簡單說說何為之STL的記憶體管理Allocator
簡單說說何為之STL的記憶體管理Allocator
1. 概述
STL Allocator是STL的記憶體管理器，也是最低調的部分之一，你可能使用了3年stl，但卻不知其為何物。
STL標準如下介紹Allocator
the STL includes some low-level mechanisms for allocating and deallocating memory. Allocators are very specialized, and you can safely ignore them for almost all purposes. Allocators encapsulate allocation and deallocation of memory. They provide a low-level interface that permits efficient allocation of many small objects; different allocator types represent different schemes for memory management.
將其描述為空間配置器，理由是allocator可以將其它儲存介質（例如硬碟）做為stl 容器的儲存空間。由於記憶體是allocator管理的主要部分，因此，本文以STL記憶體管理為出發點介紹allocator。
Allocator就在我們身邊，通常使用STL的方式：
#include
std::vectorArray(100);
本質上，呼叫的是：
#include
std::vectorArray(100);
std::allocator就是一個簡單的Allocator
2. 使用
針對不同的應用場合，STL中實現了不同的'Allocator，如下（gcc-3.4：http://www.cs.huji.ac.il/~etsman/Docs/gcc-3.4-base/libstdc++/html/20_util/allocator.html）:
__gnu_cxx::new_allocatorSimply wraps ::operator new and ::operator .
__gnu_cxx::malloc_allocatorSimply wraps malloc and free. There is also a hook for an out-of-memory handler
__gnu_cxx::debug_allocatorA wrapper around an arbitrary allocator A. It passes on slightly increased size requests to A, and uses the extra memory to store size information.
__gnu_cxx::__pool_allocA high-performance, single pool allocator. The reusable memory is shared among identical instantiations of this type.
__gnu_cxx::__mt_allocA high-performance fixed-size allocatorthat was initially developed specifically to suit the needs of multi threaded applications
__gnu_cxx::bitmap_allocato A high-performance allocator that uses a bit-map to keep track of the used and unused memory locations
例如，在多執行緒環境下，可以使用：
複製程式碼 程式碼如下:
#include
#include
std::vector
3.一個簡單的Allocator實現
我們可以實現自己的allocator
複製程式碼 程式碼如下:
#include
template
class my_allocator : public std::allocator
{
public:
typedef std::allocatorbase_type;
// 必須要重新定義
template
struct rebind
{
typedef my_allocatorother;
};
// 記憶體的分配與釋放可以實現為自定義的演算法
pointer allocate(size_type count)
{
return (base_type::allocate(count));
}
void deallocate(pointer ptr, size_type count)
{
base_type::deallocate(ptr, count);
}
// 建構函式
my_allocator()
{}
my_allocator(my_allocatorconst&)
{}
my_allocator& operator=(my_allocatorconst&)
{
return (*this);
}
template
my_allocator(my_allocatorconst&)
{}
template
my_allocator& operator=(my_allocatorconst&)
{
return (*this); }
};