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:http://blog.csdn.net/yfkiss/article/details/6537234
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:
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- #include <vector>
- #include <mt_allocator.h>
- std::vector<int, __gnu_cxx::__mt_alloc<int>> Array(100);
-
#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
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- #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); }
- };
-
#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); }
-
};