簡單說說STL的內存管理

2020-01-26 15:52:44

字體：大中小

來源：轉載

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1. 概述
STL Allocator是STL的內存管理器，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.
<STL 源碼剖析>將其描述為空間配置器，理由是allocator可以將其它存儲介質（例如硬盤）做為stl 容器的存儲空間。由于內存是allocator管理的主要部分，因此，本文以STL內存管理為出發點介紹allocator。

Allocator就在我們身邊，通常使用STL的方式：
#include <vector>
std::vector<int> Array(100);

本質上，調用的是：

#include <vector>
std::vector<int, std::allocator> Array(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_allocator<T> Simply wraps ::operator new and ::operator delete.
__gnu_cxx::malloc_allocator<T> Simply wraps malloc and free. There is also a hook for an out-of-memory handler
__gnu_cxx::debug_allocator<T> A 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_alloc<bool, int> A high-performance, single pool allocator. The reusable memory is shared among identical instantiations of this type.
__gnu_cxx::__mt_alloc<T> A 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，在多線程環境下，可以使用：

復制代碼代碼如下:

#include <vector>  
#include <mt_allocator.h>  
std::vector<int, __gnu_cxx::__mt_alloc<int>> Array(100);  

3.一個簡單的Allocator實現
我們可以實現自己的allocator

復制代碼代碼如下:

#include <memory>  

template<class T>  
class my_allocator : public std::allocator<T>  
{  
public:  
typedef std::allocator<T> base_type;  

// 必須要重新定義  
template<class Other>  
struct rebind  
{  
typedef my_allocator<Other> other;  
};  
// 內存的分配與釋放可以實現為自定義的算法  
pointer allocate(size_type count)  
{   
return (base_type::allocate(count));  
}  

void deallocate(pointer ptr, size_type count)  
{   
base_type::deallocate(ptr, count);  
}  

  
// 構造函數  
my_allocator()  
{}  

my_allocator(my_allocator<T> const&)  
{}  

my_allocator<T>& operator=(my_allocator<T> const&)  
{   
return (*this);  
 }  

template<class Other>  
my_allocator(my_allocator<Other> const&)  
{}  

template<class Other>  
my_allocator<T>& operator=(my_allocator<Other> const&)  
{   
return (*this); }  
};   