ references; the more general semantics of \CFA are an attempt to avoid the \newterm{const hell} problem, in which addition of a @const@ qualifier to one reference requires a cascading chain of added qualifiers. \subsection{Constructors and Destructors} One of the strengths of C is the control over memory management it gives programmers, allowing resource release to be more consistent and precisely timed than is possible with garbage-collected memory management. However, this manual approach to memory management is often verbose, and it is useful to manage resources other than memory (\eg file handles) using the same mechanism as memory. \CC is well-known for an approach to manual memory management that addresses both these issues, Resource Aquisition Is Initialization (RAII), implemented by means of special \emph{constructor} and \emph{destructor} functions; we have implemented a similar feature in \CFA. \CC is well-known for an approach to manual memory management that addresses both these issues, Resource Aquisition Is Initialization (RAII), implemented by means of special \newterm{constructor} and \newterm{destructor} functions; we have implemented a similar feature in \CFA. While RAII is a common feature of object-oriented programming languages, its inclusion in \CFA does not violate the design principle that \CFA retain the same procedural paradigm as C. In particular, \CFA does not implement class-based encapsulation: neither the constructor nor any other function has privileged access to the implementation details of a type, except through the translation-unit-scope method of opaque structs provided by C. \end{cfa} In the example above, a \emph{default constructor} (\ie one with no parameters besides the @this@ parameter) and destructor are defined for the @Array@ struct, a dynamic array of @int@. @Array@ is an example of a \emph{managed type} in \CFA, a type with a non-trivial constructor or destructor, or with a field of a managed type. In the example above, a \newterm{default constructor} (\ie one with no parameters besides the @this@ parameter) and destructor are defined for the @Array@ struct, a dynamic array of @int@. @Array@ is an example of a \newterm{managed type} in \CFA, a type with a non-trivial constructor or destructor, or with a field of a managed type. As in the example, all instances of managed types are implicitly constructed upon allocation, and destructed upon deallocation; this ensures proper initialization and cleanup of resources contained in managed types, in this case the @data@ array on the heap. The exact details of the placement of these implicit constructor and destructor calls are omitted here for brevity, the interested reader should consult \cite{Schluntz17}. Constructor calls are intended to seamlessly integrate with existing C initialization syntax, providing a simple and familiar syntax to veteran C programmers and allowing constructor calls to be inserted into legacy C code with minimal code changes. As such, \CFA also provides syntax for \emph{copy initialization} and \emph{initialization parameters}: As such, \CFA also provides syntax for \newterm{copy initialization} and \newterm{initialization parameters}: \begin{cfa} In addition to initialization syntax, \CFA provides two ways to explicitly call constructors and destructors. Explicit calls to constructors double as a placement syntax, useful for construction of member fields in user-defined constructors and reuse of large storage allocations. While the existing function-call syntax works for explicit calls to constructors and destructors, \CFA also provides a more concise \emph{operator syntax} for both: While the existing function-call syntax works for explicit calls to constructors and destructors, \CFA also provides a more concise \newterm{operator syntax} for both: \begin{cfa} For compatibility with C, a copy constructor from the first union member type is also defined. For @struct@ types, each of the four functions are implicitly defined to call their corresponding functions on each member of the struct. To better simulate the behaviour of C initializers, a set of \emph{field constructors} is also generated for structures. To better simulate the behaviour of C initializers, a set of \newterm{field constructors} is also generated for structures. A constructor is generated for each non-empty prefix of a structure's member-list which copy-constructs the members passed as parameters and default-constructs the remaining members. To allow users to limit the set of constructors available for a type, when a user declares any constructor or destructor, the corresponding generated function and all field constructors for that type are hidden from expression resolution; similarly, the generated default constructor is hidden upon declaration of any constructor. In rare situations user programmers may not wish to have constructors and destructors called; in these cases, \CFA provides an escape hatch'' to not call them. If a variable is initialized using the syntax \lstinline|S x @= {}| it will be an \emph{unmanaged object}, and will not have constructors or destructors called. If a variable is initialized using the syntax \lstinline|S x @= {}| it will be an \newterm{unmanaged object}, and will not have constructors or destructors called. Any C initializer can be the right-hand side of an \lstinline|@=| initializer, \eg  \lstinline|Array a @= { 0, 0x0 }|, with the usual C initialization semantics. In addition to the expressive power, \lstinline|@=| provides a simple path for migrating legacy C code to \CFA, by providing a mechanism to incrementally convert initializers; the \CFA design team decided to introduce a new syntax for this escape hatch because we believe that our RAII implementation will handle the vast majority of code in a desirable way, and we wished to maintain familiar syntax for this common case.
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