Operator Plus Problem

#include <iostream>
using namespace std;

class Tklasa
{
public:
    Tklasa( int a, int b );
    ~Tklasa();
    Tklasa operator +( Tklasa & box );
private:
    int zm_a;
    int zm_b;
   
};
Tklasa::Tklasa( int a, int b )
    : zm_a( a )
    , zm_b( b )
{
   
}

Tklasa::~Tklasa()
{
   
}

Tklasa Tklasa::operator +( Tklasa & box )
{
    Tklasa sum;
    sum.zm_a = box.zm_a + box.zm_a;
    sum.zm_b = box.zm_b + box.zm_b;
   
    return sum;
}
int main()
{
    Tklasa kla( 10, 25 );
   
    cin.get();
    return 0;
}