#include<iostream>
using  namespace std;
class  Complex
{public:
Complex(double  real=0,double  imag=0);
friend  Complex  operator  *(const  Complex  &c1,const Complex &c2);
friend  Complex  operator  /(const  Complex  &c1,const Complex &c2);

friend  bool  operator  ==(const  Complex  &c1 ,const Complex &c2);
friend  bool operator  !=(const  Complex  &c1,const Complex &c2);
friend  ostream & operator <<(ostream &,Complex &);
friend  istream & operator >>(istream &,Complex &);
 void  compare();

    double  real,imag;
};
//以上是函数声明
Complex::Complex(double r,double i):real(r),imag(i){}
Complex  operator *(const  Complex &c1,const Complex &c2 )
{return Complex(c1.real*c2.real-c1.imag*c2.imag,c1.imag*c2.real+c1.real*c2.imag);}
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));}
ostream & operator <<(ostream & output ,Complex &c)
{ output<<"("<<c.real<<"+"<<c.imag<<"i)"<<endl;
return  output;
}
istream & operator >>(istream & input ,Complex &c)
{input>>c.real>>c.imag;
return  input;
}

bool  operator ==(const  Complex &c1,const Complex &c2 )
{if((c1.real==c2.real)&&(c1.imag==c2.imag))
return  true;
else  return  false;
}
bool  operator !=(const  Complex &c1,const Complex &c2 )
{if((c1.real!=c2.real)||(c1.imag!=c2.imag))
return  true;
else  return  false;
}
void Complex::compare(const  Complex &c1,const Complex &c2 )
{if(operator== (c1,c2)==1)
    cout<<"c1==c2";
   else  cout<<"c1!=c2";
}
//以上是函数定义

int  main()
{Complex c1,c2,c3;
cin>>c1;
cin>>c2;
cout<<"c1="<<c1;
cout<<"c2="<<c2;
c3=c1*c2;
cout<<"c3="<<c3;
c3=c1/c2;
cout<<"c3="<<c3;
c1=c2;
cout<<"c1="<<c1;
cout<<"c2="<<c2;
compare(c1,c2);
return 0;
}
//我想实现的功能是修改该类，使之能用重载的>>和<<运算符输入和输出复数；
   2) 重载乘法和除法运算符，使之能执行两个复数的代数乘法和代数除法运算；
   3) 重载运算符==和!=，使之能比较两个复数；
   4) 重载运算符=，使之能进行两个复数的赋值运算。
程序运行后报错,说我compare函数没定义，晕啊

既然你定义了Complex c1,c2,c3且Complex类里你又定义了double  real,imag；那么cin>>c1和cin>>c2明显就有错误，不可以这样对变量进行赋值，好好去看下书

我学的是运算符的重载，重载了 cin  和  cout ,可以cin>>c1,cin>>c2这样使用的

我知道你想干什么，但自定义的类变量怎么可以这样赋值呢？
假设你写的没错，那问一下：cin>>c1，你想让机器读入什么数据（实数部分还是虚数部分）？

[ 本帖最后由 i80286 于 2013-11-17 19:53 编辑 ]

cin>>c1;就是让用户自己输入复数的实部 ， 虚部，你看看下面这个程序的主函数，就是这样输入数据的，我建了一个头文件两个源文件，可以运行，就是“比较两个复数大小的功能未实现”




#include<iostream>
using  namespace std;
class  Complex
{public:
Complex(double  real=0,double  imag=0);
friend  Complex  operator  *(const  Complex  &c1,const Complex &c2);
friend  Complex  operator  /(const  Complex  &c1,const Complex &c2);

friend  bool  operator  ==(const  Complex  &c1 ,const Complex &c2);
friend  bool operator  !=(const  Complex  &c1,const Complex &c2);
friend  ostream & operator <<(ostream &,Complex &);
friend  istream & operator >>(istream &,Complex &);
 
    double  real,imag;
};





/////////////////////////////////////


#include"fushu.h"
Complex::Complex(double r,double i):real(r),imag(i){}
Complex  operator *(const  Complex &c1,const Complex &c2 )
{return Complex(c1.real*c2.real-c1.imag*c2.imag,c1.imag*c2.real+c1.real*c2.imag);}
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));}
ostream & operator <<(ostream & output ,Complex &c)
{ output<<"("<<c.real<<"+"<<c.imag<<"i)"<<endl;
return  output;
}
istream & operator >>(istream & input ,Complex &c)
{input>>c.real>>c.imag;
return  input;
}

bool  operator ==(const  Complex &c1,const Complex &c2 )
{if((c1.real==c2.real)&&(c1.imag==c2.imag))
return  true;
else
 return  false;
}

bool  operator !=(const  Complex &c1,const Complex &c2 )
{if((c1.real!=c2.real)||(c1.imag!=c2.imag))
return  true;
else  
return  false;
}



//////////////////////////////////////////////////


#include"fushu.h"
int  main()
{Complex c1,c2,c3;
cin>>c1;
cin>>c2;
cout<<"c1="<<c1;
cout<<"c2="<<c2;
c3=c1*c2;
cout<<"c3="<<c3;
c3=c1/c2;
cout<<"c3="<<c3;
c1=c2;
cout<<"c1="<<c1;
cout<<"c2="<<c2;
cout<<(c1!=c2)<<endl;
cout<<(c1==c2)<<endl;
system("pause");
return 0;
}

帮你排版了一下

#include <iostream>

class Complex
{
public:
    Complex( double real=0, double imag=0 );
    friend Complex operator *( const Complex& c1, const Complex& c2 );
    friend Complex operator /( const Complex& c1, const Complex& c2 );

    friend bool operator ==( const Complex& c1, const Complex& c2 );
    friend bool operator !=( const Complex& c1, const Complex& c2 );
    friend std::ostream& operator <<( std::ostream&, const Complex& );
    friend std::istream& operator >>( std::istream&, Complex& );
    friend void compare( const Complex& c1, const Complex& c2 );

    double real,imag;
};

using namespace std;

Complex::Complex( double r, double i ) : real(r), imag(i)
{
}
Complex operator *( const Complex& c1, const Complex& c2 )
{
    return Complex( c1.real*c2.real-c1.imag*c2.imag, c1.imag*c2.real+c1.real*c2.imag );
}
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) );
}
ostream& operator <<( ostream& output, const Complex& c)
{
    return output<<"("<<c.real<<"+"<<c.imag<<"i)";
}
istream& operator >>( istream& input, Complex& c )
{
    input>>c.real>>c.imag;
    return input;
}

bool operator ==( const Complex& c1, const Complex& c2 )
{
    return c1.real==c2.real && c1.imag==c2.imag;
}
bool operator !=( const Complex& c1, const Complex& c2 )
{
    return c1.real!=c2.real || c1.imag!=c2.imag;
}
void compare( const Complex& c1, const Complex& c2 )
{
    if(operator== (c1,c2)==1)
        cout<<"c1==c2";
    else
        cout<<"c1!=c2";
}

int main()
{
    Complex c1,c2,c3;
    cin >> c1;
    cin >> c2;
    cout << "c1=" << c1 << endl;
    cout << "c2=" << c2 << endl;
    c3 = c1*c2;
    cout << "c1*c2" << c3 << endl;
    c3 = c1/c2;
    cout << "c1/c2=" << c3 << endl;
    c1=c2;
    cout<<"c1="<<c1 << endl;
    cout<<"c2="<<c2 << endl;
    compare(c1,c2);

    return 0;
}

乱糟糟的代码风格，看到人想吐

（1）你的代码木有缩进实在是很伤（2）你的程序出错是因为你在类中声明的友元函数compare()和你在类外定义的友元函数的是不匹配的（也就是这两个东西不对应，所以在编译时候出错误了，有图有真相）

void  compare();

void Complex::compare(const  Complex &c1,const Complex &c2 )

这两货参数都不对应的，而且你要在下面程序中以compare（a,b）的形式调用函数，应该在类内声明compare时候把声明为friend类型的额，不然就默认为累的成员函数了。（正确答案楼上已经给出，我就不贴了）
还有就是

if(operator== (c1,c2)==1)

既然都已经重载运算符了，这句话也可以简化成

if(c1==c2)

效果是一样的，还容易看懂。
说完了，坐等楼主给分。