从零开始学C++之运算符重载（四）：类型转换运算符、*运算符重载、->运算符重载、operator new 和 operator delete

一、类型转换运算符
必须是成员函数，不能是友元函数
没有参数
不能指定返回类型
函数原型：operator 类型名();

#ifndef _INTEGER_H_#define _INTEGER_H_class Integer{public:    Integer(int n);    ~Integer();    Integer &operator++();    //friend Integer& operator++(Integer& i);    Integer operator++(int n);    //friend Integer operator++(Integer& i, int n);    operator int();    void Display() const;private:    int n_;};#endif // _INTEGER_H_

#include "Integer.h"#include <iostream>using namespace std;Integer::Integer(int n) : n_(n){}Integer::~Integer(){}Integer &Integer::operator ++(){    //cout<<"Integer& Integer::operator ++()"<<endl;    ++n_;    return *this;}//Integer& operator++(Integer& i)//{//  //cout<<"Integer& operator++(Integer& i)"<<endl;//  ++i.n_;//  return i;//}Integer Integer::operator++(int n){    //cout<<"Integer& Integer::operator ++()"<<endl;    //n_++;    Integer tmp(n_);    n_++;    return tmp;}//Integer operator++(Integer& i, int n)//{//  Integer tmp(i.n_);//  i.n_++;//  return tmp;//}Integer::operator int(){    return n_;}void Integer::Display() const{    cout << n_ << endl;}

#include "Integer.h"#include <iostream>using namespace std;int add(int a, int b){    return a + b;}int main(void){    Integer n(100);    n = 200;    n.Display();    int sum = add(n, 100);    cout << sum << endl;    int x = n;    int y = static_cast<int>(n);    return 0;}

其中n = 200; 是隐式将int 转换成Interger类；int x = n; 是调用operator int 将Interger 类转换成int，也可以使用static_cast 办到；此外add 函数传参时也会调用operator int 进行转换。

二、->运算符重载
类* operator->();
类& operator*();

#include <iostream>using namespace std;class DBHelper{public:    DBHelper()    {        cout << "DB ..." << endl;    }    ~DBHelper()    {        cout << "~DB ..." << endl;    }    void Open()    {        cout << "Open ..." << endl;    }    void Close()    {        cout << "Close ..." << endl;    }    void Query()    {        cout << "Query ..." << endl;    }};class DB{public:    DB()    {        db_ = new DBHelper;    }    ~DB()    {        delete db_;    }    DBHelper *operator->()    {        return db_;    }    DBHelper &operator*()    {        return *db_;    }private:    DBHelper *db_;};int main(void){    DB db;    db->Open();    db->Query();    db->Close();    (*db).Open();    (*db).Query();    (*db).Close();    return 0;}

db->Open(); 等价于 (db.operator->())->Open(); 会调用operator-> 返回DBHelper类的指针，调用DBHelper的成员函数Open()。这样使用的好处是不需要知道db 对象什么时候需要释放，当生存期结束时，会调用DB类的析构函数，里面delete db_; 故也会调用DBHelper类的析构函数。
(db).Open(); 等价于(db.operator()).Open();

三、operator new 和 operator delete
在前面曾经提过：实际上new 有三种用法，包括operator new、new operator、placement new，new operator 包含operator new，而placement new 则没有内存分配而是直接调用构造函数。下面看例子：

#include <iostream>using namespace std;class Test{public:    Test(int n) : n_(n)    {        cout << "Test(int n) : n_(n)" << endl;    }    Test(const Test &other)    {        cout << "Test(const Test& other)" << endl;    }    ~Test()    {        cout << "~Test()" << endl;    }    /****************************************************************/    void *operator new(size_t size)    {        cout << "void* operator new(size_t size)" << endl;        void *p = malloc(size);        return p;    }    void operator delete(void *p) //与下面的operator delete函数类似，共存的话优先;    {        //匹配上面的operator new 函数        cout << "void operator delete(void* p)" << endl;        free(p);    }    void operator delete(void *p, size_t size)    {        cout << "void operator delete(void* p, size_t size)" << endl;        free(p);    }    /**********************************************************************/    void *operator new(size_t size, const char *file, long line)    {        cout << "   void* operator new(size_t size, const char* file, long line);" << endl;        cout << file << ":" << line << endl;        void *p = malloc(size);        return p;    }    void operator delete(void *p, const char *file, long line)    {        cout << "   void operator delete(void* p, const char* file, long line);" << endl;        cout << file << ":" << line << endl;        free(p);    }    void operator delete(void *p, size_t size, const char *file, long line)    {        cout << "void operator delete(void* p, size_t size, const char* file, long line);" << endl;        cout << file << ":" << line << endl;        free(p);    }    /**************************************************************************/    void *operator new(size_t size, void *p)    {        cout << "void* operator new(size_t size, void* p);" << endl;        return p;    }    void operator delete(void *, void *)    {        cout << "void operator delete(void *, void *);" << endl;    }    /**************************************************************************/    int n_;};/*************** global **********************************************/void *operator new(size_t size){    cout << "global void* operator new(size_t size)" << endl;    void *p = malloc(size);    return p;}void operator delete(void *p){    cout << "global void operator delete(void* p)" << endl;    free(p);}/**********************************************************************/void *operator new[](size_t size){    cout << "global void* operator new[](size_t size)" << endl;    void *p = malloc(size);    return p;}void operator delete[](void *p){    cout << "global void operator delete[](void* p)" << endl;    free(p);}/***********************************************************************/int main(void){    Test *p1 = new Test(100);   // new operator = operator new + 构造函数的调用    delete p1;    char *str1 = new char;    delete str1;    char *str2 = new char[100];    delete[] str2;    char chunk[10];    Test *p2 = new (chunk) Test(200);   //operator new(size_t, void *_Where)    // placement new，不分配内存 + 构造函数的调用    cout << p2->n_ << endl;    p2->~Test();                        // 显式调用析构函数    //Test* p3 = (Test*)chunk;    Test *p3 = reinterpret_cast<Test *>(chunk);    cout << p3->n_ << endl;#define new new(__FILE__, __LINE__)    //Test* p4 = new(__FILE__, __LINE__) Test(300);    Test *p4 = new Test(300);    delete p4;    return 0;}

从输出可以看出几点：
1、new operator 是分配内存（调用operator new） + 调用构造函数
2、operator new 是只分配内存，不调用构造函数
3、placement new 是不分配内存（调用operator new（与2是不同的函数） 返回已分配的内存地址），调用构造函数
4、delete 是先调用析构函数，再调用operator delete.
5、如果new 的是数组，对应地也需要delete [] 释放

注意：
1、如果存在继承或者对象成员，那么调用构造函数或者析构函数时将有多个，按一定顺序调用，参见这里。
2、假设存在继承，delete 基类指针；涉及到虚析构函数的问题，参见这里。

最后还存在一点疑问的是 delete p4 为什么调用的不是 void operator delete(void* p, const char* file, long line); 而是
void operator delete(void* p) ; 希望有明白的朋友告诉我一声。

参考：
C++ primer 第四版
Effective C++ 3rd
C++编程规范

转载自http://blog.csdn.net/jnu_simba/article/details/9295729