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
本文参与腾讯云自媒体分享计划,欢迎正在阅读的你也加入,一起分享。
我来说两句