Functor as function argument fails

Question

I try to give a function a functor to be more flexible to adjust certain things in my class. Here is some Code:

struct BC {
    real operator()(real x, real y, real z) {
        return sin(2 * M_PI * x) * sin(2 * M_PI * y) * sin(2 * M_PI * z);
    }
};

int main() {
        // stuff
        subgrid.initBC(BC(), subdomain);
}

and here is the Definition of initBC in a file Subgrid.cpp:

template <class Fun> 
void Subgrid::initBC(Fun f, const Subdomain& subdomain) {
        f(1, 2, 3); // more or less, just simple usage like that

}

And the declaration, which is inside Subgrid.hpp:

   template <class Fun> void initBC(Fun f, const Subdomain & subdomain );

When trying to compile and link it, here is the error message I get

Undefined symbols:
  "void Subgrid::initBC<BC>(BC, Subdomain const&)", referenced from:
      _main in main.o
ld: symbol(s) not found

I have no clue what is not working here. I think it is probably due to some templating problem, but I can't see what it can possibly be. Any help appreciated, I hope the code snippets are enough, all other code should be irrelevant for this problem I think

I made an edit. It is within a separate file, but all other class functions work fine — Chris, Apr 01 '13 at 13:44
Yes, I've seen the edit. You cannot relegate the definition of a member function template, or member function of a class template, in a separate `.cpp` file. See the answer linked above for an explanation — Andy Prowl, Apr 01 '13 at 13:45

Functor as function argument fails

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