c++ how to properly declare friend class method of another class

Question

consider the following example.

class A
{
    int member;
};

class B
{
    A& a_ref;
    void manipulate()
    {
        a_ref.member++;
    }
};

Now, obviously, B::manipulate can not access a_ref. I would like to allow (only) class B to get (reference) to A::member. I know that there exists friend keyword, but I do not know how to use it properly. My intention is, that I could change B::manipulate implementation to become this

int& A::only_B_can_call_this() // become friend of B somehow
{
    return member;
}

void B::manipulate()
{
    a_ref.only_B_can_call_this()++;
}

score 2 · Accepted Answer · answered Sep 14 '15 at 09:13

2

To make B a friend:

class A
{
    int member;
    friend /*class*/ B;  // class is optional, required if B isn't declared yet
};

Note that friends are anti-patterns - if something is private, it probably shouldn't be accessed. What are you trying to accomplish? Why isn't A self-contained? Why does another class need to access its internal data?

Use friend if you have valid answers/reasons for these questions.

answered Sep 14 '15 at 09:13

Luchian Grigore

253,575
64
457
625

Thanks, so I have to make whole A available to B? I could not make just one function available to B? – Zereges Sep 14 '15 at 09:17
Also, does accessibility specifier of friendness matter? So, does it matter if `friend` is declared in `public:`/`private:`/`protected:` section? – Zereges Sep 14 '15 at 09:19
1

Yes (whole class is friend) and no (the section doesn't matter). – Luchian Grigore Sep 14 '15 at 10:07

score 2 · Answer 2 · answered Sep 14 '15 at 09:14

2

You simply add friend class <class_name>; to the class that wants to make its members accessible, where <class_name> is the name of the class that you want to provide access for.

class A
{
    friend class B; // allow class B to access private members of A
    int member;
};

class B
{
    A& a_ref;
    void manipulate()
    {
        a_ref.member++;
    }
};

answered Sep 14 '15 at 09:14

bku_drytt

3,169
17
19

And could I make just one function available to `B`? Also, does accessibility specifier of friendness matter? So, does it matter if `friend` is declared in `public:`/`private:`/`protected:` section? – Zereges Sep 14 '15 at 09:22

c++ how to properly declare friend class method of another class

2 Answers2