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[] = {};
unsigned int ag = ;
double he = ;
double we = ; 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[];
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[];
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[] = {};
unsigned int ag = ;
double he = ;
double we = ; 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", , , );
cout << per << endl;
cin>>per;
cout << per << endl; PersionA perA("Jack", , , );
cout << perA << endl;
cin>>perA;
cout << perA << endl;
return ;
}