Question

Can anybody please help me make this work?

Donor-enumerate() doesn’t work, gcc gives no matching function error.

template < class T >
struct mesh;

template < class T >
struct meshBone
{
    friend struct mesh< T >;
private:
    T *_obj;
};

template < class T >
struct mesh
{
    template < class U >
    void enumerate( U& rcv )
    {

    }

    void connect( T* obj, mesh< T >* donor )
    {
        class object_replacement
        {
            T* _obj;
        public:
            object_replacement ( T* t ): _obj(t) {}
            bool operator()( meshBone<T> * bone )
            {
                bone->_obj = _obj;
                return true;
            }
        } obj_rpl(obj);

        donor->enumerate (obj_rpl);
    }
};

Answer 1

In C++03 you cannot use a local type as a template type argument.

object_replacement is a local type (it is local to mesh<T>::connect(T*, mesh*)) and you try to use it as the template argument U of mesh<T>::enumerate<U>. gcc gives this rather unhelpful error message that there is “no matching function.”

The “no local types” rule has been removed in C++0x, so if you have a sufficiently recent version of gcc you can compile using -std=c++0x and this should work fine (I’ve verified this with gcc 4.5.1). If that isn’t an option, you just need to make object_replacement not a local type, i.e., extract it and make it a nested type of the mesh<T> class template:

template <class T>
struct mesh
{
    template <class U> void enumerate(U& rcv) { }

    class object_replacement
    {
        T* _obj;
    public:
        object_replacement (T* t) : _obj(t) {}
        bool operator()(meshBone<T>* bone)
        {
            bone->_obj = _obj;
            return true;
        }
    };

    void connect(T* obj, mesh<T>* donor)
    {
        object_replacement obj_rpl(obj);
        donor->enumerate(obj_rpl);
    }
};