Not sure exactly what you are trying to acheive with your function, how ever you might do this

#include <iomanip>
#include <iostream>
using namespace std;

class myclass
{
public :
    int m,n;
    myclass(){m=10;n=12;}
}

void disp( myclass& obj )
{
    cout<<"Hello"<<endl
         <<obj.m;
    return;
}

void main()
{
    myclass myobjct();
    disp( myobjct );
    return;
}