Question

我在互联网上看过这样的课程，头文件

#ifndef _COMMON_ARRAY_OBJECT_POOL_H_
#define _COMMON_ARRAY_OBJECT_POOL_H_

#include <stdint.h>

namespace easynet
{

class ArrayObjectPool
{
public:
    /** construct
     * @param elem_size : element size;
     * @param elem_num  : element number
     */
    ArrayObjectPool(uint32_t elem_size, uint32_t elem_num);
    ~ArrayObjectPool();

    uint32_t ElemSize(){return m_ElemSize;}
    uint32_t Capacity(){return m_ElemNum;}
    bool IsEmpty(){return m_FreeHead==NULL;}


    void* Get();


    bool Recycle(void *elem);
private:
    void *m_Elements;
    void *m_End;
    void *m_FreeHead;
    uint32_t m_ElemSize;
    uint32_t m_ElemNum;
};

}
#endif //_COMMON_ARRAY_OBJECT_POOL_H_

cpp文件

#include <assert.h>
#include <stddef.h>
#include <stdlib.h>

#include "ArrayObjectPool.h"

namespace easynet
{

ArrayObjectPool::ArrayObjectPool(uint32_t elem_size, uint32_t elem_num)
{
    m_ElemNum = elem_num;
    if(elem_size < sizeof(void*))
        m_ElemSize = sizeof(void*);
    else
        m_ElemSize = elem_size;

    m_Elements = malloc(m_ElemSize*m_ElemNum);
    m_End = (void*)((char*)m_Elements+m_ElemSize*m_ElemNum);
    assert(m_Elements != NULL);

    //construct list
    int i;
    void *node = m_Elements;
    for(i=0; i<m_ElemNum-1; ++i)
    {
        *(void**)node = (void*)((char*)node+m_ElemSize);
        node = *(void**)node;
    }
    *(void**)node = NULL;
    m_FreeHead = m_Elements;    //list head
}

ArrayObjectPool::~ArrayObjectPool()
{
    free(m_Elements);
}

void* ArrayObjectPool::Get()
{
    if(m_FreeHead == NULL)
        return NULL;
    void *temp = m_FreeHead;
    m_FreeHead = *(void**)m_FreeHead;
    return temp;
}

bool ArrayObjectPool::Recycle(void *elem)
{
    if(elem<m_Elements || elem>=m_End)
        return false;
    *(void**)elem = m_FreeHead;
    m_FreeHead = elem;
    return true;
}

}

问题是我无法理解这意味着什么：

int i;
void *node = m_Elements;
for(i=0; i<m_ElemNum-1; ++i)
{
*(void**)node = (void*)((char*)node+m_ElemSize);
node = *(void**)node;
}

以及*(void**)的含义是什么？谢谢！

Answer 1

它将内存视为用户数据类型与union之间的void*。当块在空闲块列表中时，使用void*。

您可以将其视为：

union ObjectInObjectPool
{
    void* ptr_next_free_block;
    UserType content;
};

然后该循环基本上正在做：

ObjectInObjectPool* node = m_Elements;
for(i=0; i<m_ElemNum-1; ++i) {
    node->ptr_next_free_block = node + 1;
    node = node->ptr_next_free_block;
}

除了程序员手工完成编译器的类型检查器通常所做的所有指针算法。

Answer 2

void*是指向无类型内存的指针值。当你*(void**)node = ...时，它真正做的是*node = ...。但是，对于后者，你试图给void赋予一些东西，这对C ++的类型系统没有意义;你必须像前者一样把它投射到void**，这样*node将是void*，而不是void，你可以分配给它。 / p>

node = *(void**)node只是node = *node，但强制类型系统正常工作。它只是“node将*node内存的值分配为void*”。

*（void **）的含义是什么

2 个答案: