C++中的new和delete操作符重载

1 new和delete操作符重载

1.1 new和delete操作符重载简介

new/delete的本质是C++预定义的操作符，C++对这两个操作符做了严格的行为定义：

• new：
  1. 获取足够大的内存空间（默认为堆空间）。
  2. 在获取的空间中调用构造函数创建对象。
• delete：
  1. 调用析构函数销毁对象。
  2. 归还对象所占用的空间（默认为堆空间）。

在C++中能够重载new/delete操作符：

• 全局重载（不推荐）
• 局部重载（针对具体类进行重载）

重载new/delete的意义在于改变动态对象创建时的内存分配方式。

new和delete的重载方式：

需要注意的点：

• new操作符重载函数返回后编译器会自动调用构造函数。
• delete操作符重载函数调用前编译器会自动调用析构函数。

1.2 静态存储区中创建动态对象

#include <iostream>
#include <string>

using namespace std;

class Test
{
    static const unsigned int COUNT = 4;
    static char c_buffer[];
    static char c_map[];
    
    int m_value;
public:
    void* operator new (unsigned int size)
    {
        void* ret = NULL;
        
        for(int i=0; i<COUNT; i++)
        {
            if( !c_map[i] )
            {
                c_map[i] = 1;
                
                ret = c_buffer + i * sizeof(Test);
                
                cout << "succeed to allocate memory: " << ret << endl;
                
                break;
            }
        }
        
        return ret;
    }
    
    void operator delete (void* p)
    {
        if( p != NULL )
        {
            char* mem = reinterpret_cast<char*>(p);
            int index = (mem - c_buffer) / sizeof(Test);
            int flag = (mem - c_buffer) % sizeof(Test);
            
            if( (flag == 0) && (0 <= index) && (index < COUNT) )
            {
                c_map[index] = 0;
                
                cout << "succeed to free memory: " << p << endl;
            }
        }
    }
};

char Test::c_buffer[sizeof(Test) * Test::COUNT] = {0};
char Test::c_map[Test::COUNT] = {0};

int main(int argc, char *argv[])
{
    cout << "===== Test Single Object =====" << endl;
     
    Test* pt = new Test;
    
    delete pt;
    
    cout << "===== Test Object Array =====" << endl;
    
    Test* pa[5] = {0};
    
    for(int i=0; i<5; i++)
    {
        pa[i] = new Test;
        
        cout << "pa[" << i << "] = " << pa[i] << endl;
    }
    
    for(int i=0; i<5; i++)
    {
        cout << "delete " << pa[i] << endl;
        
        delete pa[i];
    }
    
    return 0;
}

1.3 在指定的地址上创建C++对象

解决方案：

• 在类中重载new/delete操作符。
• 在new的操作符重载函数中返回指定的地址。
• 在delete操作符重载中标记对应的地址可用。

编程实验：自定义动态对象的存储空间

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

class Test
{
    static unsigned int c_count;
    static char* c_buffer;
    static char* c_map;
    
    int m_value;
public:
    static bool SetMemorySource(char* memory, unsigned int size)
    {
        bool ret = false;
        
        c_count = size / sizeof(Test);
        
        ret = (c_count && (c_map = reinterpret_cast<char*>(calloc(c_count, sizeof(char)))));
        
        if( ret )
        {
            c_buffer = memory;
        }
        else
        {
            free(c_map);
            
            c_map = NULL;
            c_buffer = NULL;
            c_count = 0;
        }
        
        return ret;
    }
    
    void* operator new (unsigned int size)
    {
        void* ret = NULL;
        
        if( c_count > 0 )
        {
            for(int i=0; i<c_count; i++)
            {
                if( !c_map[i] )
                {
                    c_map[i] = 1;
                    
                    ret = c_buffer + i * sizeof(Test);
                    
                    cout << "succeed to allocate memory: " << ret << endl;
                    
                    break;
                }
            }
        }
        else
        {
            ret = malloc(size);
        }
        
        return ret;
    }
    
    void operator delete (void* p)
    {
        if( p != NULL )
        {
            if( c_count > 0 )
            {
                char* mem = reinterpret_cast<char*>(p);
                int index = (mem - c_buffer) / sizeof(Test);
                int flag = (mem - c_buffer) % sizeof(Test);
                
                if( (flag == 0) && (0 <= index) && (index < c_count) )
                {
                    c_map[index] = 0;
                    
                    cout << "succeed to free memory: " << p << endl;
                }
            }
            else
            {
                free(p);
            }
        }
    }
};

unsigned int Test::c_count = 0;
char* Test::c_buffer = NULL;
char* Test::c_map = NULL;

int main(int argc, char *argv[])
{
    char buffer[12] = {0};
    
    Test::SetMemorySource(buffer, sizeof(buffer));
    
    cout << "===== Test Single Object =====" << endl;
     
    Test* pt = new Test;
    
    delete pt;
    
    cout << "===== Test Object Array =====" << endl;
    
    Test* pa[5] = {0};
    
    for(int i=0; i<5; i++)
    {
        pa[i] = new Test;
        
        cout << "pa[" << i << "] = " << pa[i] << endl;
    }
    
    for(int i=0; i<5; i++)
    {
        cout << "delete " << pa[i] << endl;
        
        delete pa[i];
    }
    
    return 0;
}

---

2 new[]和delete[]操作符重载

new[]/delete[]和new/delete完全不同：

• 动态对象数组创建通过new[]完成。
• 动态对象数组的销毁通过delete[]完成。
• new[]/delete[]能够被重载，进而改变内存管理方式。

new[]/delete[]的重载方式：

注意事项：

• new[]实际需要返回的内存空间可能比期望的要多。
• 对象数组占用的内存中需要保存数组信息。
• 数组信息用于确定构造函数和析构函数的调用次数。

编程实验：动态数组的内存管理

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

class Test
{
    int m_value;
public:
    Test()
    {
        m_value = 0;
    }
    
    ~Test()
    {
    }
    
    void* operator new (unsigned int size)
    {
        cout << "operator new: " << size << endl;
        
        return malloc(size);
    }
    
    void operator delete (void* p)
    {
        cout << "operator delete: " << p << endl;
        
        free(p);
    }
    
    void* operator new[] (unsigned int size)
    {
        cout << "operator new[]: " << size << endl;
        
        return malloc(size);
    }
    
    void operator delete[] (void* p)
    {
        cout << "operator delete[]: " << p << endl;
        
        free(p);
    }
};

int main(int argc, char *argv[])
{
    Test* pt = NULL;
    
    pt = new Test;
    
    delete pt;
    
    pt = new Test[5];
    
    delete[] pt;
    
    return 0;
}

---

参考资料：

1. C++深度解析教程