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  • doc/papers/general/Paper.tex

    r3d5f2ef r77acd07d  
    12721272However, 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.
    12731273\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.
    1274 
    1275 \TODO{Fill out section. Mention field-constructors and at-equal escape hatch to C-style initialization. Probably pull some text from Rob's thesis for first draft.}
    1276 
     1274While 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.
     1275In 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.
     1276
     1277In \CFA, a constructor is a function named @?{}@, while a destructor is a function named @^?{}@; like other \CFA operators, these names represent the syntax used to call the constructor or destructor, \eg @S s = { ... };@ or @^(s){};@.
     1278Every constructor and destructor must have a return type of @void@, and its first parameter must have a reference type whose base type is the type of the object the function constructs or destructs.
     1279This first parameter is informally called the @this@ parameter, as in many object-oriented languages, though a programmer may give it an arbitrary name.
     1280Destructors must have exactly one parameter, while constructors allow passing of zero or more additional arguments along with the @this@ parameter.
     1281
     1282\begin{cfa}
     1283struct Array {
     1284        int * data;
     1285        int len;
     1286};
     1287
     1288void ?{}( Array& arr ) {
     1289        arr.len = 10;
     1290        arr.data = calloc( arr.len, sizeof(int) );
     1291}
     1292
     1293void ^?{}( Array& arr ) {
     1294        free( arr.data );
     1295}
     1296
     1297{
     1298        Array x;
     1299        `?{}(x);`       $\C{// implicitly compiler-generated}$
     1300        // ... use x
     1301        `^?{}(x);`      $\C{// implicitly compiler-generated}$
     1302}
     1303\end{cfa}
     1304
     1305In 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@.
     1306@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.
     1307As 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.
     1308The exact details of the placement of these implicit constructor and destructor calls are omitted here for brevity, the interested reader should consult \cite{Schluntz17}.
     1309
     1310Constructor 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.
     1311As such, \CFA also provides syntax for \emph{copy initialization} and \emph{initialization parameters}:
     1312
     1313\begin{cfa}
     1314void ?{}( Array& arr, Array other );
     1315
     1316void ?{}( Array& arr, int size, int fill );
     1317
     1318Array y = { 20, 0xDEADBEEF }, z = y;
     1319\end{cfa}
     1320
     1321Copy constructors have exactly two parameters, the second of which has the same type as the base type of the @this@ parameter; appropriate care is taken in the implementation to avoid recursive calls to the copy constructor when initializing this second parameter.
     1322Other constructor calls look just like C initializers, except rather than using field-by-field initialization (as in C), an initialization which matches a defined constructor will call the constructor instead.
     1323
     1324In addition to initialization syntax, \CFA provides two ways to explicitly call constructors and destructors.
     1325Explicit calls to constructors double as a placement syntax, useful for construction of member fields in user-defined constructors and reuse of large storage allocations.
     1326While the existing function-call syntax works for explicit calls to constructors and destructors, \CFA also provides a more concise \emph{operator syntax} for both:
     1327
     1328\begin{cfa}
     1329Array a, b;
     1330(a){};                                  $\C{// default construct}$
     1331(b){ a };                               $\C{// copy construct}$
     1332^(a){};                                 $\C{// destruct}$
     1333(a){ 5, 0xFFFFFFFF };   $\C{// explicit constructor call}$
     1334\end{cfa}
     1335
     1336To provide a uniform type interface for @otype@ polymorphism, the \CFA compiler automatically generates a default constructor, copy constructor, assignment operator, and destructor for all types.
     1337These default functions can be overridden by user-generated versions of them.
     1338For compatibility with the standard behaviour of C, the default constructor and destructor for all basic, pointer, and reference types do nothing, while the copy constructor and assignment operator are bitwise copies; if default zero-initialization is desired, the default constructors can be overridden.
     1339For user-generated types, the four functions are also automatically generated.
     1340@enum@ types are handled the same as their underlying integral type, and unions are also bitwise copied and no-op initialized and destructed.
     1341For compatibility with C, a copy constructor from the first union member type is also defined.
     1342For @struct@ types, each of the four functions are implicitly defined to call their corresponding functions on each member of the struct.
     1343To better simulate the behaviour of C initializers, a set of \emph{field constructors} is also generated for structures.
     1344A 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.
     1345To 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.
     1346These semantics closely mirror the rule for implicit declaration of constructors in \CC\cite[p.~186]{ANSI98:C++}.
     1347
     1348In 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.
     1349If 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.
     1350Any C initializer can be the right-hand side of an \lstinline|@=| initializer, \eg  \lstinline|Array a @= { 0, 0x0 }|, with the usual C initialization semantics.
     1351In 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.
    12771352
    12781353\subsection{Default Parameters}
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