Declaration of specialized function in template class - c++

Question

The problem I am struggling with is the declaration of specialized template function inside template class (I keep class declaration in header file and define member functions in associated .C file).
I have template class representing Points. The header file is presented below:
//...
template<typename T, int dim=3> // T - coords. type, int dim - no. of dimensions
class Point {
public:
// ...
// function below sets val at the given position in array m_c and returns reference
template<int position> Point& set(T val);
private:
T m_c[dim]; // coordinates
};
//...
definition of function set is placed in .C file:
template<typename T, int dim> template<int position> Point<T, dim>& Point<T, dim>::set(T val){
// ...
return *this;
}
As I understand this is the most general form of its definition.
In main function I create Point with float as T and try to set some values in the array:
int main(int argc, char** argv) {
Point<float> p1;
p1.set<0>(3).set<1>(3.6).set<2>(3);
//...
}
In order to make this possible with definition of the member functions of template outside header file I need to inform compiler about specialization in .C file:
template class Point<float>;
and I need as well to declare usage of set function, which I try to accomplish this way
(and this piece of code is the problem):
template<> template<int> Point<float>& Point<float>::set(float);
That unfortunately doesn't do the job and I get errors:
/tmp/ccR7haA5.o: In function `main':
.../pdim.C:32: undefined reference to `Point<float, 3>& Point<float, 3>::set<0>(float)'
.../pdim.C:32: undefined reference to `Point<float, 3>& Point<float, 3>::set<1>(float)'
.../pdim.C:32: undefined reference to `Point<float, 3>& Point<float, 3>::set<2>(float)'
I would really appreciate an explanation from someone who may know how to cope with this problem. Thanks.

Answer 1

In order to provide the definition of a function template specialization in a different TU, you need an explicit instantiation declaration:
[Point.hpp]
template<typename T, int dim=3>
struct Point
{
template<int position> Point& set(T val);
};
// `extern` makes this an explicit instantiation _declaration_
extern template Point<float,3>& Point<float,3>::set<0>(float);
extern template Point<float,3>& Point<float,3>::set<1>(float);
extern template Point<float,3>& Point<float,3>::set<2>(float);
[Point.cpp]
#include <iostream>
#include "Point.hpp"
template<typename T, int dim>
template<int position>
Point<T,dim>& Point<T,dim>::set(T val)
{
// note: non-standard macro
std::cout << __PRETTY_FUNCTION__ << std::endl;
return *this;
}
// no `extern`: this is an explicit instantiation _definition_
// which instantiates the function template, and therefore requires the definition
// to be available in this TU
template Point<float,3>& Point<float,3>::set<0>(float);
template Point<float,3>& Point<float,3>::set<1>(float);
template Point<float,3>& Point<float,3>::set<2>(float);
[main.cpp]
#include "Point.hpp"
int main()
{
// in this TU, there's no definition for the function template
// hence, it cannot be instantiated
// however, we can use the explicit instantiations
Point<float,3>().set<0>(0);
Point<float,3>().set<1>(0);
Point<float,3>().set<2>(0);
// does not compile (linker error):
//Point<int,3>().set<0>(0);
//Point<float,4>().set<0>(0);
//Point<float,3>().set<4>(0);
}