不得不说的故事：STL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. 为什么需要了解Allocator

项目中遇到的两个case

1）memory高位

线上使用vector保存待处理的数据，在停服务追数据的过程中，由于vector::clear并不释放内存，造成内存始终处于高位。

参考：/article/1410089.html

2）多线程使用vector、map

使用map保存状态，在多线程环境下，出现性能问题

在实际使用STL的过程中，会遇到很多问题，需要了解STL Allocator

3. 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);

4.一个简单的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); }
};

5. 参考文献

STL源码剖析

http://gcc.gnu.org/onlinedocs/libstdc++/manual/ext_allocators.html

http://www.codeguru.com/cpp/cpp/cpp_mfc/stl/article.php/c4079

http://blog.163.com/dengminwen@126/blog/static/870226720097189486788/