friend declaration block an external function access to the private section of a class

Question

I'm trying to force function caller from a specific class. For example this code bellow demonstrate my problem. I want to make 'use' function would be called only from class A. I'm using a global namespace all over the project.

a.h

#include "b.h"
namespace GLOBAL{

class A{
public:
    void doSomething(B);
}
}

a.cpp

#include "a.h"
using namespace GLOBAL;

void A::doSomething(B b){

    b.use();
}

b.h

namespace GLOBAL{

class B{
public:
    friend void GLOBAL::A::doSomething(B);
private:
    void use();
}

Compiler says:

‘GLOBAL::A’ has not been declared

‘void GLOBAL::B::use()’ is private

Can anyone help here ?

Thanks a lot,

Mike.

Answer 1

The following compiles fine from a cpp file:

namespace GLOBAL
{
    class B;

    class A
    {
    public:
        void doSomething(B& b);
    };
};


namespace GLOBAL
{
    class B
    {
    public:
        friend void GLOBAL::A::doSomething(B&);
    private:
        void use()
        {
        }
    };
};

void GLOBAL::A::doSomething(B& b)
{
    b.use();
}

As best I can tell your problem arises from the fact that you include "b.h" from "a.h" that defines the B class before the A class is defined yet the B class makes a reference to the A class. So you have problems. However you can't forward declare an object of type B because you are copying via the stack. Hence why I use a reference to class B (as this doesn't require the B object to be known in advance).

Basically you have some fundamental problems with your structure that needs working out. You need to do some reading up on forward declarations and circular dependencies

Edit: Specifying purely that class A is a friend of B (rather than a specific function in A that references B) is actually a possibility as the friend definition provides a kind of forward declaration. As such the following code compiles:

namespace GLOBAL
{
    class B
    {
    public:
        friend class A;
    private:
        void use()
        {
        }
    };
};

namespace GLOBAL
{
    class A
    {
    public:
        void doSomething(B b);
    };
};

void GLOBAL::A::doSomething(B b)
{
    b.use();
}

As such in your code, originally posted, chaging the friend statement to, simply

friend class A;

should allow your code to compile.