What is going on here?

This doesn't compile,

#include <boost/intrusive_ptr.hpp>

class X
{
public:
 void intrusive_ptr_add_ref(X* blah)
 {
 }

void intrusive_ptr_release(X * blah)
{
}

};



int main()
{
  boost::intrusive_ptr<X> ex(new X);
}

But this does :

#include <boost/intrusive_ptr.hpp>

class X
{
public:
  friend void intrusive_ptr_add_ref(X* blah)
  {
  }

  friend void intrusive_ptr_release(X * blah)
  {
  }

};



int main()
{
  boost::intrusive_ptr<X> ex(new X);
}

and this :

    #include <boost/intrusive_ptr.hpp>

    class X
    {
    public:


    };


    void intrusive_ptr_add_ref(X* blah)
      {
      }

      void intrusive_ptr_release(X * blah)
      {
      }

int main()
{
  boost::intrusive_ptr<X> ex(new X);
}

I suppose it has something to do with SFINAE (which I haven't as of yet bothered to understand) ? Does the friend qualifier put the defined function as a free function in the enclosed namespace ?

edit

Whoever removed their post, member functions non-friend as add_ref and release (these specific member functions are not mentioned in the documention...) did solve the problem. What happens with the nested definition with the friend qualifier ?

From the documentation of boost::intrusive_ptr:

Every new intrusive_ptr instance increments the reference count by using an unqualified call to the function intrusive_ptr_add_ref, passing it the pointer as an argument. Similarly, when an intrusive_ptr is destroyed, it calls intrusive_ptr_release; this function is responsible for destroying the object when its reference count drops to zero. The user is expected to provide suitable definitions of these two functions. On compilers that support argument-dependent lookup, intrusive_ptr_add_ref and intrusive_ptr_release should be defined in the namespace that corresponds to their parameter; otherwise, the definitions need to go in namespace boost.

This means that intrusive_ptr_add_ref and intrusive_ptr_release should not be member functions, but free functions (friend functions behave as such). Furthermore they are called without qualification, so they should be in the global namespace or somewhere found by ADL.

Edit: To your question about nested definitions with friend qualifier: friend functions are defined as non memberfunctions, so friend void intrusive_ptr_add_ref(X* blah) will be called as intrusive_ptr_add_ref(my_x_ptr) instead of as my_x_ptr->intrusive_ptr_add_ref().