#include "b.h"
namespace GLOBAL{

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

#include "a.h"
using namespace GLOBAL;

void A::doSomething(B b){

    b.use();
}

namespace GLOBAL{

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

‘GLOBAL::A’ has not been declared

‘void GLOBAL::B::use()’ is private

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();
}

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();
}

friend class A;

// #include "b.h"   // remove this line it is not needed.
namespace GLOBAL{

class B; // Forward declare the class here.

class A{
public:
    void doSomething(B&); // Note: This should probably be a reference.
                          // Change to doSomething(B&);
}
}

// Add this line it is needed for the friend declaration.
// To have a member as a friend we need the definition of 'A'
#include "a.h"

namespace GLOBAL{

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

#include "a.h"
// Add this line to include the B definition so you know how to call use()
#include "b.h"
using namespace GLOBAL;

void A::doSomething(B& b){ // b should be a reference otherwise you are copying it.

    b.use();
}