Problem with .CPP program output HELP please!!!

I am a beginner in C++ :D .i wrote this code in which operator is overloaded on matrices.Go thru the program;

Code:

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                                /*prog.cpp*/
#include<iostream.h>
#include<conio.h>
int k=1;
class matrix
{
        private:
                        static int a[20][20];
                        int m,n;
        public:
                        friend istream& operator>>(istream &cin,matrix&);
                        friend ostream& operator<<(ostream &cout,matrix);
                        friend matrix operator+(matrix,matrix);
                        friend matrix operator-(matrix,matrix);
                        friend matrix operator*(matrix,matrix);
                        friend matrix operator/(matrix,matrix);
};

istream& operator>>(istream &cin,matrix &a)
{
        int i,j;
        cout<<"Enter the size of the matrix m,n"<<endl;
        cin>>a.m>>a.n;
        cout<<"Enter the matrix"<<endl;
        for(i=0;i<=a.m-1;i++)
        for(j=0;j<=a.n-1;j++)
          cin>>a.a[i][j];
        return(cin);
}
ostream& operator<<(ostream &cout,matrix a)
{
        int i,j;
        cout<<"The matrix is as follows"<<endl;
        for(i=0;i<=a.m-1;i++)
        {
          for(j=0;j<=a.n-1;j++)
          cout<<a.a[i][j]<<"\t";
          cout<<"\n";
        }
        return(cout);
}
matrix operator+(matrix a,matrix b)
{
        matrix c;
        int i,j;
        c.m=a.m;
        c.n=a.n;
        for(i=0;i<=a.m-1;i++)
        for(j=0;j<=a.n-1;j++)
        c.a[i][j]=a.a[i][j]+b.a[i][j];
        return(c);
}

matrix operator-(matrix a,matrix b)
{
        matrix c;
        int i,j;
        c.m=a.m;
        c.n=a.n;
        for(i=0;i<=a.m-1;i++)
        for(j=0;j<=a.n-1;j++)
        c.a[i][j]=a.a[i][j]-b.a[i][j];
        return(c);
}
matrix operator*(matrix a,matrix b)
{
        matrix c;
        int i,j;
        c.m=a.m;
        c.n=b.n;
        if(a.n==b.m)
        {
        for(i=0;i<=a.m-1;i++)
        for(j=0;j<=b.n-1;j++)
        for(k=0;k<=a.n-1;k++)
          c.a[i][j]+=a.a[i][k]+b.a[k][j];
        }
        else
          k=0;
        return(c);
}
matrix operator/(matrix a,matrix b)
{
        matrix c;
        int i,j;
        c.m=a.m;
        c.n=a.n;
        for(i=0;i<=a.m-1;i++)
        for(j=0;j<=a.n-1;j++)
        c.a[i][j]=(float)a.a[i][j]/float(b.a[i][j]);
        return(c);
}
main()
{
        matrix a,b,c,d,e,f;
        cin>>a>>b;
        c=a+b;
        d=a-b;
        e=a*b;
        f=a/b;
        cout<<"The addition of matrices is:"<<"\n"<<c;
        cout<<"The subtraction of matrices is:"<<"\n"<<d;
        if(k==1)
        cout<<"The multiplication of matrices is:"<<"\n"<<e;
        else
        cout<<"The multiplication of matrices is not possible"<<endl;
        cout<<"The division of matrices is:"<<"\n"<<f;
        getch();
        return(0);
}

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When i compile the above prog i get an error :confused: :
"Linker error:undefined symbol matrix::a in module prog.cpp"

please help me out with the error. :(

Why is the member array a static?

Code:

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class matrix
{
        private:
                        static int a[20][20];

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u need to declare static int a[20][20] outside the class and use it directly.

thanks 4 helping me out.The main problem was that a clas cannot have a static member.
thx

A class can have a static member, you just didn't implement it completely. In this case, though, the array shouldn't have been static anyway since each matrix instance should have its own data.