Unresolved externals with explicit template instantiations. What is the declaration syntax?

Question

Here's some simplified code to demonstrate the problem I have.

I have a template function for which I only wish to compile certain fixed instantiations.

The function declarations are:

// *** template.h ***
int square (int x);
double square (double x);

The definitions are:

// *** template.cpp ***
#include "template.h"

// (template definition unusually in a code rather than header file)
template <typename T>
T square (T x)
{
    return x*x;
}

// explicit instantiations
template int square (int x);
template float square (float x);

And, an example use is:

// *** main.cpp ***

#include <iostream>
using namespace std;

#include "template.h"

int main (void)
{
    cout << square(2) << endl;
    cout << square(2.5) << endl;
}

An attempt to compile this results in a link errors, roughly:

main.obj : unresolved external symbol "int square(int)" referenced in function main

I understand what the problem is: the function signatures of my explicit template instantiations do not match those in the header file.

What is the syntax for the (forward) declaration of the explicit template instantiations please? I do not wish to forward declare the template definition, or to move the template definition into a header file.

For what it's worth, I do have a workaround, which is to use wrapper functions, adding the following to the above files:

// *** template.cpp ***

// ...

// wrap them [optionally also inline the templates]
int square (int x) { return square<> (x); }
double square (double x) { return square<> (x); }

That compiles and works as expected. However, this seems like a hack to me. There should be something more elegant than this available in C++ and template syntax.

Any help or hints would be much appreciated.

Answer 1

You need to declare the function template in your header:

template <typename T>
T square(T x);

As you have it now, you declare two nontemplate functions in the header, which are never defined.