Binary Search Tree access within a std::bind function, called in another class

General explanation of program design.

It is a student registration program. Registration is the top object in the hierarchy it contains a vector of result objects. Result object contains a mark, date object and a string unitId.

This string is used to access a binary search tree. The binary search tree is of a object unit, the operator are overloaded to compare the id.

My problem is such. I'm looking for a way for registration class to the data it has access to std::string unitId (via a result object) to set a temporary unit object to run calculations on.

I've tried doing this by passing the a (BSTree& Tree) as a parameter but my map.. excuse lack of precise explanation, has a cry and says that all the functions have already been defined and I also get errors that my unit comparison operators don't deal with (unit U1, const Unit u2) - it gets really messy it seems.

As such I've tried to learn and used std::bind and placeholders.

Main pieces of code are as follows:

using namespace std::placeholders;

Unit& accessTree(std::string id, BSTree<Unit>& tree);

---

template<typename Ft>
unsigned Registration::GetCredits(Ft func)
{
    unsigned sum = 0;
    for(unsigned i = 0; i < GetSize(); i++)
        sum += results[i].GetCredits(func(results[i].GetUnit()));

    return sum;
}

---

int main()
{
    BSTree<Unit> uTree;

    Result Re;
    Re.SetUnit("ICT210");
    Re.SetMark(70);

    Registration R;
    R.SetResult(Re, 0);

    Unit u0;
    u0.SetId("ICT210");
    u0.SetName("Rikki Testing");
    u0.SetCredits(4);

    uTree.insert(u0);

    R.GetCredits(std::bind(accessTree, _1, std::ref(uTree)));

    return 0;
}

The Error I get:

undefined reference to `unsigned int Registration::GetCredits(std::_Placeholder<1>, std::reference_wrapper >))(std::string, BSTree&)> >(std::_Bind(std::_Placeholder<1>, std::reference_wrapper >))(std::string, BSTree&)>)'|