理解allocator

參考：http://oss.org.cn/?action-viewnews-itemid-3744

allocator 是一個類,有着叫allocate()和deallocate()成員函數（相當於malloc和free）。它還有用於維護所分配的內存的輔助函數和指示如何使用這些內存的typedef（指針或引用類型的名字）。

舉個例子vector::get_allocator(代碼來自http://www.cplusplus.com/reference/stl/vector/get_allocator/)

// vector::get_allocator
#include <iostream>
#include <vector>

using namespace std;

int main ()
{
  vector<int> myvector;
  int * p;
  unsigned int i;

  // allocate an array of 5 elements using vector's allocator:
  p=myvector.get_allocator().allocate(5);

  // assign some values to array
  for (i=0; i<5; i++) p[i]=i;

  cout << "The allocated array contains:";
  for (i=0; i<5; i++) cout << " " << p[i];
  cout << endl;

  myvector.get_allocator().deallocate(p,5);

  return 0;
}

Returns the allocator object used to construct the vector.

allocator也參考(You can see an example of a pool allocator in the open source SGIPro64TM?compiler)：http://oss.sgi.com/projects/Pro64/

也參考：http://www.cplusplus.com/reference/std/memory/allocator/

Default allocator

Allocators are classes that define memory models to be used by some parts of the Standard Library, and most specifically, by STL containers.

This section describes the default allocator template allocator (lowercase). This is the allocator that all standard containers will use if their last (and optional) template parameter is not specified, and is the only predefined allocator in the standard library.

Other allocators may be defined. Any class having the same members as this default allocator and following its minimum requirements can be used as an allocator -- notice that only under very specific circumstances this is needed.

Technically, a memory model described by allocators might be specialized for each type of object to be allocated and even may store local data for each container they work with. Although this does not happen with the defaultallocator.

類模板allocator的定義如下：

template < class T > class allocator;

Taking one template parameter (which is assumed to be T in this entire reference).

其成員變量和成員函數如下：

Member types

member	definition in allocator	represents
value_type	T	Element type
pointer	T*	Pointer to element
reference	T&	Reference to element
const_pointer	const T*	Constant pointer to element
const_reference	const T&	Constant reference to element
size_type	size_t	Quantities of elements
difference_type	ptrdiff_t	Difference between two pointers

Member functions

(constructor)

Construct allocator object (public member function)

(destructor)

Allocator destructor (public member function)

address

Return address (public member function)

allocate

Allocate block of storage (public member function)

deallocate

Release block of storage (public member function)

max_size

Maximum size possible to allocate (public member function)

construct

Construct an object (public member function)

destroy

Destroy an object (public member function)

同時也可以看看MSDN中的定義：

template<class T>
    class allocator {
    typedef size_t size_type;
    typedef ptrdiff_t difference_type;
    typedef T *pointer;
    typedef const T *const_pointer;
    typedef T& reference;
    typedef const T& const_reference;
    typedef T value_type;
    pointer address(reference x) const;
    const_pointer address(const_reference x) const;
    allocator();
    allocator<T>& operator=(const allocator<T>);
    pointer allocate(size_type n, const void *hint);
    void deallocate(pointer p, size_type n);
    void construct(pointer p, const T& val);
    void destroy(pointer p);
    size_type max_size() const;
    };