Can a friend class invoke a private constructor in c++? (and what's Singleton)

Question

Consider class A and B. Class A is befriended by class B. Class B has a private constructor. Can class A create class B instances, or is a private constructor an indication to the compiler that the class cannot be instantiated, even by a friend class?
Here's some sample code:

class B;

class A {
    void myFunction() {
        B newBobject;
    }
};

class B {
    private:
        B() {}
    public:
        int someData;
        friend class A;
};

Also, note I'm using C++03. If it's not valid in C++03, is it allowed in C++11?

As a side question, how is the Singleton method related? Does it deal specifically with instantiating one and only one instance of an object, or is it something else?

@OliCharlesworth Did that compile I would consider a very poor judge in these cases... — paul23, Jan 09 '13 at 01:32
@paul23: It's in no way definitive, but it would have been a start... ;) — Oliver Charlesworth, Jan 09 '13 at 01:33

Karthik T · Accepted Answer · 2013-01-09T01:41:15.170

3

Your code(more like your idea) is actually error free and valid on both C++03 and c++11.

There are 2 errors in your code however. To create an object of type B, you need to see the entire definition of B, and that means the definitions of B and A need to be exchanged.

Secondly, you need to make myfunction public, or call it from within class A.

To answer your other question.. Most singletons are implemented somewhat like this..

class Singleton{
private:

   Singleton(){}
public:
   static Singleton& GetInstance(){
       static Singleton instance;
       return instance;
   }
};

This would prevent anybody but the Singleton class (and a friend as you have discovered) from making an instance of the class, so it is easier to enforce the 1 object rule. The Singleton will be accessed as Singleton::GetInstance().

This and other possible implementations are shown at Wikipedia.

edited Jan 09 '13 at 01:41

answered Jan 09 '13 at 01:26

Karthik T

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thx! But would I _have_ to make myFunction public? I could add a call to it in some public function in A - but I guess that doesn't matter. – Codesmith Jan 09 '13 at 01:32
@AUTO yup, you dont HAVE to make it public, i had to, to call it from main. – Karthik T Jan 09 '13 at 01:34
For your Singleton example, is it required or optional to put: `Singleton& GetInstance() const`? – Codesmith Jan 09 '13 at 01:38
@AUTO Ah!! I missed it, the function has to be static, so const doesnt make sense. – Karthik T Jan 09 '13 at 01:41

Can a friend class invoke a private constructor in c++? (and what's Singleton)

1 Answers1