Friend function and protected data

Question

I have class

    class ScoreBoard: public die
{ //..//
    protected:
    bool mSetValue[6];
    public:
//...//
        friend void ValueSet();
};

and I would like to use that friendship to get access to mSetValue. So what I do in ScoreBoard.cpp is: I predifine a funcion (void ValueSet();) and then define it like this:

void ValueSet()
{ char lPick;
std::cin >> lPick;
if (lPick == 1) mSetValue[0] = true; }

But the debugger says:

'mSetValue' was not declared in this scope.

So, my question is - how to properly set a friendship so ValueSet can get access to mSetValue array?

On an unrelated note: in what way is a scoreboard similar to a die? — molbdnilo, May 31 '16 at 14:06
Why should it be similiar? Scoreboard inheritances from die class to have access to it's public methods. — , May 31 '16 at 14:25
@PawełMotyka: What `molbdnilo` is trying to say is that inheritance is usually used to indicate an `is-a` relationship for the types. So your inheritance indicates that `ScoreBoard is-a die` — Martin York, May 31 '16 at 14:30
I'd be quite surprised that this program compiles. Are you sure that the **debugger** says anything? — eerorika, May 31 '16 at 14:48
You don't use inheritance for accessing methods. In particular not the public methods, as they are accessible from anywhere. — molbdnilo, Jun 01 '16 at 05:36

eerorika · Answer 1 · 2016-05-31T14:54:05.343

Member variables^† exist inside instances (objects). They are also called "instance variables". If there is no instance of the class, then there is no instance variable.

mSetValue is a member variable of ScoreBoard. Therefore ScoreBoard::mSetValue instances exists only within an instance of ScoreBoard.

how to properly set a friendship so ValueSet can get access to mSetValue array?

In function void ValueSet() you don't have any instances of ScoreBoard. You cannot access ScoreBoard::mSetValue - regardless of its access specifier or friendship - because it doesn't exist. What you need is an instance of ScoreBoard.

I use this function inside the ScoreBoard class so I can't create any instance of ScoreBoard inside it

Nothing prevents you from creating an instance of ScoreBoard within a member function of ScoreBoard. Although, within a member function of ScoreBoard you already have access to an instance, which is pointed by this, so there might be no need to create a new instance. What you should do depends on your intention.

Given your comment, I suspect that a member function would be more appropriate for you, rather than a free function.

^† I use the generic term "Member variable" to refer non-static member variables for simplicity. Static member variables are different. They are also called class variables.

Ok, but I use this function inside the ScoreBoard class so I can't create any instance of ScoreBoard inside it. I though of static variable ( because they are "immortal") but this idea is not working either. Is there any way I can you it inside the function or I have to use the ValueSet funcion in main ? — , May 31 '16 at 14:29
@PawełMotyka see the edited answer. I suspect that a member function would be more appropriate for you, rather than a free function. — eerorika, May 31 '16 at 14:45

score 1 · Answer 2 · edited May 31 '16 at 14:34

Friend functions are not member of class. So in your case if you want to use mSetValue in ValueSet then you have to provide some access to mSetValue via an instance or "object" of that class. This can be done by declaring your ValueSet() function as

friend void ValueSet(ScoreBoard &sb);

And your definition as

void ValueSet(ScoreBoard &sb)
{
    char lPick;
    std::cin >> lPick;
    if (lPick == 1)
        sb.mSetValue[0] = true; 
}

Friend function and protected data

2 Answers2