【一天一篇CPP】重载运算符的实例

1.重载双目运算符

未重载版本

#include <iostream>using namespace std;class String{public: String() { p = NULL; }String(char *str) { p = str; }void display() { cout << p<< endl;}private:char *p;};int main(){String str1("Hello"),str2("Book");str1.display();str2.display();return 0;}

增加友元函数: ' > ' '==' '<' 的尝试

#include <iostream>#include <cstring>using namespace std;class String{public: String() { p = NULL; }String(char *str) { p = str; }void display() { cout << p;}friend bool operator >(String &s1,String &s2);friend bool operator ==(String &s1,String &s2);friend bool operator <(String &s1,String &s2);private:char *p;};bool operator >(String &s1, String &s2){if( strcmp( s1.p, s2.p) > 0 )return true;return false;}bool operator ==(String &s1, String &s2){if( strcmp( s1.p, s2.p) == 0 )return true;return false;}bool operator <(String &s1, String &s2){if( strcmp( s1.p, s2.p) < 0 )return true;return false;}void compare(String &s1, String &s2){if(s1 > s2){s1.display() ;cout << " > " ;s2.display();cout << endl;}else if(s1 < s2){s1.display() ;cout << " < " ;s2.display();cout << endl;}else if(s1 == s2){s1.display() ;cout << " == " ;s2.display();cout << endl;}}int main(){String str1("Hello"),str2("book");str1.display();str2.display();compare(str1,str2);return 0;}

双目运算符要注意的地方：若作为成员函数，则" 2.5 + c1 "一定是错误的【因为只可能是 " c1 + 2.5 "】。作为友元函数，则要声明和定义两种情况的才能通用。

2.重载单目运算符
常见的单目运算符： !a , -b ,&c , *p 还有 ++，等。单目运算符只有一个操作数，因此运算符只有一个参数，如果运算符重载函数作为成员函数，还可以省略这个参数！

一个例子：【前置++运算符的实现】

#include <iostream>#include <cstring>using namespace std;class Time{public:Time(int m = 0, int s = 0) {minute = m; sec = s;} Time operator++();//前置"++"，由于前置++和后置++的返回值不同，所以需要区分void display() {cout << minute << " : " << sec  << endl;}private:int minute,sec;};Time Time::operator++(){if( ++sec >= 60){sec -= 60;++ minute;}return *this;}int main(){Time t1(34);for( int i = 0; i < 61 ; i++){++t1;//t1++; wrong!!!!t1.display();}return 0;}

单目运算符常作为成员函数，双目运算符常为友元函数。
如何重载后置++运算符呢？Time operator++(int) ，加上int修饰，虽然int并没有实际作用。

 

3.重载 " << "  和 " >> " 【流插入、流提取运算符】

返回值是流操作符对象本身。

#include <iostream>#include <cstring>using namespace std;class Complex{public:Complex(double r = 0,double i = 0) { real = r; imag = i;}friend ostream& operator <<( ostream &, Complex & c);friend istream& operator >>( istream &, Complex & c);Complex operator +(Complex &c){return Complex(real + c.real, imag + c.imag);}private:double real,imag;};/*ostream & operator  << (ostream &output, Complex &c){output << "(" << c.real << "+" << c.imag << "i)";}*/ostream & operator  << (ostream &output, Complex &c){output << "(" <<c.real;if(c.imag >= 0) output << "+";output << c.imag << "i)";return output;}//修正了当imag为负数时，显示为(4+-10i)的错误istream & operator  >> (istream &input, Complex &c){cout << "please input a compex ...\n";input >> c.real >> c.imag;return input;}int main(){Complex c3;cin >> c3;cout << c3  <<endl;}