Compatibility layer in C++ to access template functions from C

Question

I have some code implemented in template variadic functions that uses modern c++17 features. Being templates, they are implemented in the .h files.

// .H FILE
template <typename... T>
inline constexpr void foo(const T& ...values){
    // Do stuff
}

Is there a way to create a compatibility layer that would allow users to access this functions from C?

I think you'll need to provide a subset of the functionality in the C API. When I've done similar things I've just picked the most important things first and made wrapper functions and wrapper types that only support a very specific subset of everything the real C++ functions could handle. — Ted Lyngmo
– Ted Lyngmo, Commented Dec 21, 2021 at 17:32
You can declare C-compatible functions to call your template. Like extern "C" void foo_int(int v) { foo(v); }. — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Dec 21, 2021 at 17:42
@MarcosAlvarez "It seems like C has fairly nice support for variadic functions as well" - yes, however C variadic functions and C++ variadic templates are VERY different beasts. A C++ variadic template can call a C variadic function, but I don't think the reverse is possible since the C function doesn't know what it is called with in order to populate the template parameters. — Remy Lebeau
– Remy Lebeau, Commented Dec 21, 2021 at 18:05
@TedLyngmo I agree. However, that is a business decision that I have no control over. The problem is solved though. The wrapper may have a tiny bit of overhead, but it is not a deal breaker. — Marcos Alvarez
– Marcos Alvarez, Commented Dec 22, 2021 at 14:07

Marcos Alvarez · Accepted Answer · 2021-12-21 23:43:05Z

0

The way I actually solved may not be valid to all particular cases!! I found that trying to pass arguments directly to a c++ variadic function was not possible (to the best of my knowledge). Instead, a void vector would be passed, and the results will not be the ones expected. In this case, stringifying the c input, and passing it to the c++ function worked just fine.

#ifdef __cplusplus
extern "C" {
#endif
    void cfoo(const char * fmt, ...)
    {
        va_list args
        va_start(args, fmt);
        char str[1024];
        vsprintf(str, fmt, args);
        cpp::foo(str); // My c++ function
        va_end(args);
    }
#ifdef __cplusplus
}
#endif

edited Dec 21, 2021 at 23:43

answered Dec 21, 2021 at 23:36

Marcos Alvarez

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