Assign templated implementation to previously declared function

I've got an .h file containing 2 different functions declaration:

#ifdef MY_HEADER
#define MY_HEADER

void a();
void b();

#endif

Now into .cpp file I want to implement those functions as different instances of another templated function:

#include "my_header.h"

namespace {
    template<size_t N>
    void c()
    {
        ...
    }
}

void (*a)() = c<42>;
void (*b)() = c<265>;

I'm getting an error message error: 'void (* a)()' redeclared as different kind of symbol. I've also tried a = c<42> and auto a = c<42> with no luck.

I know I can do it like this:

void a() {c<42>();}
void b() {c<265>();}

and I'm almost sure compiler will optimize this extra function call for me, but I'm wondering if there's a better way to declare this. I don't want to put c function itself into .h file either because this function is quite heavy and I don't want to have it recompiled in every source file using my header.