面向对象的程序设计-12-类型转换和运算符重载的实例

//分数的计算
#include<iostream>
using namespace std;
class rational
{
public:
    rational(long n=0,long d=0);
    friend rational operator +(rational r1,rational r2);
    operator double();//类型转换函数
    void print() const;
private:
    long numerator,denominator;
};
rational::rational(long n,long d)
{
    numerator=n;denominator=d;
}
rational operator +(rational r1,rational r2)
{
    long n,d;
    n=r1.numerator*r2.denominator+r1.denominator*r2.numerator;
    d=r1.denominator*r2.denominator;
    return rational(n,d);//会调用一次构造函数
}
rational::operator double()
{
    return double(numerator)/double(denominator);
}
void rational::print() const
{cout<<numerator<<"/"<<denominator<<endl;}
void display(double d);//普通函数，向前调用原型
void main()
{
    rational a(2,4),b(1,2);
    a=a+b;//使用友元函数重载“+”和缺省重载“=”
    a.print();
    display(a);//此处调用类型转换
}
void display(double d)
{cout<<d<<endl;}