Placement syntax: Difference between revisions

Revision as of 14:32, 2 July 2025

Revision as of 14:34, 2 July 2025

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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 />

Any <code>new</code> expression that uses the placement syntax is a placement <code>new</code> expression, and any <code>operator new</code> or <code>operator delete</code> function that takes more than the mandatory first parameter (<syntaxhighlight lang="cpp" inline>std::size_t</syntaxhighlight>) is a placement new or placement delete function.<ref name=Meyers1998 /> A placement new function takes two input parameters: <syntaxhighlight lang="cpp" inline>std::size_t</syntaxhighlight> and <syntaxhighlight lang="cpp" inline>void *</syntaxhighlight>.

== History ==

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</syntaxhighlight>

In all of the overloads, the first parameter to the <code>operator new</code> function is of type <syntaxhighlight lang="cpp" inline>std::size_t</syntaxhighlight>, which when the function is called will be passed as an argument specifying the amount of memory, in bytes, to allocate. All of the functions must return type <syntaxhighlight lang="cpp" inline>void *</syntaxhighlight>, which is a [[pointer (computer 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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</syntaxhighlight>

In all of the overloads, the first parameter to the <code>operator delete</code> function is of type <syntaxhighlight lang="cpp" inline>void *</syntaxhighlight>, which is the address of the storage to deallocate.<ref name=Lischner2003 />

For both the new and the delete functions, the functions are global, are not in any namespace, and do not have static linkage.<ref name=Lischner2003 />

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=== Default placement ===

The placement overloads of <code>operator new</code> and <code>operator delete</code> that employ an additional <syntaxhighlight lang="cpp" inline>void *</syntaxhighlight> parameter are used for default placement, also known as ''pointer placement''. Their definitions by the Standard C++ library, which it is not permitted for a C++ program to replace or override, are:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><ref name=Anderson1998a />

<syntaxhighlight lang="cpp">

void* operator new(std::size_t, void* p) noexcept { return p; }

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Normally, the (non-placement) new functions throw an exception, of type <code>std::bad_alloc</code>, if they encounter an error, such as exhaustion of all available memory. This was not how the functions were defined by Stroustrup's ''Annotated C++ Reference Manual'', but was a change made by the standardization committee when the C++ language was standardized. The original behaviour of the functions, which was to return a <syntaxhighlight lang="cpp" inline>NULL</syntaxhighlight> pointer when an error occurred, is accessible via placement syntax.<ref name=Lippman1997 /><ref name=Meyers1998 /><ref name=Loudon2003 />

Programmers who wish to do this in their programs must include the Standard C++ library header <code><new></code> (or import the <code>std</code> module) in the source code. This header declares the global <code>std::nothrow</code> object, which is of type <code>std::nothrow_t</code> (also declared in the header), which is used to call the overloaded new functions that are declared as taking <syntaxhighlight lang="cpp" inline>const std::nothrow_t &</syntaxhighlight> as their second parameter. For example:<ref name=Anderson1998a />

<syntaxhighlight lang="cpp">

import std;

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int main() {

// Call the function operator new(std::size_t, const std::nothrow_t &) and (if successful) construct the object.

T* p = new (std::nothrow) T;

if (p) {

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/* ... */

p->~T(); // First invoke the destructor explicitly.

operator delete(p, arena); // Then call the deallocator function indirectly via operator delete(void*, A &).

</syntaxhighlight>

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The placement delete functions are called from placement <code>new</code> expressions. In particular, they are called if the [[constructor (computer science)|constructor]] of the object throws an exception. In such a circumstance, in order to ensure that the program does not incur a [[memory leak]], the placement delete functions are called. A placement new expression first calls the placement <code>operator new</code> function, then calls the constructor of the object upon the raw storage returned from the allocator function. If the constructor throws an exception, it is necessary to deallocate that storage before propagating the exception back to the code that executed the placement new expression, and that is the purpose of the placement delete functions.<ref name=Lischner2003 /><ref name=Meyers1998 /><ref name=SolterKleper2005 /><ref name=Anderson1998b />

The placement delete function that is called matches the placement new function that was invoked by the placement new expression. So, for example, if the following code is executed, the placement delete function that is called will be <code>operator delete(void *, const A &)</code>:<ref name=Lischner2003 /><ref name=SolterKleper2005 /><ref name=Anderson1998b />

<syntaxhighlight lang="cpp">

import std;