Wrapper function for template<> in C++

Question

I am currently trying to create a wrapper interface that can translate C++ to C, and while studying on the possibilities to do that, I came across the template functions (and classes). Knowing that these functions can take any data type and return any data type as well, I find it hard to create a corresponding caller function name that C could read. A simple example is the adder.

template <class typesToAdd>
typesToAdd addStuff(typesToAdd a, typesToAdd b) {
    return a + b;
}

My interface includes the extern "C" command to avoid the C++ name mangling in C.

Answer 1

You may use function templates to simplify both implementing and maintaining the actual code that does the work but to provide a C interface, you'll still need to explicitly instantiate the functions for the types you aim to support.

If you'd like a C++ user to have the same restricted access to the functions as a C user will have, you can move the template implementation into the .cpp file and do the explicit instantiation there. A C++ user trying to use the function with types for which you have not explicitly instantiated the template will get a linking error.

It could look something like this:

// a.hpp
#pragma once

template <class T>
T addStuff(const T& a, const T& b); // no implementation here.

// a.cpp
#include "a.hpp"
#include "a.h"

template <class T>
T addStuff(const T& a, const T& b) {
    T rv = a;
    rv += b;
    return rv;
}

// C interface - note: it's inside the .cpp file
extern "C" {

int add_ints(int a, int b) {
    return addStuff(a, b);
}

double add_doubles(double a, double b) {
    return addStuff(a, b);
}

}

/* a.h */
#pragma once

#ifdef __cplusplus
extern "C" {
#endif

int add_ints(int, int);
double add_doubles(double, double);

#ifdef __cplusplus
}
#endif

A C user can now include the .h file and call the two functions for which you've provided an implementation.

Example:

// main.c
#include "a.h"

#include <stdio.h>

int main() {
    printf("%d\n", add_ints(10, 20));
    printf("%f\n", add_doubles(3., .14159));
}

Compilation:

g++ -c a.cpp
gcc -o main main.c a.o

Answer 2

Template is not a function that works for any data type. It is a template for a function that is created at compilation time. Each type you use creates a new function with a new symbol in the binary.

To export to C, you will have to specialize the type you want to use from C, like:

template <class typesToAdd>
typesToAdd addStuff(typesToAdd a, typesToAdd b) {
    return a + b;
}

extern "C" {

    int addStuffInt(int a, int b) {
        return addStuff(a, b);
    }

}