How to apply constant reference parameter on this code?

This code is passing object to a function and I want to apply constant reference parameter on this code.

class Test
{
    int num;
public :
    void setNum(int z)
    {
        num=z;
    }//end mutator

    int getNum()
    {
        return num;
    }//end accessor
};//end class
void storeNumber(Test &n)
{
    int a=10;
    n.setNum(a);
}
void displayNumber(Test n)
{
    cout<<"number="<<n.getNum()<<endl;
}
int main()
{
    Test t;
    t.setNum(5);
    cout<<t.getNum()<<endl;
    storeNumber(t);//pass by reference;
    displayNumber(t); //pass by value
    cout<<t.getNum();
}

This code is passing object to a function and I want to apply constant reference parameter on this code.
Any help!!

You can do for example void displayNumber(const Test &n) but then if you call methods of the Test class, they must have the const qualifier too (for example int getNum() const). If you pass Test as const & to a function, then you won't be able to use void setNum(int z) function as it violates the constantness of the class.

Edit:

Here is the full code:

class Test
{
    int num;
public :
    void setNum(int z)
    {
        num=z;
    }//end mutator

    int getNum() const
    {
        return num;
    }//end accessor
};//end class
void storeNumber(Test &n)
{
    int a=10;
    n.setNum(a);
}
void displayNumber(const Test &n)
{
    cout<<"number="<<n.getNum()<<endl;
}
int main()
{
    Test t;
    t.setNum(5);
    cout<<t.getNum()<<endl;
    storeNumber(t);//pass by reference;
    displayNumber(t); //pass by value
    cout<<t.getNum();
}