Changes in doc/rob_thesis/ctordtor.tex [0eb18557:f92aa32]
- File:
-
- 1 edited
-
doc/rob_thesis/ctordtor.tex (modified) (28 diffs)
Legend:
- Unmodified
- Added
- Removed
-
doc/rob_thesis/ctordtor.tex
r0eb18557 rf92aa32 7 7 8 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.9 As well, \CFA is not an object-oriented programming language, i.e., structures cannot have routine members. 10 10 Nevertheless, one important goal is to reduce programming complexity and increase safety. 11 11 To that end, \CFA provides support for implicit pre/post-execution of routines for objects, via constructors and destructors. … … 30 30 Next, @x@ is assigned the value of @y@. 31 31 In 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 ( \ie, an object that contains data).32 The key difference between assignment and initialization being that assignment occurs on a live object (i.e., an object that contains data). 33 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. 34 34 Use of uninitialized variables yields undefined behaviour, which is a common source of errors in C programs. … … 79 79 80 80 In \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, \eg, @struct S = { ... };@.81 Like other operators in \CFA, the name represents the syntax used to call the constructor, e.g., @struct S = { ... };@. 82 82 Every 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). 83 83 The @this@ parameter must have a pointer type, whose base type is the type of object that the function constructs. … … 114 114 In other words, a default constructor is a constructor that takes a single argument: the @this@ parameter. 115 115 116 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it take sonly one argument.117 A destructor for the @Array@ type can be defined as :116 In \CFA, a destructor is a function much like a constructor, except that its name is \lstinline!^?{}! and it take only one argument. 117 A destructor for the @Array@ type can be defined as such. 118 118 \begin{cfacode} 119 119 void ^?{}(Array * arr) { … … 167 167 } 168 168 \end{cfacode} 169 170 169 In \CFA, constructors are called implicitly in initialization contexts. 171 170 \begin{cfacode} 172 171 Array x, y = { 20, 0xdeadbeef }, z = y; 173 172 \end{cfacode} 174 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. 175 One downside of reusing C initialization syntax is that it is not 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 in the program. 173 174 In \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. 175 One 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. 176 176 177 177 This example generates the following code … … 246 246 \end{cfacode} 247 247 Finally, constructors and destructors support \emph{operator syntax}. 248 Like other operators in \CFA, the function name mirrors the use-case, in that the question marks are placeholders for the first $N$ arguments.248 Like 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. 249 249 This syntactic form is similar to the new initialization syntax in \CCeleven, except that it is used in expression contexts, rather than declaration contexts. 250 250 \begin{cfacode} … … 272 272 Like other operators, the function name @?{}@ matches its operator syntax. 273 273 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 @malloc@.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@. 275 275 \begin{cfacode} 276 276 struct X { ... }; 277 277 void ?{}(X *, double); 278 X * x = malloc( ){ 1.5 };278 X * x = malloc(sizeof(X)){ 1.5 }; 279 279 \end{cfacode} 280 280 In this example, @malloc@ dynamically allocates storage and initializes it using a constructor, all before assigning it into the variable @x@. 281 281 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. 282 282 \begin{cfacode} 283 X * x = malloc( );283 X * x = malloc(sizeof(X)); 284 284 x{ 1.5 }; 285 285 \end{cfacode} … … 291 291 struct X *_tmp_ctor; 292 292 struct X *x = ?{}( // construct result of malloc 293 _tmp_ctor=malloc _T(sizeof(struct X), _Alignof(struct X)), // store result of malloc293 _tmp_ctor=malloc(sizeof(struct X)), // store result of malloc 294 294 1.5 295 295 ), _tmp_ctor; // produce constructed result of malloc … … 297 297 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. 298 298 299 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. 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@. 300 302 301 303 \subsection{Function Generation} … … 374 376 The field constructors are constructors that consume a prefix of the structure's member-list. 375 377 That 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. 376 The addition of field constructors allows structures in \CFA to be used naturally in the same ways as used in C ( \ie, to initialize any prefix of the structure), \eg, @A a0 = { b }, a1 = { b, c }@.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 }@. 377 379 Extending the previous example, the following constructors are implicitly generated for @A@. 378 380 \begin{cfacode} … … 427 429 428 430 \subsection{Using Constructors and Destructors} 429 Implicitly generated constructor and destructor calls ignore the outermost type qualifiers, \eg@const@ and @volatile@, on a type by way of a cast on the first argument to the function.431 Implicitly generated constructor and destructor calls ignore the outermost type qualifiers, e.g. @const@ and @volatile@, on a type by way of a cast on the first argument to the function. 430 432 For example, 431 433 \begin{cfacode} … … 446 448 Here, @&s@ and @&s2@ are cast to unqualified pointer types. 447 449 This mechanism allows the same constructors and destructors to be used for qualified objects as for unqualified objects. 448 This ruleapplies only to implicitly generated constructor calls.450 This applies only to implicitly generated constructor calls. 449 451 Hence, explicitly re-initializing qualified objects with a constructor requires an explicit cast. 450 452 … … 487 489 Instead, @a2->x@ is initialized to @0@ as if it were a C object, because of the explicit initializer. 488 490 489 In addition to freedom, \ateq provides a simple path formigrating legacy C code to \CFA, in that objects can be moved from C-style initialization to \CFA gradually and individually.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. 490 492 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. 491 493 It is recommended that most objects be managed by sensible constructors and destructors, except where absolutely necessary. … … 501 503 { 502 504 void ?{}(S * s, int i) { s->x = i*2; } // locally hide autogen constructors 503 S s4; // error , no default constructor504 S s5 = { 3 }; // okay , local constructor505 S s6 = { 4, 5 }; // error , no field constructor505 S s4; // error 506 S s5 = { 3 }; // okay 507 S s6 = { 4, 5 }; // error 506 508 S s7 = s5; // okay 507 509 } … … 511 513 In this example, the inner scope declares a constructor from @int@ to @S@, which hides the default constructor and field constructors until the end of the scope. 512 514 513 When defining a constructor or destructor for a struct ure@S@, any members that are not explicitly constructed or destructed are implicitly constructed or destructed automatically.515 When defining a constructor or destructor for a struct @S@, any members that are not explicitly constructed or destructed are implicitly constructed or destructed automatically. 514 516 If an explicit call is present, then that call is taken in preference to any implicitly generated call. 515 517 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. … … 595 597 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. 596 598 597 More precisely, constructor calls cannot have a nesting depth greater than the number of array dimensions in the type of the initialized object, plus one.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. 598 600 For example, 599 601 \begin{cfacode} … … 607 609 { {14 }, { 15 } } // a2[1] 608 610 }; 609 A a3[4] = { // 1 dimension => max depth 2610 { { 11 }, { 12 } }, // error , three levels deep611 A a3[4] = { 612 { { 11 }, { 12 } }, // error 611 613 { 80 }, { 90 }, { 100 } 612 614 } … … 620 622 \label{sub:implicit_dtor} 621 623 Destructors are automatically called at the end of the block in which the object is declared. 622 In addition to this, destructors are automatically called when statements manipulate control flow to leave a block in which the object is declared, \eg, with return, break, continue, and goto statements.624 In addition to this, destructors are automatically called when statements manipulate control flow to leave a block in which the object is declared, e.g., with return, break, continue, and goto statements. 623 625 The example below demonstrates a simple routine with multiple return statements. 624 626 \begin{cfacode} … … 745 747 Exempt from these rules are intrinsic and built-in functions. 746 748 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. 747 That is, since the parameter is not marked as an unmanaged object using \ateq, it isbe 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.748 Th ese semantics are importantto bear in mind when using unmanaged objects, and could produce unexpected results when mixed with objects that are explicitly constructed.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. 750 This 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. 749 751 \begin{cfacode} 750 752 struct A; … … 761 763 identity(z); // copy construct z into x 762 764 \end{cfacode} 763 Note that unmanaged argument @z@ is logically copy constructed into managed parameter @x@; however, the translator must copy construct into a temporary variable to be passed as an argument, which is also destructed after the call. 764 A compiler could by-pass the argument temporaries since it is in control of the calling conventions and knows exactly where the called-function's parameters live. 765 Note that @z@ is copy constructed into a temporary variable to be passed as an argument, which is also destructed after the call. 765 766 766 767 This generates the following … … 858 859 This transformation provides @f@ with the address of the return variable so that it can be constructed into directly. 859 860 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}. 860 A 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, \eg861 A 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. 861 862 \begin{cfacode} 862 863 struct A { int v; }; … … 873 874 Furthermore, it is not possible to overload C functions, so using @extern "C"@ to declare functions is of limited use. 874 875 875 It would be possible to regain some control by adding an attribute to struct ures 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.876 Ideally, struct ures should be manageable by default, since otherwise the default case becomes more verbose.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. 877 Ideally, structs should be manageable by default, since otherwise the default case becomes more verbose. 877 878 This means that in general, function signatures would have to be rewritten, and in a select few cases the signatures would not be rewritten. 878 879 \begin{cfacode} … … 885 886 C h(); // rewritten void h(C *); 886 887 \end{cfacode} 887 An alternative is to make the attribute \emph{identifiable}, which states that objects of this type use the @this@ parameter as an identity.888 An alternative is to instead make the attribute \emph{identifiable}, which states that objects of this type use the @this@ parameter as an identity. 888 889 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. 889 890 Furthermore, no restrictions would need to be placed on whether objects can be constructed. … … 1014 1015 1015 1016 \subsection{Global Initialization} 1016 In standard C, global variables can only be initialized to compile-time constant expressions, which places strict limitations on the programmer's ability to control the default values of objects. 1017 In standard C, global variables can only be initialized to compile-time constant expressions. 1018 This places strict limitations on the programmer's ability to control the default values of objects. 1017 1019 In \CFA, constructors and destructors are guaranteed to be run on global objects, allowing arbitrary code to be run before and after the execution of the main routine. 1018 1020 By default, objects within a translation unit are constructed in declaration order, and destructed in the reverse order. 1019 1021 The default order of construction of objects amongst translation units is unspecified. 1020 It is, however, guaranteed that any global objects in the standard library are initialized prior to the initialization of any object in auser program.1022 It is, however, guaranteed that any global objects in the standard library are initialized prior to the initialization of any object in the user program. 1021 1023 1022 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}. … … 1051 1053 % https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes 1052 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 1053 GCC provides an attribute @init_priority@ in \CC, which allows specifying the relative priority for initialization of global objects on a per-object basis.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. 1054 1056 A similar attribute can be implemented in \CFA by pulling marked objects into global constructor/destructor-attribute functions with the specified priority. 1055 1057 For example, … … 1074 1076 In standard C, it is possible to mark variables that are local to a function with the @static@ storage class. 1075 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. 1076 Much like global variables, @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.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. 1077 1079 1078 1080 Yet again, this rule is too restrictive for a language with constructors and destructors. 1079 Since the initializer expression is not necessarily a compile-time constant and can depend on the current execution state of the function, \CFA modifies the definition of a @static@ local variable so that objects are guaranteed to be live from the time control flow reaches their declaration, until the end of the program.1080 Since standard C does not allow access to a @static@ local variable before the first time control flow reaches the declaration, this changedoes not preclude any valid C code.1081 Instead, \CFA modifies the definition of a @static@ local variable so that objects are guaranteed to be live from the time control flow reaches their declaration, until the end of the program, since the initializer expression is not necessarily a compile-time constant, but can depend on the current execution state of the function. 1082 Since standard C does not allow access to a @static@ local variable before the first time control flow reaches the declaration, this restriction does not preclude any valid C code. 1081 1083 Local objects with @static@ storage class are only implicitly constructed and destructed once for the duration of the program. 1082 1084 The object is constructed when its declaration is reached for the first time. … … 1088 1090 Since the parameter to @atexit@ is a parameter-less function, some additional tweaking is required. 1089 1091 First, the @static@ variable must be hoisted up to global scope and uniquely renamed to prevent name clashes with other global objects. 1090 If necessary, a local structure may need to be hoisted, as well. 1091 Second, a function is built that calls the destructor for the newly hoisted variable. 1092 Second, a function is built which calls the destructor for the newly hoisted variable. 1092 1093 Finally, the newly generated function is registered with @atexit@, instead of registering the destructor directly. 1093 1094 Since @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. … … 1155 1156 void f(T); 1156 1157 \end{cfacode} 1157 This allows easily specifying constraints that are common to all complete object -types very simply.1158 1159 Now that \CFA has constructors and destructors, more of a complete object's behaviour can be specified than was previously possible.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. 1160 1161 As such, @otype@ has been augmented to include assertions for a default constructor, copy constructor, and destructor. 1161 1162 That is, the previous example is now equivalent to 1162 1163 \begin{cfacode} 1163 forall(dtype T | sized(T) | 1164 { T ?=?(T *, T); void ?{}(T *); void ?{}(T *, T); void ^?{}(T *); }) 1164 forall(dtype T | sized(T) | { T ?=?(T *, T); void ?{}(T *); void ?{}(T *, T); void ^?{}(T *); }) 1165 1165 void f(T); 1166 1166 \end{cfacode} 1167 Th ese 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.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 and prevent potential reuse.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.
Note:
See TracChangeset
for help on using the changeset viewer.
