STL空间配置器解析和实现

　　STL空间配置器的强大和借鉴作用不言而喻，查阅资料，发现了Dawn_sf已经对其有了极其深入和详细的描述，所以决定偷下懒借用其内容，只提供自己实现STL空间配置器的源码，具体解析内容参考：
（一）STL — 浅析一级空间配置器
（二）STL — 浅析二级空间配置器

1. 一级空间配置器实现

1.1 接口

// 完全解析STL空间配置器
/***** 一级配置区 ****************************/ 
// 1. 采用mallo/relloc/free申请和释放内存
// 2. 处理内存申请失败的处理
//      (1)设置set_new_handle，若为NULL抛出__THROW_BAD_ALLOC;
//      (2)尝试重复申请
/**********************************************/
#pragma once

class CFirstLevelAlloc;
class CSecondLevelAlloc;

#ifdef _CHUNK_ALLOC
typedef CFirstLevelAlloc SelfAlloc;
#else
typedef CSecondLevelAlloc SelfAlloc;
#endif

typedef void(*CallBackFunc)();
class CFirstLevelAlloc
{
public:
    CFirstLevelAlloc();
    
    static CallBackFunc m_CallBackFunc;
    static void* Allocate(size_t nSize);
    static void* Allocate(void *p, size_t nSize);
    static void Deallocate(void *p, size_t nSize = 0);
    static void SetCallBackHandle(CallBackFunc cb);

private:
    static void* ReAllocate(size_t nSize);
    static void* ReAllocate(void *p, size_t nSize);
};

enum {ALIGN = 8};  // 小型区块的上调边界
enum {MAX_BYTES = 128}; // 小型区块的上限
enum {FREELISTNUM = MAX_BYTES/ALIGN}; // free-lists个数

// 空闲内存链表结构
union FreeList
{
    union FreeList *pFreeList;
    char client_data[1];
};

1.2 实现

#include "stdio.h"
#include "alloc_define.h"
#include <stdlib.h>
#include <iostream>
using namespace std;

CallBackFunc CFirstLevelAlloc::m_CallBackFunc = NULL;
CFirstLevelAlloc::CFirstLevelAlloc()
{
    
}

void* CFirstLevelAlloc::Allocate(size_t nSize)
{
    void *result = malloc(nSize);
    if (NULL == result)
    {
        result = ReAllocate(nSize);
    }
    return result;
}

void* CFirstLevelAlloc::Allocate(void *p, size_t nSize)
{
    void *result = realloc(p, nSize);
    if (NULL == result)
    {
        result = ReAllocate(p, nSize);
    }
    return result;
}


void* CFirstLevelAlloc::ReAllocate(size_t nSize)
{
    while (1)
    {
        if (NULL == m_CallBackFunc)
        {
            cout << "bad alloc!" << endl;
            return NULL;
        }
        else
        {
            m_CallBackFunc();
            void *result = Allocate(nSize);
            if (result)
            {
                return result;
            }
        }
    }
}

void* CFirstLevelAlloc::ReAllocate(void *p, size_t nSize)
{
    while (1)
    {
        if (NULL == m_CallBackFunc)
        {
            cout << "bad alloc!" << endl;
            return NULL;
        }
        else
        {
            m_CallBackFunc();
            void *result = Allocate(p, nSize);
            if (result)
            {
                return result;
            }
        }
    }
}

void CFirstLevelAlloc::Deallocate(void *p, size_t nSize)
{
    free(p);
}
void CFirstLevelAlloc::SetCallBackHandle(CallBackFunc cb)
{
    m_CallBackFunc = cb;
}

2. 二级空间配置器实现

2.1 接口

class CSecondLevelAlloc
{
public:
    CSecondLevelAlloc();
    static void* Allocate(size_t nSize);
    static void Deallocate(void *p, size_t nSize);
    static void SetCallBackHandle(CallBackFunc cb);

private:
    static size_t FreeListIndex(int nBytes); // 根据区块大小得到freelist索引
    static size_t Round_Up(int nBytes);  // 将bytes按内存对齐上调至ALIGN的倍数
    static char *ChunkAlloc(size_t nSize, int& nObjs);
    static void* Refill(size_t nSize);
    
private:
    static FreeList *m_pFreeList[FREELISTNUM];
    static char *m_pStartFree;
    static char *m_pEndFree;
    static size_t m_nHeapSize;
};

2.2 实现

FreeList* CSecondLevelAlloc::m_pFreeList[FREELISTNUM] = { 0 };
char* CSecondLevelAlloc::m_pStartFree = NULL;
char* CSecondLevelAlloc::m_pEndFree = NULL;
size_t CSecondLevelAlloc::m_nHeapSize = 0;
CSecondLevelAlloc::CSecondLevelAlloc()
{
}


void* CSecondLevelAlloc::Allocate(size_t nSize)
{
    // 首先判断nSize，若大于128则调用一级配置器，否则调用二级配置器
    if (nSize > (size_t)MAX_BYTES)
    {
        cout << "调用1级配置器配置内存空间,空间大小：" << nSize << endl;
        return (CFirstLevelAlloc::Allocate(nSize));
    }

    cout << "调用2级配置器配置内存空间,空间大小：" << nSize << endl;
    FreeList **pFreeList = m_pFreeList + FreeListIndex(nSize);
    if (*pFreeList == NULL)
    {
        return Refill(Round_Up(nSize));
    }

    FreeList *p = *pFreeList;
    *pFreeList = p->pFreeList;
    return p;
}

void CSecondLevelAlloc::Deallocate(void *p, size_t nSize)
{
    // 首先判断nSize，若大于128则调用一级配置器，否则调用二级配置器
    if (nSize > MAX_BYTES)
    {
        cout << "调用1级配置器释放内存空间,空间大小：" << nSize << endl;
        return CFirstLevelAlloc::Deallocate(p);
         
    }

    cout << "调用2级配置器释放内存空间,空间大小：" << nSize << endl;
    FreeList **pFreeList = m_pFreeList + FreeListIndex(Round_Up(nSize));
    ((FreeList*)p)->pFreeList = *pFreeList;
    *pFreeList = (FreeList*)p;
}
size_t CSecondLevelAlloc::FreeListIndex(int nBytes)
{
    return ((nBytes + ALIGN) / ALIGN - 1);
}
size_t CSecondLevelAlloc::Round_Up(int nBytes)
{
    return ((nBytes + ALIGN - 1) & (~(ALIGN - 1)));
}
char* CSecondLevelAlloc::ChunkAlloc(size_t nSize, int& nObjs)
{
    char *pResult = NULL;
    size_t nTotalBytes = nSize * nObjs;
    size_t nBytesLeft = m_pEndFree - m_pStartFree;
    if (nBytesLeft >= nTotalBytes)
    {
        // 内存池剩余空间完全满足需求量
        pResult = m_pStartFree;
        m_pStartFree += nTotalBytes;
        return pResult;
    }
    else if (nBytesLeft >= nSize)
    {
        // 内存池剩余空间不能完全满足需求量，但足够供应一个或一个以上的区块
        nObjs = nBytesLeft / nSize;
        pResult = m_pStartFree;
        m_pStartFree += (nObjs * nSize);
        return pResult;
    }
    else
    {
        // 内存池剩余空间连一个区块的大小都无法提供,就调用malloc申请内存，新申请的空间是需求量的两倍
        // 与随着配置次数增加的附加量，在申请之前，将内存池的残余内存回收
        size_t nBytesToGet = 2 * nTotalBytes + Round_Up(m_nHeapSize >> 4);
        // 以下试着让内存池中的残余零头还有价值
        if (nBytesLeft > 0)
        {
            // 内存池内还有一些零头，先配给适当的freelist
            // 首先寻找适当的freelist
            FreeList *pFreeList = m_pFreeList[FreeListIndex(nBytesLeft)];
            // 调整freelist,将内存池中的残余空间编入
            ((FreeList*)m_pStartFree)->pFreeList = pFreeList;
            pFreeList = (FreeList*)m_pStartFree;

        }
        // 配置heap空间
        m_pStartFree = (char *)malloc(nBytesToGet);
        if (NULL == m_pStartFree)
        {
            //如果没有申请成功，如果free_list当中有比n大的内存块，这个时候将free_list中的内存块释放出来.  
            //然后将这些内存编入自己的free_list的下标当中.调整nobjs.
            int i;
            FreeList **pFreeList, *p;
            for (i = nSize; i < MAX_BYTES; i += ALIGN)
            {
                pFreeList = m_pFreeList+FreeListIndex(i);
                p = *pFreeList;
                if (NULL != p)
                {
                    // freelist内尚有未用区块
                    // 调整freelist以释放未用区块
                    *pFreeList = p->pFreeList;
                    m_pStartFree = (char *)p;
                    m_pEndFree = m_pStartFree + i;
                    // 调整自己，为了修正nobjs
                    return (ChunkAlloc(nSize, nObjs));
                }
            }

            m_pEndFree = NULL; // 如果出现意外（山穷水尽，到处都没有内存可用）
            // 调用1级配置器，看out-of-range机制能不能出点力
            m_pStartFree = (char*)CFirstLevelAlloc::Allocate(nBytesToGet);
        }

        m_nHeapSize += nBytesToGet;
        m_pEndFree = m_pStartFree + nBytesToGet;
        return (ChunkAlloc(nSize, nObjs));
    }
}

// 当freelist中没有可用的区块了时，就调用ReFill重新填充空间
// 新的空间将取自内存池，缺省为20个新节点
// 但万一内存池空间不足，获得的节点数可能小于20
void* CSecondLevelAlloc::Refill(size_t nSize)
{
    int nObjs = 20; // 默认每个链表组右20个区块
    char *pChunk = ChunkAlloc(nSize, nObjs);
    if (1 == nObjs)
    {
        // 如果获得一个区块，这个区块就分配给调用者，freelist无新节点
        return pChunk;
    }

    // 若有多余的区块，则将其添加到对应索引的freelist中
    FreeList **pFreeList = m_pFreeList + FreeListIndex(nSize);
    FreeList *pResult = (FreeList *)pChunk;  // 这一块准备返回给客户端

    FreeList *pCurrent = NULL; 
    FreeList *pNext = NULL;
    *pFreeList = pNext = (FreeList*)(pChunk + nSize);
    for (int i = 1; i < nObjs; i++)
    {
        pCurrent = pNext;
        pNext = (FreeList*)((int)pNext + nSize);
        pCurrent->pFreeList = pNext;
    }
    pCurrent->pFreeList = NULL;

    return pResult;
}

void CSecondLevelAlloc::SetCallBackHandle(CallBackFunc cb)
{
    CFirstLevelAlloc::m_CallBackFunc = cb;
}

3. 配置器标准接口

#pragma once

#include "alloc_define.h"
template<typename T, typename Alloc = SelfAlloc>
class CSimpleAlloc
{
public:
    static T* Allocate(size_t n)
    {
        if (n == 0)
        {
            return NULL;
        }
        return (T *)Alloc::Allocate(n * sizeof(T));
    }

    static T* Allocate(void)
    {
        return (T *)Alloc::Allocate(sizeof(T));
    }

    static void Deallocate(T *p, size_t n)
    {
        if (n != 0)
        {
            Alloc::Deallocate(p, n * sizeof(T));
        }
    }

    static void Deallocate(T *p)
    {
        Alloc::Deallocate(p, sizeof(T));
    }

    static void SetCallBackHandle(CallBackFunc cb)
    {
        Alloc::SetCallBackHandle(cb);
    }
};

4. 测试

#include "stdio.h"

#include<iostream>
using namespace std;

#include"stl_test.h"
#include "simple_alloc.h"
#include<vector>

void Func()
{
    cout << "调用回调函数处理内存不足的情况" << endl;
    // 为了防止死循环，该函数应该加上一个判断条件如果它本次没有清理出空间，那么就抛出异常
}

template <class T, class Alloc = SelfAlloc>
class A
{
public:
    A() :m_ptr(NULL), m_nSize(0){}
    A(size_t nSize)
    {
        DataAllocator::SetCallBackHandle(Func);
        m_nSize = nSize;
        m_ptr = DataAllocator::Allocate(nSize);
        for (int i = 0; i < (int)nSize; i++)
        {
            m_ptr[i] = i;
            cout << m_ptr[i] << " ";
        }
        cout << endl;
    }
    ~A()
    {
        DataAllocator::Deallocate(m_ptr, m_nSize);
    }

private:
    T *m_ptr;
    size_t m_nSize;
    typedef CSimpleAlloc<T, Alloc> DataAllocator;
};
void main()
{
    A<int> a(11);
    A<int> b(50);
    a.~A();
    b.~A();
}