运算符重载的定义
运算符重载的方法是定义一个重载运算符的函数，在需要执行被重载的运算符的时候，系统就自动调用该函数，实现相应的运算；也就是说，运算符重载是通过定义函数实现的。运算符重载实质上就是函数的重载。
运算符重载的规则：
- C++中的运算符除了少数几个之外，全部可以重载。而且只能重载C++中已有的运算符
- 重载之后运算符的优先级和结合性不会改变
- 运算符重载是针对新类型数据的实际需要，对原有运算符进行适当的改造。一般来讲，重载的功能应当与原邮件功能相类似，不能改变原有运算符的操作对象个数，同时至少要有一个操作对象是自定义类型。
- 不能重载的运算符只有5个，他们是：成员运算符“.”、指针运算符“*”、作用运算符“::"、sizeof、条件运算符"?:"
运算符重载形式：
- 重载为类的成员函数
- 重载为类的友元函数
运算符重载为类的成员函数的一般语法形式
函数类型 operator 运算符（形参表）
{
函数体；
}
运算符重载为类的友元函数的一般语法形式
friend 函数类型 operator 运算符（形参表）
{
函数体；
}
实例
1 重载为类的成员函数
class complex
{
public:
complex()
{
real=imag=0;
}
complex(double r,double i)
{
real=r;
imag=i;
}
complex operator + (const complex &c);
complex operator - (const complex &c);
complex operator * (const complex &c);
complex operator / (const complex &c);
friend void print(const complex &c);
private:
double real,imag;
};
inline complex complex::operator+(const complex &c)
{
return complex(real+c.real,imag+c.imag);
}
inline complex complex::operator-(const complex &c)
{
return complex(real-c.real,imag-c.imag);
}
inline complex complex::operator*(const complex &c)
{
return complex(real * c.real - imag * c.imag, real * c.imag + imag * c.real);
}
inline complex complex::operator/(const complex &c)
{
return complex((real * c.real + imag + c.imag) / (c.real * c.real + c.imag * c.imag),
(imag * c.real - real * c.imag) / (c.real * c.real + c.imag * c.imag));
}
void print(const complex &c)
{
if(c.imag<0)
cout<<c.real<<c.imag<<'i';
else
cout<<c.real<<'+'<<c.imag<<'i';
}
int _tmain(int argc, _TCHAR* argv[])
{
complex c1(2.0, 3.0), c2(4.0, -2.0), c3;
c3 = c1 + c2;
cout<<"c1+c2=
";
print(c3);
c3 = c1 - c2;
cout<<"c1-c2=
";
print(c3);
c3 = c1 * c2;
cout<<"c1*c2=
";
print(c3);
c3 = c1 / c2;
cout<<"c1/c2=
";
print(c3);
c3 = (c1+c2) * (c1-c2) * c2/c1;
cout<<"(c1+c2)*(c1-c2)*c2/c1=
";
print(c3);
cout<<endl;
return 0;
}
输出结果：
c1+c2=6+1i
c1-c2=-2+5i
c1*c2=14+8i
c1/c2=0.45+0.8i
(c1+c2)*(c1-c2)*c2/c1=9.61538+25.2308i
2 重载为类的友元函数
#include ＜iostream.h＞
class complex
{
public:
complex() { real=imag=0; }
complex(double r, double i)
{
real = r, imag = i;
}
friend complex operator +(const complex &c1, const complex &c2);
friend complex operator -(const complex &c1, const complex &c2);
friend complex operator *(const complex &c1, const complex &c2);
friend complex operator /(const complex &c1, const complex &c2);
friend
void print(const complex &c);
private:
double real, imag;
};
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);
}
complex operator *(const complex &c1, const complex &c2)
{
return complex(c1.real * c2.real - c1.imag * c2.imag, c1.real * c2.imag + c1.imag * c2.real);
}
complex operator /(const complex &c1, const complex &c2)
{
return complex((c1.real * c2.real + c1.imag * c2.imag) / (c2.real * c2.real + c2.imag * c2.imag),
(c1.imag * c2.real - c1.real * c2.imag) / (c2.real * c2.real + c2.imag * c2.imag));
}
void print(const complex &c)
{
if(c.imag<0)
cout<<c.real<<c.imag<<'i';
else
cout<<c.real<<'+'<<c.imag<<'i';
}
void main()
{
complex c1(2.0, 3.0), c2(4.0, -2.0), c3;
c3 = c1 + c2;
cout<<"c1+c2= ";
print(c3);
c3 = c1 - c2;
cout<<"c1-c2= ";
print(c3);
c3 = c1 * c2;
cout<<"c1*c2= ";
print(c3);
c3 = c1 / c2;
cout<<"c1/c2= ";
print(c3);
c3 = (c1+c2) * (c1-c2) * c2/c1;
cout<<"(c1+c2)*(c1-c2)*c2/c1= ";
print(c3);
cout<<endl;
return 0;
}
该程序的运行结果与上例相同。前面已讲过，对又目运算符，重载为成员函数时，仅一个参数，另一个被隐含；重载为友元函数时，有两个参数，没有隐含参数。
两种运算符重载的比较
一般说来，单目运算符最好被重载为成员；对双目运算符最好被重载为友元函数，双目运算符重载为友元函数比重载为成员函数更方便此，但是，有的双目运算符还是重载为成员函数为好，例如，赋值运算符。因为，它如果被重载为友元函数，将会出现与赋值语义不一致的地方。