c++笔记2-操作符重载

操作符重载：
操作符重载为操作符提供不同的语义
1.通过operator关键字可以利用函数扩展操作符，operator的本质是通过函数重载实现操作符重载
  例1：：扩展“+”，使他可以支持复数相加
#include <cstdlib>
#include <iostream>

using namespace std;

struct Complex
{
    int a;
    int b;
};

Complex operator+(const Complex& c1, const Complex& c2)
{
    Complex ret;
   
    ret.a = c1.a + c2.a;
    ret.b = c1.b + c2.b;
   
    return ret;
}

int main(int argc, char *argv[])
{
    Complex c1 = {1, 2};
    Complex c2 = {3, 4};
    Complex c3 = c1+c2;
   
    cout<<"c3.a = "<<c3.a<<endl;
    cout<<"c3.b = "<<c3.b<<endl;
   
    cout << "Press the enter key to continue ...";
    cin.get();
    return EXIT_SUCCESS;
}

2.用operator关键字扩展的操作符可以用于类：
private声明使得类的成员不能被外界访问，但是通过friend关键字可以例外的开放权限
例二：
#include <cstdlib>
#include <iostream>

using namespace std;

class Complex
{
    int a;
    int b;
public:
    Complex(int a = 0, int b = 0)
    {
        this->a = a;
        this->b = b;
    }
   
    int getA()
    {
        return a;
    }
   
    int getB()
    {
        return b;
    }
   
    friend Complex operator+ (const Complex& c1, const Complex& c2);
};

Complex operator+ (const Complex& c1, const Complex& c2)
{
    Complex ret;
   
    ret.a = c1.a + c2.a;
    ret.b = c1.b + c2.b;
   
    return ret;
}

int main(int argc, char *argv[])
{
    Complex c1(1, 2);
    Complex c2(3, 4);
    Complex c3 = c1 + c2;
   
    cout << "Press the enter key to continue ...";
    cin.get();
    return EXIT_SUCCESS;
}
例三：重载“<<”
#include <cstdlib>
#include <iostream>

using namespace std;

class Complex
{
    int a;
    int b;
public:
    Complex(int a = 0, int b = 0)
    {
        this->a = a;
        this->b = b;
    }
   
    int getA()
    {
        return a;
    }
   
    int getB()
    {
        return b;
    }
   
    friend Complex operator+ (const Complex& c1, const Complex& c2);
    friend ostream& operator<< (ostream& out, const Complex& c);
};

ostream& operator<< (ostream& out, const Complex& c)
{
    out<<c.a<<" + "<<c.b<<"i";
   
    return out;
}

Complex operator+ (const Complex& c1, const Complex& c2)
{
    Complex ret;
   
    ret.a = c1.a + c2.a;
    ret.b = c1.b + c2.b;
   
    return ret;
}

int main(int argc, char *argv[])
{
    Complex c1(1, 2);
    Complex c2(3, 4);
    Complex c3 = c1 + c2;
   
    cout<<c1<<endl;
    cout<<c2<<endl;
    cout<<c3<<endl;
   
    cout << "Press the enter key to continue ...";
    cin.get();
    return EXIT_SUCCESS;
}
3.用成员函数重载的操作符,比全局操作符重载函数少一个参数，即左操作数,不需要使用friend关键字
例4：
#include <cstdlib>
#include <iostream>

using namespace std;

class Complex
{
    int a;
    int b;
public:
    Complex(int a, int b)
    {
        this->a = a;
        this->b = b;
    }
   
    int getA()
    {
        return a;
    }
   
    int getB()
    {
        return b;
    }
   
    Complex operator+ (const Complex& c2);
   
    friend ostream& operator<< (ostream& out, const Complex& c);
};

ostream& operator<< (ostream& out, const Complex& c)
{
    out<<c.a<<" + "<<c.b<<"i";
   
    return out;
}

Complex Complex::operator+ (const Complex& c2)
{
    Complex ret(0, 0);
   
    ret.a = this->a + c2.a;
    ret.b = this->b + c2.b;
   
    return ret;
}

int main(int argc, char *argv[])
{
    Complex c1(1, 2);
    Complex c2(3, 4);
    Complex c3 = c1 + c2;
   
    cout<<c1<<endl;
    cout<<c2<<endl;
    cout<<c3<<endl;
   
    cout << "Press the enter key to continue ...";
    cin.get();
    return EXIT_SUCCESS;
}
4.什么时候使用全局函数重载操作符？什么时候使用成员函数重载操作符？
  当无法修改左操作数的类时，使用全局函数进行重载
   =, [], ()和->操作符只能通过成员函数进行重载

5.不要重载&&和||操作符
  &&和||是C++中非常特殊的操作符
  &&和||内置实现了短路规则
  操作符重载是靠函数重载来完成的
  操作数作为函数参数传递
  C++的函数参数都会被求值，无法实现短路规则

6.++操作符的重载
  ++操作符只有一个操作数
  ++操作符有前缀和有后缀的区分,区分前后缀：C++中通过一个占位参数来区分前置运算和后置运算
例：
obj++:
Complex operator++(int)
{
   Complex ret = *this;
   a++;
   b++;
   return ret;
}
++obj:
Complex& operator++()
{
   ++a;
   ++b;
   return *this;
}