C++多态性

一、运算符重载

将双目运算符重载为成员函数：

#include <iostream>
using namespace std;

class Complex {
public:
    Complex(double r = 0.0, double i = 0.0) : real(r), imag(i) { }
    //运算符+重载成员函数
    Complex operator + (const Complex &c2) const;
    //运算符-重载成员函数
    Complex operator - (const Complex &c2) const;
    void display() const;   //输出复数
private:
    double real;    //复数实部
    double imag;    //复数虚部
};

Complex Complex::operator+(const Complex &c2) const{
    //创建一个临时无名对象作为返回值 
    return Complex(real + c2.real, imag + c2.imag);
}

Complex Complex::operator-(const Complex &c2) const{
    //创建一个临时无名对象作为返回值
    return Complex(real - c2.real, imag - c2.imag);
}

void Complex::display() const {
    cout << "(" << real << ", " << imag << ")" << endl;
}

int main() {
    Complex c1(5, 4), c2(2, 10), c3;
    cout << "c1 = "; c1.display();
    cout << "c2 = "; c2.display();
    c3 = c1 - c2;   //使用重载运算符完成复数减法
    cout << "c3 = c1 - c2 = "; c3.display();
    c3 = c1 + c2;   //使用重载运算符完成复数加法
    cout << "c3 = c1 + c2 = "; c3.display();
    return 0;
}

将单目运算符重载为成员函数：

为了在程序中区分前置运算符（前置++）和后置++，需要在后置++的参数列表里面加东西来区分。

#include <iostream>
using namespace std;

class Clock {//时钟类定义
public:
    Clock(int hour = 0, int minute = 0, int second = 0);
    void showTime() const;
    //前置单目运算符重载
    Clock& operator ++ ();
    //后置单目运算符重载
    Clock operator ++ (int);
private:
    int hour, minute, second;
};

Clock::Clock(int hour, int minute, int second) {
    if (0 <= hour && hour < 24 && 0 <= minute && minute < 60
        && 0 <= second && second < 60) {
        this->hour = hour;
        this->minute = minute;
        this->second = second;
    }
    else
        cout << "Time error!" << endl;
}
void Clock::showTime() const {  //显示时间
    cout << hour << ":" << minute << ":" << second << endl;
}

Clock & Clock::operator ++ () {
    second++;
    if (second >= 60) {
        second -= 60;  minute++;
        if (minute >= 60) {
            minute -= 60; hour = (hour + 1) % 24;
        }
    }
    return *this;
}

Clock Clock::operator ++ (int) {
    //注意形参表中的整型参数
    Clock old = *this;
    ++(*this);  //调用前置“++”运算符
    return old;
}

int main() {
    Clock myClock(23, 59, 59);
    cout << "First time output: ";
    myClock.showTime();
    cout << "Show myClock++:    ";
    (myClock++).showTime();
    cout << "Show ++myClock:    ";
    (++myClock).showTime();
    return 0;
}

 运算符重载为非成员函数：

#include <iostream>
using namespace std;

class Complex {
public:
    Complex(double r = 0.0, double i = 0.0) : real(r), imag(i) { }
    friend Complex operator+(const Complex &c1, const Complex &c2);
    friend Complex operator-(const Complex &c1, const Complex &c2);
    friend ostream & operator<<(ostream &out, const Complex &c);
private:
    double real;  //复数实部
    double imag;  //复数虚部
};

Complex operator+(const Complex &c1, const Complex &c2){
    return Complex(c1.real + c2.real, c1.imag + c2.imag);
}

Complex operator-(const Complex &c1, const Complex &c2){
    return Complex(c1.real - c2.real, c1.imag - c2.imag);
}

ostream & operator<<(ostream &out, const Complex &c){
    out << "(" << c.real << ", " << c.imag << ")";
    return out;
}

int main() {
    Complex c1(5, 4), c2(2, 10), c3;
    cout << "c1 = " << c1 << endl;
    cout << "c2 = " << c2 << endl;
    c3 = c1 - c2;   //使用重载运算符完成复数减法
    cout << "c3 = c1 - c2 = " << c3 << endl;
    c3 = c1 + c2;   //使用重载运算符完成复数加法
    cout << "c3 = c1 + c2 = " << c3 << endl;
    return 0;
}

二、虚函数

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