//重载双目运算符，双目运算符有两个操作数，通常在运算符的左右两侧，在重载双目运算符时，在友元函数中应该有两个参数//思想：要想将运算符用于用户自定义类上，用户必须自己对运算符进行重载。要对“==”,“<”和“>”三个运算符进行重载，需要重新定义3个运算符重载函数。可以将重载函数定义成类成员函数（ 1个参数），也可以定义成类的友元函数（两个参数）。//先建立一个string类#include <iostream>using namespace std;class String{public://友元类#include <iostream>#include <cstring>using namespace std;class String{public: String(){p=NULL;} String(char *str); void display(); friend bool operator>(String &string1,String &string2);private: char *p;};String::String(char *str){ p=str;}void String::display(){ cout<<p;}bool operator>(String &string1,String &string2){ if(strcmp(string1.p,string2.p)>0) return true; else return false;}int main(){ String string1(“Hello”),string2(“Book”); cout<<(string1>string2)<<endl;}String(){p=NULL;}String(char *str);void display();private:char *p;};String::String(char *str){p=str;}void String::display(){cout<<p;//此时输出的是字符串，如果cout<<*p;则输出的是字符踹的首字母}int main(){String string1(“Hello”),string2(“Book”);string1.display();cout<<endl;string2.display();cout<<endl;return 0;} //再扩展到三个#include <iostream>#include <cstring>using namespace std;class String{public: String(){p=NULL;} String(char *str); void display(); friend bool operator>(String &string1,String &string2); friend bool operator<(String &string1,String &string2); friend bool operator==(String &string1,String &string2);private: char *p;};String::String(char *str){ p=str;}void String::display(){ cout<<p;}bool operator>(String &string1,String &string2){ if(strcmp(string1.p,string2.p)>0) return true; else return false;}bool operator<(String &string1,String &string2){ if(strcmp(string1.p,string2.p)<0) return true; else return false;}bool operator==(String &string1,String &string2){ if(strcmp(string1.p,string2.p)==0) return true; else return false;}void compare(String &string1,String &string2){ if(operator>(string1,string2)==1) string1.display();cout<<‘>’;string2.display(); if(operator<(string1,string2)==1) string1.display();cout<<‘<‘;string2.display(); if(operator==(string1,string2)==1) string1.display();cout<<‘=’;string2.display(); cout<<endl;}int main(){ String string1(“Hello”),string2(“Book”),string3(“Computer”),string4(“Hello”); compare(string1,string2); compare(string2,string3); compare(string3,string4); return 0;}