Restricting one class not create object

[viddi]

Hi,
i am a beginner in c++ programming.
i have 3 independent classes A,B,C.i do not want B to create A's object but C can create A's object.how to implement this scinario?

[Disch]

B only creates an A object if you tell it to. So if you don't want it to do that, then don't do that.

class B
{
 // this is an example of B which doesn't create an A object
};

[viddi]

thanks for your answer...
but my question is that if you try to create A's object in class B.it should not create?

[Disch]

er... what?

I don't understand what you're asking. Can you clarify?

If you don't want class B to create class A, then just don't put an A object in class B. If you put an A in B, B is going to create an A.

[Bazzy]

Maybe viddi wants this:

class A
{
    A(){} // a private constructor
    friend class C; // grant C access to the constructor
};
class B
{
    A a; // error
};
class C
{
    A a; // OK
};

[m4ster r0shi]

When I saw the title I thought he was talking about the singleton pattern, but I guess he wants something else. In general, restrictions on the instantiation of an object can be imposed by making its constructor(s) and its destructor private, and declaring friendship to a desired set of classes:

class A
{
    friend class C;

private:
    A(){}
    ~A(){}
};

class B
{
public:
    //uncommenting results in
    //compilation error

    //void b_func() {A a;}
};

class C
{
public:
    void c_func() {A a;}
};

int main()
{
    //uncommenting results in
    //compilation error

    //A a;

    return 0;
}

Though, I don't see how that would be useful... (but maybe I'm just missing something)
As Disch said, if you don't want B to create an A object, just don't tell it to :P

EDIT: That's what I get for checking my posts two and three times before submitting them... -.-

[Kyon]

That would only be useful for inheritance, when using abstract classes. In this context, it's not really useful in any way.

[LB]

m4ster r0shi wrote:

class A
{
    friend class C;

private:
    A(){}
    ~A(){}
};

Why is there a private keyword there? Aren't classes private by default?

EDIT: v OK

[m4ster r0shi]

Yes, they are. I put the keyword there for emphasis.