为了与IO标准库一致，重载输入输出运算符函数的第一个行参应该是流的引用，第二个行参是对象的引用。
如果重载为类的成员函数，第一个行参应该是对象的引用，第二个行参是流的引用。
使用方式是 ClassObj << cout 这样与标准IO库就不一致了，所以输入输出运算符不能重载为类的成员函数，可以重载为类的友元函数和普通函数。
通常重载输出运算符的第二个行参是const的,因为输出一个类不许要更改它；
但是重载输入运算符的第二个行参必须是非const的,否则无法赋值。
重载的基本方法如下：
//重载输出运算符 ostream& operator<<(ostream& out, const ClassType& obj) { out << /*想要输出的内容1*/ << /*想要输出的内容2*/ <<...; return out; } //重载输入运算符 istream& operator<<(istream& in, ClassType& obj) { in >> /*想要输入的内容1*/ >> /*想要输入的内容2*/ >>...;
//检查错误 和 文件结束的可能性 return in; }
例子：类Persion使用友元函数的方式重载了输入输出运算符，类PersionA使用了普通函数重载了输入输出运算符。
#include <cstring> #include <iostream> using namespace std; class Persion { public: //constructor Persion(const char *pname, unsigned int ag, double he,double we):age(ag),height(he),weight(we){strcpy(name,pname);} //operator overload : << friend ostream& operator<<(ostream& out, const Persion& ref) { out<<ref.name<<"\t"<<ref.age<<"\t"<<ref.height<<"\t"<< ref.weight; return out; } //operator overload : >> friend istream& operator>>(istream& in, Persion& ref) { char name[40] = {0}; unsigned int ag = 0; double he = 0; double we = 0; in>>name>>ag>>he>>we; //check that if the inputs succeeded if (in) {//Input Succeeded strcpy(ref.name, name); ref.age = ag; ref.height = he; ref.weight = we; } else {//Input Failed } return in; } private: char name[40]; unsigned int age; double height; double weight; }; class PersionA { public: //constructor PersionA(const char *pname, unsigned int ag, double he,double we):age(ag),height(he),weight(we){strcpy(name,pname);} //GetData char* GetName(void){return name;} unsigned int& GetAge(void){return age;} double& GetHeight(void){return height;} double& GetWeight(void){return weight;} private: char name[40]; unsigned int age; double height; double weight; }; //operator overload : << ostream& operator<<(ostream& out, PersionA& ref) { out<<ref.GetName()<<"\t"<<ref.GetAge()<<"\t"<<ref.GetHeight()<<"\t"<<ref.GetWeight(); return out; } //operator overload : >> istream& operator>>(istream& in, PersionA& ref) { char name[40] = {0}; unsigned int ag = 0; double he = 0; double we = 0; in>>name>>ag>>he>>we; //check that if the inputs succeeded if (in) {//Input Succeeded strcpy(ref.GetName(), name); ref.GetAge() = ag; ref.GetHeight() = he; ref.GetWeight() = we; } else {//Input Failed } return in; } int main(void) { Persion per("Jack", 20, 175, 65); cout << per << endl; cin>>per; cout << per << endl; PersionA perA("Jack", 20, 175, 65); cout << perA << endl; cin>>perA; cout << perA << endl; return 0; }