Question

I am trying to learn C++ myself by reading a book and do exercise from that book.

Now I am trying to declare an instance of function template inside the .cpp file. I know that I can declare/define that function template in header file, but I am still curious about how to do that inside the .cpp file.

Here is piece of code which is trivial but demonstrate my problem:

// temp_func.h
#ifndef GUARD_temp_func
#define GUARD_temp_func

#include <iostream>

using std::cout;    using std::endl;

int addone(int);
int addtwo(int);

template<typename F>
void call_adds(int, F);

#endif

Header file

// temp_func.cpp
#include "temp_func.h"

using std::cout;    using  std::endl;

int addone(int n)
{
    return n + 1;
}

int addtwo(int n)
{
    return n + 2;
}

template<typename F>
void call_adds(int n, F f)
{
    cout << f(n) << endl;
}
template void call_adds<addone>(int n, F addone);

.cpp file, and obviously the last line doesn’t work.

Edit:
Based on solutions provided by n.m.

template<int F(int)>
void call_adds(int n)
{
    cout << F(n) << endl;
}
template void call_adds<addtwo>(int);
template void call_adds<addone>(int);

Answer 1

Your call_ads template requires a type parameter. addone is not a type, it’s a function. You can specialize call_adds for a type like int(int), but not for individual functions of that type.

You can create a function template with a non-type template parameter:

template <int F(int)>
void call_adds(int n)
{
    cout << F(n) << endl;
}

and specialize it:

template<> void call_adds<addtwo> (int n) { ... }

Note that call_adds doesn’t have a regular function parameter any more, only a template parameter.

The compiler doesn’t really care whether you declare something in a header file or in a source file, as long as the declaration is visible where is should be.