how do i remove the constant expression required in this program?

[deepak123]

#include<iostream.h>
#include<conio.h>

class mtr
{
int m,n,i,j;
int ar[m][n];

public:
mtr()
{
for(int p=0;p<m ; p++)
{
for(int q=0;q<n;q++)
{
ar[p][q]=0;
}
}
}
void read();
void display();
};

void mtr::read()
{
cout<<"enter dimensions of matrix \n";
cin>>m>>n;
cout<<"now enter elements of matrix: \n";
for(i=0;i<m ; i++)
{
for(j=0;j<n;j++)
{
cin>>ar[i][j];
}
}
}

void mtr::display()
{
cout<<"the matrix entered by you is : \n";
for(i=0;i<m ; i++)
{
for(j=0;j<n;j++)
{
cout<< ar[i][j]=0;
}
}
cout<<"thank you";
}

void main()
{
mtr r;
r.read();
clrscr();
r.display();
getch();
}

[weaknessforcats]

I have no idea what const expression you are referring to.