std::bind used in function template issue

Question

Consider the follow code:

Unit& accessTree(std::string unitId2Find,  BSTree<Unit> &tree)
{
    Unit searchU;
    searchU.SetId(unitId2Find);
    return tree.search(searchU);
}

int main()
{
    using namespace std::placeholders;
    BSTree<Unit> tesTree;
    auto getUnit = std::bind(accessTree, _1, std::cref(tesTree));'

    Registration R;

..... //Setting required data

int i = R.GetCredits(getUnit); //Causes the error.

Now I've tried two different types of implementation: 1

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

    return sum;
}

template<typename Fn>
float Registration::CalculateGPA(Fn access)
{
    float total = 0;

    for (int i = 0; i < results.getLength(); i++)
    {
        total = total + results[i].GetGPAvalue() * results[i].GetCredits(access(results[i].GetUnit()));
    }

    return total / GetCredits(access);
}

2

unsigned Registration::GetCredits(Unit& (*access) (std::string & uId))
{
    unsigned sum = 0;
    for(unsigned i = 0; i < GetSize(); i++)
        sum += results[i].GetCredits(*access(results[i].GetUnit()));

    return sum;
}

float Registration::CalculateGPA(Unit& (*access) (std::string & uId))
{
    float total = 0;

    for (int i = 0; i < results.getLength(); i++)
    {
        total = total + results[i].GetGPAvalue() * results[i].GetCredits(*access(results[i].GetUnit()));
    }

    return total / GetCredits(func);
}

Compiler: ||=== Build: Debug in TesterProgram (compiler: GNU GCC Compiler) ===| E:\CODE\DSA\TesterProgram\BSTree.h||In member function 'elemType& BSTree::search(const elemType&) const [with elemType = Unit]':| E:\CODE\DSA\TesterProgram\BSTree.h|189|warning: control reaches end of non-void function [-Wreturn-type]| E:\CODE\DSA\TesterProgram\src\Regist.cpp||In member function 'Vector& Registration::GetHighestResults()':| E:\CODE\DSA\TesterProgram\src\Regist.cpp|100|warning: reference to local variable 'vec' returned [-Wreturn-local-addr]| E:\CODE\DSA\TesterProgram\src\Regist.cpp||In member function 'Vector& Registration::GetLowestResults()':| E:\CODE\DSA\TesterProgram\src\Regist.cpp|125|warning: reference to local variable 'vec' returned [-Wreturn-local-addr]| ..\TesterProgram\vector.h||In member function 'const T& Vector::operator[](unsigned int) const [with T = Result]':| ..\TesterProgram\vector.h|277|warning: control reaches end of non-void function [-Wreturn-type]| obj\Debug\main.o||In function main':| E:\CODE\DSA\TesterProgram\main.cpp|81|undefined reference tounsigned int Registration::GetCredits(std::_Placeholder<1>, std::reference_wrapper const>))(std::string, BSTree&)> >(std::_Bind(std::_Placeholder<1>, std::reference_wrapper const>))(std::string, BSTree&)>)'| ||=== Build failed: 1 error(s), 4 warning(s) (0 minute(s), 0 second(s)) ===|

What is going on here, I've had a look at related stuff on this site but nothing seems to directly answer this question.

I don't know how to use boost:: Is this something that is required, how would it be used?

Or is this a run time/ compiler issue? Is it not define in the compilation process when it needs to be? Is there a work around?

Thanks SO

Working on MVCE now.. update soon