Go4Expert - how to overload unary operator '!' as member funtion

Hi folks,
I am learning operator overloading concept in c++, wrote sample program to test overloading of unary operator '!' and '-'.
Code will work if i use them as friend function but not for member function. Can anybody tell where am i going wrong in function bool operator!(const co_ordi &a) and co_ordi operator-(const co_ordi &x); . Is that correct way to declare and define.

I am getting following error
Error:
op_ovld_unary.cc:18: error: ‘bool co_ordi:: operator!(const co_ordi&)’ must take ‘void’
op_ovld_unary.cc: In function ‘int main()’:
op_ovld_unary.cc:45: error: no match for ‘operator!’ in ‘!a’
op_ovld_unary.cc:45: note: candidates are: operator!(bool) <built-in>
op_ovld_unary.cc:50: error: no match for ‘operator-’ in ‘-b’
op_ovld_unary.cc:22: note: candidates are: co_ordi co_ordi:: operator-(const co_ordi&)

Code:

#include<iostream>

using namespace std;



class co_ordi

{

        int cx,cy,cz;

        public:

        co_ordi(int x=0,int y=0,int z=0):cx(x),cy(y),cz(z)

        {

                cout<<"in c_tor="<<cx<<cy<<cz<<endl;

        }

        void get_coordi()

        {

                cout<<"x="<<cx<<"y="<<cy<<"z="<<cz<<endl;

        }

 // friend co_ordi operator- (const co_ordi &a);

 // friend bool operator! (const co_ordi &a);

 bool operator!(const co_ordi &a)

        {

                return (a.cx == 0 && a.cy == 0 && cz==0);

        }

        co_ordi operator-(const co_ordi &x)

        {

                co_ordi k(-x.cx,-x.cy,-x.cz);

                return k;

        }

                     

};

/**co_ordi operator- (const co_ordi &x)

{

        cout<<"me minus"<<endl;

        return co_ordi(-x.cx,-x.cy,-x.cz);

}

bool operator! (const co_ordi &a)

{



        return (a.cx == 0 && a.cy == 0 && a.cz == 0);

}**/





int main()

{

        co_ordi a,b(4,5,6);



        if(!a)

        cout<<"a at origin"<<endl;

        else

                cout<<"a not at origin"<<endl;

        b.get_coordi();

    b =        -b;

    b.get_coordi();

        return 0;

}