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# 运算符重载

### 作为成员函数

下面的代码定义了一个复数类，通过运算符重载，可以用`+`号实现复数的加法运算：

```
#include <iostream>

using namespace std;

class complex{
public:
    complex();
    complex(double real, double imag);
public:
    //声明运算符重载
    complex operator+(const complex &A) const;
    void display() const;
private:
    double m_real;  //实部
    double m_imag;  //虚部
};

complex::complex(): m_real(0.0), m_imag(0.0){ }
complex::complex(double real, double imag): m_real(real), m_imag(imag){ }

//实现运算符重载
complex complex::operator+(const complex &A) const{
    complex B;
    B.m_real = this->m_real + A.m_real;
    B.m_imag = this->m_imag + A.m_imag;
    return B;
}

void complex::display() const{
    cout<<m_real<<" + "<<m_imag<<"i"<<endl;
}

int main(){
    complex c1(4.3, 5.8);
    complex c2(2.4, 3.7);
    complex c3;
    c3 = c1 + c2;
    c3.display();
    return 0;
}
```

运行结果： 6.7 + 9.5i

本例中义了一个复数类 complex，m\_real 表示实部，m\_imag 表示虚部，第 10 行声明了运算符重载，第 21 行进行了实现（定义）。认真观察这两行代码，可以发现运算符重载的形式与函数非常类似。

运算符重载其实就是定义一个函数，在函数体内实现想要的功能，当用到该运算符时，编译器会自动调用这个函数。也就是说，运算符重载是通过函数实现的，它本质上是函数重载。

运算符重载的格式为：

```
返回值类型 operator 运算符名称 (形参表列){
    //TODO:
}
```

`operator`是关键字，专门用于定义重载运算符的函数。我们可以将`operator 运算符名称`这一部分看做函数名，对于上面的代码，函数名就是`operator+`。\
\
运算符重载函数除了函数名有特定的格式，其它地方和普通函数并没有区别。\
\
上面的例子中，我们在 complex 类中重载了运算符`+`，该重载只对 complex 对象有效。当执行`c3 = c1 + c2;`语句时，编译器检测到`+`号左边（`+`号具有左结合性，所以先检测左边）是一个 complex 对象，就会调用成员函数`operator+()`，也就是转换为下面的形式：

```
c3 = c1.operator+(c2);
```

c1 是要调用函数的对象，c2 是函数的实参。

上面的运算符重载还可以有更加简练的定义形式：

```
complex complex::operator+(const complex &A)const{
    return complex(this->m_real + A.m_real, this->m_imag + A.m_imag);
}
```

return 语句中的`complex(this->m_real + A.m_real, this->m_imag + A.m_imag)`会创建一个临时对象，这个对象没有名称，是一个匿名对象。在创建临时对象过程中调用构造函数，return 语句将该临时对象作为函数返回值。

### 作为全局函数

运算符重载函数不仅可以作为类的成员函数，还可以作为全局函数。这相当于不是直接为类本身实现了一个新的方法，而是为某个运算符重载了一个函数（能以新类为参数的函数）。

更改上面的代码，在全局范围内重载`+`，实现复数的加法运算：

```
#include <iostream>

using namespace std;

class complex{
public:
    complex();
    complex(double real, double imag);
public:
    void display() const;
    //声明为友元函数
    friend complex operator+(const complex &A, const complex &B);
private:
    double m_real;
    double m_imag;
};

complex operator+(const complex &A, const complex &B);
complex::complex(): m_real(0.0), m_imag(0.0){ }
complex::complex(double real, double imag): m_real(real), m_imag(imag){ }

void complex::display() const{
    cout<<m_real<<" + "<<m_imag<<"i"<<endl;
}

//在全局范围内重载+
complex operator+(const complex &A, const complex &B){
    complex C;
    C.m_real = A.m_real + B.m_real;
    C.m_imag = A.m_imag + B.m_imag;
    return C;
}

int main(){
    complex c1(4.3, 5.8);
    complex c2(2.4, 3.7);
    complex c3;
    c3 = c1 + c2;
    c3.display();
    return 0;
}
```

运算符重载函数不是 complex 类的成员函数，但是却用到了 complex 类的 private 成员变量，所以必须在 complex 类中将该函数声明为友元函数。\
\
当执行`c3 = c1 + c2;`语句时，编译器检测到`+`号两边都是 complex 对象，就会转换为类似下面的函数调用：

```
c3 = operator+(c1, c2);
```

以上两种方案都说明了，运算符本身和函数的行为并无大异，使用`operator运算符`来获取运算符对应的“函数”。


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