[转]关于Memory Pool的一些想法及实现

http://blog.csdn.net/xiaocai0001/archive/2006/04/14/663752.aspx

在《Effective C++》(Second Edition)中Item 10 "Write operator delete if you write operator new"中提到一个Memory Pool技术, 并给出一个例子
void *Airplane::operator new(size_t size)
{
if(size != sizeof(Airplane))
return ::operator new(size);
Airplane *p = headOfFreeList;
if(p)
headOfFreeList = p->next;
else
{
Airplane * newBlock = static_cast<Airplane *>(::operator new(BLOCK_SIZE*sizeof(Airplane)));
}
for(int i=1; i<BLOCK_SIZE-1; ++i)
newBlock[i].next = &newBlock[i+1];
newBlock[BLOCK_SIZE-1].next = 0;
p= newBlock;
headOfFreeList = &newBlock[1];
}
书中提到, 用这种技术的好处有二: 一. 节省空间, 普通的new操作由于需要记住申请的大小, 一般需要更多的空间. 而应用这种技术则不需要额外的空间. 二. 操作速度快, 此种的new和delete操作只是对链表的添加和删除操作, 不需要做真正的系统空间分配, 速度更快.

但明显, 例子中给出的操作只能针对Airplane类进行操作, 试想, 能不能扩展到对任何类进行这样的Memory Pool操作呢?

第一想法就是用template.
花了一中午的时间写了一个CMemoryPool模板类. 如下所示

#ifndef __MEMORYPOOL_H__
#define __MEMORYPOOL_H__

#include <list>

template <class _T>
class CMemoryPool 
{

public:
CMemoryPool(size_t init_size = 100) : INIT_SIZE(init_size), m_memoryHeadList(5)
{
headOfFreeList = NULL;
}

~CMemoryPool()
{
std::list<Envelope *>::iterator iter;
for(iter = m_memoryHeadList.begin(); iter != m_memoryHeadList.end(); ++iter)
{
Envelope *block = *iter;
::operator delete(block);
}
}

_T *alloc()
{
Envelope *p = headOfFreeList;
if(p)
headOfFreeList = p->next;
else
{
const size_t ObjectSize = (sizeof(_T) > sizeof(Envelope)) ? sizeof(_T) : sizeof(Envelope);
// 申请空间
Envelope *newBlock = static_cast<Envelope *>(::operator new(INIT_SIZE * ObjectSize));
// 初始化空闲空间列表
for(int i=1; i<INIT_SIZE; ++i)
((Envelope *)((char *)newBlock + i * ObjectSize))->next = (Envelope *)((char *)newBlock + (i+1) * ObjectSize);

((Envelope*)((char *)newBlock + (INIT_SIZE - 1) * ObjectSize))->next = NULL;
// 保存首址
m_memoryHeadList.push_back(newBlock);

p = newBlock;
headOfFreeList = (Envelope*)((char *)newBlock + ObjectSize);
}
return (_T *)p;
}

void free(_T *p)
{
if(p==NULL)
return;

Envelope *freeBlock = (Envelope *)p;
freeBlock->next = headOfFreeList;
headOfFreeList = freeBlock;
}

private:
union Envelope
{
_T *object;
union Envelope *next;
};
const size_t INIT_SIZE;// 默认每次分配多少初始大小的空间
Envelope *headOfFreeList; // 当前自由空间首址
std::list<Envelope *> m_memoryHeadList; // 申请空间首址向量
};

#endif // !defined __MEMORYPOOL_H__

此模板类对于内建类型, 可做如下使用.
#include <iostream>
#include "MemoryPool.h"

int main()
{
CMemoryPool<double> d_Pool;
double *p[10];
int i;
for(i=0; i<10; ++i)
{
p[i] = d_Pool.alloc();
std::cout<<p[i]<<std::endl;
}
for(i=0; i<5; ++i)
d_Pool.free(p[i]);

for(i=0; i<5; ++i)
p[i] = d_Pool.alloc();

for(i=0; i<10; ++i)
std::cout<<p[i]<<std::endl;

return 0;
}

如果仅仅这样使用, 还没有达到所要求的目的.
关键是对于自己的自定义类, 通过重载operator new与operator delete来使用Memory Pool
如下:
#include <iostream>
#include "MemoryPool.h"

class MyClass
{
public:
MyClass();
~MyClass();
static void * operator new(size_t t);
static void operator delete(void *p);

private:
static CMemoryPool<MyClass> st_objectPool;
};

CMemoryPool<MyClass> MyClass::st_objectPool(10);

MyClass::MyClass()
{
std::cout<<"A() called"<<std::endl;
}

MyClass::~MyClass()
{
std::cout<<"~A() called"<<std::endl;
}

void * MyClass::operator new(size_t t)
{
if(t != sizeof(MyClass))
return ::operator new(t);
return MyClass::st_objectPool.alloc();
}

void MyClass::operator delete(void *p)
{
MyClass::st_objectPool.free((MyClass *)p);
}

int main()
{
MyClass *ptr;
ptr = new MyClass;
delete ptr;
return 0;
}

至此, 已经完成了对于任何类的一个CMemoryPool类的写法, 自然会想到, 怎么样去写一个基类CMemoryPoolBaseClass, 让所有继承该类的子类都具有这种内存池的动态内存分配方式呢?