Placement syntax: Difference between revisions

Revision as of 05:46, 8 December 2010

Revision as of 19:18, 14 December 2011

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In the [[C++]] [[programming language]], '''placement syntax''' allows programmers to explicitly specify the [[memory management]] of individual objects&nbsp;— i.e. their "placement" in [[memory (computing)|memory]]. Normally, when an object is created dynamically, an allocation function is invoked in such a way that it will both allocate memory for the object, and [[Constructor (object-oriented programming)|initialize]] the object within the newly allocated memory.<!--5.3.5--> The placement syntax allows the programmer to supply additional arguments to the allocation function. A common use is to supply a [[pointer (computingcomputer programming)|pointer]] to a suitable region of storage where the object can be initialized, thus separating memory allocation from object construction.{{Citation needed|date=October 2010}}

The "placement" versions of the <code>[[new (C++)|new]]</code> and <code>[[delete (C++)|delete]]</code> operators and functions are known as placement <code>new</code> and placement <code>delete</code>.<ref name=McCluskey2000 /> A <code>new</code> ''expression'', placement or otherwise, calls a <code>new</code> ''function'', also known as an allocator function, whose name is <code>operator new</code>. Similarly, a <code>delete</code> ''expression'' calls a <code>delete</code> ''function'', also known as a deallocator function, whose name is <code>operator delete</code>.<ref name=Lischner2003 /><ref name=Lippman1997 />

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In all of the overloads, the first parameter to the <code>operator new</code> function is of type <source lang="cpp" enclose=none>size_t</source>, which when the function is called will be passed as an argument the amount of memory, in bytes, to allocate. All of the functions must return type <source lang="cpp" enclose=none>void *</source>, which is a [[pointer (computingcomputer programming)|pointer]] to the storage that the function allocates.<ref name=Lischner2003 />

There are also placement delete functions. They are overloaded versions of the non-placement delete functions. The non-placement delete functions are declared as:<ref name=Vermeir2001 /><ref name=Stroustrup1997b />

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If no matching placement delete function exists, no deallocation function is called in the event of an exception being thrown by a constructor within a placement <code>new</code> expression. There are also some (older) C++ implementations that do not support placement delete (which, like the exception-throwing allocator functions, were an addition made to C++ when it was standardized) at all. In both such situations, an exception being thrown by a constructor when allocating using a custom allocator will result in a memory leak. (In the case of the older C++ implementations, a memory leak will also occur with ''non-''placement <code>new</code> expressions.)<ref name=Meyers1998 /><ref name=Anderson1998b />

== ReferencesNotes ==

{{reflist|3|refs=

<ref name=Anderson1998a>{{harvnb|Anderson|1998a|pp=345–356}}</ref>

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}}

== References ==

=== Reference bibliography ===

{{refbegin|1}}

*{{cite book|ref=harv|title=Navigating C++ and Object-oriented Design|first=Gail|last=Anderson|chapter=Object Storage Management|publisher=Prentice Hall|year=1998a|isbn=0135327482|isbn13=9780135327487}}