STL的内存管理

2020-02-24 14:31:51

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STL提供了很多泛型容器，很多程序员是使用这些容器的时候都只关心何时往容器内塞对象，而不用关心如何管理内存，其实管理内存也是非常重要的，那么我们现在就去看看STL的内存管理。

1. 概述
STL Allocator是STL的内存管理器，也是最低调的部分之一，你可能使用了3年stl，但却不知其为何物。

STL标准如下介绍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 源码剖析>将其描述为空间配置器，理由是allocator可以将其它存储介质（例如硬盘）做为stl 容器的存储空间。由于内存是allocator管理的主要部分，因此，本文以STL内存管理为出发点介绍allocator。

Allocator就在我们身边，通常使用STL的方式：
#include <vector>
std::vector<int> Array(100);

本质上，调用的是：

#include <vector>
std::vector<int, std::allocator> Array(100);
std::allocator就是一个简单的Allocator

2. 使用
针对不同的应用场合，STL中实现了不同的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，在多线程环境下，可以使用：

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

3.一个简单的Allocator实现
我们可以实现自己的allocator

#include <memory> 

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

// 必须要重新定义 
template<class Other> 
struct rebind 
{ 
typedef my_allocator<Other> other; 
}; 
// 内存的分配与释放可以实现为自定义的算法 
pointer allocate(size_type count) 
{  
return (base_type::allocate(count)); 
} 

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

 
// 构造函数 
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); } 
};  