STL的內(nèi)存管理

2020-02-24 14:31:51

字體：大中小

供稿：網(wǎng)友

STL提供了很多泛型容器，很多程序員是使用這些容器的時(shí)候都只關(guān)心何時(shí)往容器內(nèi)塞對(duì)象，而不用關(guān)心如何管理內(nèi)存，其實(shí)管理內(nèi)存也是非常重要的，那么我們現(xiàn)在就去看看STL的內(nèi)存管理。

1. 概述
STL Allocator是STL的內(nèi)存管理器，也是最低調(diào)的部分之一，你可能使用了3年stl，但卻不知其為何物。

STL標(biāo)準(zhǔn)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(chǔ)介質(zhì)（例如硬盤）做為stl 容器的存儲(chǔ)空間。由于內(nèi)存是allocator管理的主要部分，因此，本文以STL內(nèi)存管理為出發(fā)點(diǎn)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(zhì)上，調(diào)用的是：

#include <vector>
std::vector<int, std::allocator> Array(100);
std::allocator就是一個(gè)簡(jiǎn)單的Allocator

2. 使用
針對(duì)不同的應(yīng)用場(chǎng)合，STL中實(shí)現(xiàn)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(huán)境下，可以使用：

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

3.一個(gè)簡(jiǎn)單的Allocator實(shí)現(xiàn)
我們可以實(shí)現(xiàn)自己的allocator

#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(nèi)存的分配與釋放可以實(shí)現(xiàn)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(gòu)造函數(shù)?
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); }?
};??