有关函数重载的问题！！

[abcxyz83]

我编了一个很简单的类，想练习一下函数重载。可是不知为什么加上重载函数总是出错，请高手赐教。谢谢！
程序如下：

class complex
{
private:
double rel,img;
public:
complex(double r=0,double i=0);
~complex();
void disp();
};

complex::complex(double r,double i)
{
rel=r;
img=i;
}

complex::~complex()
{
cout<<"bye"<<endl;
}

void complex::disp()
{
cout<<"real is :"<<rel<<endl;
cout<<"img is :"<<img<<endl;
}

complex operator+(complex c1,complex c2)//为什么加上此函数就报错，
//书上都这么的写
{
complex temp;
temp.rel=c1.rel+c2.rel;
temp.img=c1.img+c2.img;
return temp;
}

int main()
{
complex c1(33,89.3),c2(83,22),c3;
c1.disp();
c2.disp();
c3.disp();
c3=c1+c2;
c3.disp();
return 1;
}