361 lines
13 KiB
ReStructuredText
361 lines
13 KiB
ReStructuredText
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.. role:: block-term
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=================================
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Language Specification for Blocks
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=================================
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.. contents::
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:local:
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Revisions
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=========
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- 2008/2/25 --- created
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- 2008/7/28 --- revised, ``__block`` syntax
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- 2008/8/13 --- revised, Block globals
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- 2008/8/21 --- revised, C++ elaboration
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- 2008/11/1 --- revised, ``__weak`` support
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- 2009/1/12 --- revised, explicit return types
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- 2009/2/10 --- revised, ``__block`` objects need retain
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Overview
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========
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A new derived type is introduced to C and, by extension, Objective-C,
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C++, and Objective-C++
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The Block Type
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==============
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Like function types, the :block-term:`Block type` is a pair consisting
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of a result value type and a list of parameter types very similar to a
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function type. Blocks are intended to be used much like functions with
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the key distinction being that in addition to executable code they
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also contain various variable bindings to automatic (stack) or managed
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(heap) memory.
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The abstract declarator,
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.. code-block:: c
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int (^)(char, float)
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describes a reference to a Block that, when invoked, takes two
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parameters, the first of type char and the second of type float, and
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returns a value of type int. The Block referenced is of opaque data
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that may reside in automatic (stack) memory, global memory, or heap
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memory.
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Block Variable Declarations
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===========================
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A :block-term:`variable with Block type` is declared using function
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pointer style notation substituting ``^`` for ``*``. The following are
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valid Block variable declarations:
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.. code-block:: c
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void (^blockReturningVoidWithVoidArgument)(void);
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int (^blockReturningIntWithIntAndCharArguments)(int, char);
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void (^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);
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Variadic ``...`` arguments are supported. [variadic.c] A Block that
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takes no arguments must specify void in the argument list [voidarg.c].
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An empty parameter list does not represent, as K&R provide, an
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unspecified argument list. Note: both gcc and clang support K&R style
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as a convenience.
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A Block reference may be cast to a pointer of arbitrary type and vice
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versa. [cast.c] A Block reference may not be dereferenced via the
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pointer dereference operator ``*``, and thus a Block's size may not be
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computed at compile time. [sizeof.c]
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Block Literal Expressions
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=========================
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A :block-term:`Block literal expression` produces a reference to a
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Block. It is introduced by the use of the ``^`` token as a unary
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operator.
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.. code-block:: c
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Block_literal_expression ::= ^ block_decl compound_statement_body
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block_decl ::=
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block_decl ::= parameter_list
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block_decl ::= type_expression
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where type expression is extended to allow ``^`` as a Block reference
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(pointer) where ``*`` is allowed as a function reference (pointer).
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The following Block literal:
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.. code-block:: c
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^ void (void) { printf("hello world\n"); }
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produces a reference to a Block with no arguments with no return value.
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The return type is optional and is inferred from the return
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statements. If the return statements return a value, they all must
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return a value of the same type. If there is no value returned the
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inferred type of the Block is void; otherwise it is the type of the
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return statement value.
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If the return type is omitted and the argument list is ``( void )``,
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the ``( void )`` argument list may also be omitted.
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So:
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.. code-block:: c
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^ ( void ) { printf("hello world\n"); }
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and:
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.. code-block:: c
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^ { printf("hello world\n"); }
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are exactly equivalent constructs for the same expression.
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The type_expression extends C expression parsing to accommodate Block
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reference declarations as it accommodates function pointer
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declarations.
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Given:
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.. code-block:: c
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typedef int (*pointerToFunctionThatReturnsIntWithCharArg)(char);
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pointerToFunctionThatReturnsIntWithCharArg functionPointer;
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^ pointerToFunctionThatReturnsIntWithCharArg (float x) { return functionPointer; }
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and:
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.. code-block:: c
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^ int ((*)(float x))(char) { return functionPointer; }
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are equivalent expressions, as is:
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.. code-block:: c
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^(float x) { return functionPointer; }
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[returnfunctionptr.c]
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The compound statement body establishes a new lexical scope within
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that of its parent. Variables used within the scope of the compound
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statement are bound to the Block in the normal manner with the
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exception of those in automatic (stack) storage. Thus one may access
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functions and global variables as one would expect, as well as static
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local variables. [testme]
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Local automatic (stack) variables referenced within the compound
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statement of a Block are imported and captured by the Block as const
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copies. The capture (binding) is performed at the time of the Block
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literal expression evaluation.
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The compiler is not required to capture a variable if it can prove
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that no references to the variable will actually be evaluated.
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Programmers can force a variable to be captured by referencing it in a
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statement at the beginning of the Block, like so:
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.. code-block:: c
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(void) foo;
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This matters when capturing the variable has side-effects, as it can
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in Objective-C or C++.
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The lifetime of variables declared in a Block is that of a function;
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each activation frame contains a new copy of variables declared within
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the local scope of the Block. Such variable declarations should be
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allowed anywhere [testme] rather than only when C99 parsing is
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requested, including for statements. [testme]
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Block literal expressions may occur within Block literal expressions
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(nest) and all variables captured by any nested blocks are implicitly
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also captured in the scopes of their enclosing Blocks.
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A Block literal expression may be used as the initialization value for
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Block variables at global or local static scope.
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The Invoke Operator
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===================
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Blocks are :block-term:`invoked` using function call syntax with a
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list of expression parameters of types corresponding to the
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declaration and returning a result type also according to the
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declaration. Given:
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.. code-block:: c
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int (^x)(char);
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void (^z)(void);
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int (^(*y))(char) = &x;
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the following are all legal Block invocations:
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.. code-block:: c
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x('a');
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(*y)('a');
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(true ? x : *y)('a')
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The Copy and Release Operations
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===============================
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The compiler and runtime provide :block-term:`copy` and
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:block-term:`release` operations for Block references that create and,
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in matched use, release allocated storage for referenced Blocks.
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The copy operation ``Block_copy()`` is styled as a function that takes
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an arbitrary Block reference and returns a Block reference of the same
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type. The release operation, ``Block_release()``, is styled as a
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function that takes an arbitrary Block reference and, if dynamically
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matched to a Block copy operation, allows recovery of the referenced
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allocated memory.
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The ``__block`` Storage Qualifier
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=================================
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In addition to the new Block type we also introduce a new storage
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qualifier, :block-term:`__block`, for local variables. [testme: a
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__block declaration within a block literal] The ``__block`` storage
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qualifier is mutually exclusive to the existing local storage
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qualifiers auto, register, and static. [testme] Variables qualified by
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``__block`` act as if they were in allocated storage and this storage
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is automatically recovered after last use of said variable. An
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implementation may choose an optimization where the storage is
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initially automatic and only "moved" to allocated (heap) storage upon
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a Block_copy of a referencing Block. Such variables may be mutated as
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normal variables are.
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In the case where a ``__block`` variable is a Block one must assume
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that the ``__block`` variable resides in allocated storage and as such
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is assumed to reference a Block that is also in allocated storage
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(that it is the result of a ``Block_copy`` operation). Despite this
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there is no provision to do a ``Block_copy`` or a ``Block_release`` if
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an implementation provides initial automatic storage for Blocks. This
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is due to the inherent race condition of potentially several threads
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trying to update the shared variable and the need for synchronization
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around disposing of older values and copying new ones. Such
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synchronization is beyond the scope of this language specification.
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Control Flow
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============
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The compound statement of a Block is treated much like a function body
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with respect to control flow in that goto, break, and continue do not
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escape the Block. Exceptions are treated *normally* in that when
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thrown they pop stack frames until a catch clause is found.
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Objective-C Extensions
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======================
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Objective-C extends the definition of a Block reference type to be
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that also of id. A variable or expression of Block type may be
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messaged or used as a parameter wherever an id may be. The converse is
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also true. Block references may thus appear as properties and are
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subject to the assign, retain, and copy attribute logic that is
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reserved for objects.
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All Blocks are constructed to be Objective-C objects regardless of
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whether the Objective-C runtime is operational in the program or
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not. Blocks using automatic (stack) memory are objects and may be
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messaged, although they may not be assigned into ``__weak`` locations
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if garbage collection is enabled.
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Within a Block literal expression within a method definition
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references to instance variables are also imported into the lexical
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scope of the compound statement. These variables are implicitly
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qualified as references from self, and so self is imported as a const
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copy. The net effect is that instance variables can be mutated.
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The :block-term:`Block_copy` operator retains all objects held in
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variables of automatic storage referenced within the Block expression
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(or form strong references if running under garbage collection).
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Object variables of ``__block`` storage type are assumed to hold
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normal pointers with no provision for retain and release messages.
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Foundation defines (and supplies) ``-copy`` and ``-release`` methods for
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Blocks.
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In the Objective-C and Objective-C++ languages, we allow the
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``__weak`` specifier for ``__block`` variables of object type. If
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garbage collection is not enabled, this qualifier causes these
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variables to be kept without retain messages being sent. This
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knowingly leads to dangling pointers if the Block (or a copy) outlives
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the lifetime of this object.
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In garbage collected environments, the ``__weak`` variable is set to
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nil when the object it references is collected, as long as the
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``__block`` variable resides in the heap (either by default or via
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``Block_copy()``). The initial Apple implementation does in fact
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start ``__block`` variables on the stack and migrate them to the heap
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only as a result of a ``Block_copy()`` operation.
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It is a runtime error to attempt to assign a reference to a
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stack-based Block into any storage marked ``__weak``, including
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``__weak`` ``__block`` variables.
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C++ Extensions
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==============
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Block literal expressions within functions are extended to allow const
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use of C++ objects, pointers, or references held in automatic storage.
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As usual, within the block, references to captured variables become
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const-qualified, as if they were references to members of a const
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object. Note that this does not change the type of a variable of
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reference type.
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For example, given a class Foo:
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.. code-block:: c
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Foo foo;
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Foo &fooRef = foo;
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Foo *fooPtr = &foo;
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A Block that referenced these variables would import the variables as
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const variations:
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.. code-block:: c
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const Foo block_foo = foo;
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Foo &block_fooRef = fooRef;
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Foo *const block_fooPtr = fooPtr;
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Captured variables are copied into the Block at the instant of
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evaluating the Block literal expression. They are also copied when
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calling ``Block_copy()`` on a Block allocated on the stack. In both
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cases, they are copied as if the variable were const-qualified, and
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it's an error if there's no such constructor.
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Captured variables in Blocks on the stack are destroyed when control
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leaves the compound statement that contains the Block literal
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expression. Captured variables in Blocks on the heap are destroyed
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when the reference count of the Block drops to zero.
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Variables declared as residing in ``__block`` storage may be initially
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allocated in the heap or may first appear on the stack and be copied
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to the heap as a result of a ``Block_copy()`` operation. When copied
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from the stack, ``__block`` variables are copied using their normal
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qualification (i.e. without adding const). In C++11, ``__block``
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variables are copied as x-values if that is possible, then as l-values
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if not; if both fail, it's an error. The destructor for any initial
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stack-based version is called at the variable's normal end of scope.
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References to ``this``, as well as references to non-static members of
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any enclosing class, are evaluated by capturing ``this`` just like a
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normal variable of C pointer type.
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Member variables that are Blocks may not be overloaded by the types of
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their arguments.
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