Content deleted Content added
m →References: Task 16: replaced (1×) / removed (0×) deprecated |dead-url= and |deadurl= with |url-status=; |
m Task 70: Update syntaxhighlight tags - remove use of deprecated <source> tags |
||
Line 3:
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 (<
== History ==
Line 16:
== Functions ==
The placement new functions are overloads of the non-placement new functions. The declaration of the non-placement new functions, for non-array and array <code>new</code> expressions respectively, are:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><
void * operator new (std::size_t) throw(std::bad_alloc);
void * operator new[] (std::size_t) throw(std::bad_alloc);
</syntaxhighlight>
The Standard C++ library provides two placement overloads each for these functions. Their declarations are:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><
void * operator new (std::size_t, const std::nothrow_t &) throw();
void * operator new (std::size_t, void *) throw();
void * operator new[] (std::size_t, const std::nothrow_t &) throw();
void * operator new[] (std::size_t, void *) throw();
</syntaxhighlight>
In all of the overloads, the first parameter to the <code>operator new</code> function is of type <
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 /><
void operator delete (void *) throw();
void operator delete[] (void *) throw();
</syntaxhighlight>
The Standard C++ library provides two placement overloads each for these functions. Their declarations are:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><
void operator delete (void *, const std::nothrow_t &) throw();
void operator delete (void *, void *) throw();
void operator delete[] (void *, const std::nothrow_t &) throw();
void operator delete[] (void *, void *) throw();
</syntaxhighlight>
In all of the overloads, the first parameter to the <code>operator delete</code> function is of type <
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 />
Line 50:
=== Default placement ===
The placement overloads of <code>operator new</code> and <code>operator delete</code> that employ an additional <
void * operator new (std::size_t, void * p) throw() { return p ; }
void * operator new[] (std::size_t, void * p) throw() { return p ; }
void operator delete (void *, void *) throw() { }
void operator delete[] (void *, void *) throw() { }
</syntaxhighlight>
There are various uses for default placement.
Line 64:
The C++ language does allow a program to call a [[destructor (computer science)|destructor]] directly, and, since it is not possible to destroy the object using a <code>delete</code> expression, that is how one destroys an object that was constructed via a pointer placement new expression. For example:<ref name=SolterKleper2005 /><ref name=SeedCooper2001 />
: <
p->~T() ;
</syntaxhighlight>
Placement new is used when you do not want operator new to allocate memory (you have pre-allocated it and you want to place the object there), but you do want the object to be constructed. Examples of typical situations where this may be required are:
Line 75:
The basic problem is that the constructor is a peculiar function; when it starts off, there is no object, only raw memory. And by the time it finishes, you have a fully initialized object. Therefore, i) The constructor cannot be called on an object ii) However, it needs to access (and initialize) non-static members. This makes calling the constructor directly an error. The solution is the placement form of operator new.
This operator is implemented as:<
inline void* operator new( size_t sz, void* here )
Line 81:
inline void* operator new[]( size_t sz, void* here )
{ return here ; }
</syntaxhighlight>
=== Preventing exceptions ===
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 <
Programmers who wish to do this in their programs must include the Standard C++ library header <code><new></code> 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 <
#include <new>
Line 102:
return 0 ;
}
</syntaxhighlight>
=== Custom allocators ===
Placement syntax is also employed for custom [[allocator (C++)|allocators]]. This does not use any of the allocator and deallocator functions from the Standard C++ library header <code><new></code>, but requires that programmers write their own allocation and deallocation functions, overloaded for user-defined types. For example, one could define a memory management class as follows:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><
#include <cstdlib>
class A {
Line 112:
void deallocate ( void * ) ;
} ;
</syntaxhighlight>
And define custom placement allocation and deallocation functions as follows:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><
void *
operator new (std::size_t size, A & arena)
Line 125:
arena.deallocate(p) ;
}
</syntaxhighlight>
The program would employ the placement syntax to allocate objects using different instances of the <code>A</code> class as follows:<ref name=Vermeir2001 /><ref name=Stroustrup1997b /><
A first_arena, second_arena ;
T * p1 = new (first_arena) T ;
T * p2 = new (second_arena) T ;
</syntaxhighlight>
Destroying an object whose storage is allocated in such a fashion requires some care. Because there is no placement delete expression, one cannot use it to invoke the custom deallocator. One must either write a destruction function that invokes the custom deallocator, or call the placement delete function directly, as a function call.<ref name=SolterKleper2005 /><ref name=Vermeir2001 /><ref name=Stroustrup1997b />
The former would resemble:<ref name=Stroustrup1997b /><
void
destroy (T * p, A & arena)
Line 142:
arena.deallocate(p) ; // Then call the deallocator function directly.
}
</syntaxhighlight>
which would be invoked from a program as:<
A arena ;
T * p = new (arena) T ;
/* ... */
destroy(p, arena) ;
</syntaxhighlight>
The latter would involve simply writing the destructor invocation and delete function call into the program:<ref name=Vermeir2001 /><ref name=Dewhurst2003 /><
A arena ;
T * p = new (arena) T ;
Line 156:
p->~T() ; // First invoke the destructor explicitly.
operator delete(p, arena) ; // Then call the deallocator function indirectly via operator delete(void *, A &) .
</syntaxhighlight>
A common error is to attempt to use a delete expression to delete the object. This results in the wrong <code>operator delete</code> function being called. Dewhurst recommends two strategies for avoiding this error. The first is to ensure that any custom allocators rely upon the Standard C++ library's global, non-placement, <code>operator new</code>, and are thus nothing more than simple wrappers around the C++ library's memory management. The second is to create new and delete functions for individual classes, and customize memory management via class function members rather than by using the placement syntax.<ref name=Dewhurst2003 />
=== Debugging ===
Placement new can also be used as a simple debugging tool, to enable programs to print the filename and line number of the source code where a memory allocation has failed. This does not require the inclusion of the Standard C++ library header <code><new></code>, but does require the inclusion of a header that declares four placement functions and a macro replacement for the <code>new</code> keyword that is used in new expressions. For example, such a header would contain:<ref name=Anderson1998a /><ref name=Yongwei2007 /><
#if defined(DEBUG_NEW)
void * operator new (std::size_t size, const char* file, int line);
Line 171:
#define New new
#endif
</syntaxhighlight>
This would be employed in a program as follows:<ref name=Anderson1998a /><ref name=Yongwei2007 /><
T * p = New T ;
</syntaxhighlight>
The custom-written placement new functions would then handle using the supplied file and line number information in the event of an exception. For example:<ref name=Anderson1998a /><ref name=Yongwei2007 /><
#include <new>
#include <cstdlib>
Line 194:
throw NewError(file, line) ;
}
</syntaxhighlight>
== Placement delete ==
Line 201:
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 /><
#include <cstdlib>
#include <iostream>
Line 227:
}
</syntaxhighlight>
This is why the ''pointer placement'' delete functions are defined as no-operations by the Standard C++ library. Since the pointer placement new functions do not allocate any storage, there is no storage to be deallocated in the event of the object's constructor throwing an exception.<ref name=SolterKleper2005 />
| |||