Part8 多态性 8.1运算符重载

1运算符重载的规则
C++ 几乎可以重载全部的运算符，而且只能够重载C++中已经有的。
不能重载的运算符：“.”、“.*”、“::”、“?:”
重载之后运算符的优先级和结合性都不会改变。

重载的两种形式：
　　1 重载为类的非静态成员函数；
　　2 重载为非成员函数。

函数类型 operator 运算符（形参）
{
......
}
参数个数=原操作数个数-1 （后置++、--除外）

双目运算符重载规则：
　　如果要重载 B 为类成员函数，使之能够实现表达式 oprd1 B oprd2，其中 oprd1 为A 类对象，则 B 应被重载为 A 类的成员函数，
　　形参类型应该是 oprd2 所属的类型。
　　经重载后，表达式 oprd1 B oprd2 相当于 oprd1.operator B(oprd2)

//例8-1复数类加减法运算重载为成员函数
#include<iostream>
using namespace std;
class Complex{
public:
    Complex(double r = 0.0, double i = 0.0):real(r), image(i){}
    Complex operator +(const Complex &c2) const;
    Complex operator -(const Complex &c2) const;
    void display() const;
private:
    double real;
    double image;
};
Complex Complex::operator+(const Complex &c2) const{
    return Complex(real+c2.real, image+c2.image);//创建一个临时无名对象作为返回值
}
Complex Complex::operator-(const Complex &c2) const{
    return Complex(real-c2.real, image-c2.image);
}
void Complex::display() const{
    cout << "(" << real << ", " << image << ")" << endl;
}
int main(){
    Complex c1(5,4),c2(2,10),c3;
    cout << "c1 = "; c1.display();
    cout << "c2 = "; c2.display();
    c3 = c1 - c2;
    cout << "c3 = c1 - c2 = "; c3.display();
    c3 = c1 + c2;
    cout << "c3 = c1 + c2 = "; c3.display();
    return 0;
}

//例8-2重载前置++和后置++为时钟类成员函数
#include<iostream>
using namespace std;
class Clock{
public:
    Clock(int hour = 0, int minute = 0, int second = 0);
    void showTime() const;
    Clock& operator ++();//前置单目运算符重载
    Clock operator ++ (int);//用参数表来区分两个重载函数，函数体中不适用
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{
    cout << hour << " : " << minute << " : " << second << endl;
}
Clock &Clock::operator ++ (){//返回本类对象的引用
    second++;
    if(second >= 60){
        second-=60;
        minute++;
        if(minute >= 60){
            minute-=60;
            hour = (hour+1)%24;
        }
    }
    return *this;
}
Clock Clock::operator ++(int){
    Clock old = *this;
    ++(*this);//调用前置++运算符（形参）
    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;
}

双目运算符 B重载后，
　　表达式oprd1 B oprd2
　　等同于operator B(oprd1,oprd2)
前置单目运算符 B重载后，
　　表达式 B oprd
　　等同于operator B(oprd )
后置单目运算符 ++和--重载后，
　　表达式 oprd B
　　等同于operator B(oprd,0)

//例8-3 重载Complex的加减法和“<<”运算符为非成员函数
#include<iostream>
using namespace std;
class Complex{
public:
    Complex(double r = 0.0, double i = 0.0):real(r), image(i){}
    friend Complex operator+(const Complex &c1, const Complex &c2);
    friend Complex operator-(const Complex &c1, const Complex &c2);
    //它的左操作数是std::ostream引用，右操作数为复数类的常引用，返回std::ostream引用
    friend ostream &operator<<(ostream &out, const Complex &c);
private:
    double real;
    double image;
};

Complex operator+(const Complex &c1, const Complex &c2){
    return Complex(c1.real+c2.real, c1.image+c2.image);
}
Complex operator-(const Complex &c1, const Complex &c2){
    return Complex(c1.real-c2.real, c1.image-c2.image);
}
ostream &operator<<(ostream &out, const Complex &c){
    out << "(" << c.real << ", " << c.image << ")";
    return out;
}
int main(){
    Complex c1(5,4),c2(2,10),c3;
    cout << "c1 = " << c1 << endl;
    cout << "c2 = " << c2 << endl;
    c3 = c1 - c2;
    cout << "c3 = c1 - c2 = " << c3 << endl;
    c3 = c1 + c2;
    cout << "c3 = c1 + c2 = " << c3 << endl;
    return 0;
}