Calling a template function from a C file

Question

I know that templates are not defined in C. However in my case, i have an API written in C++, which is used by an application written in C. I wish to add a template function in the API. The function is defined as follows in abc.cpp:

template<typename T> T function_name(T param1){
  ...
  ...
  return val;
}

the declaration in abc.hpp as follows:

template<typename T> T function_name(T);

and this function is called from xyz.c as:

int a ,b = 5;
a = function_name(b);

However, it shows the following error in both abc.cpp and abc.hpp:

 error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘<’ token
 template<typename T>

Even using extern "C" doesn't help(leads to error: template with C linkage). My doubt is, is it even possible to call this template function in such a way? If yes, how can this be achieved? Thank you.

The "error: template with C linkage" error is the hint you should be taking; it is not intended that it can be done with C calling C++ directly. You'd have to know the mangled C++ name for an instantiation of the template function, and then the C code would call that. That ties you to a particular C++ compiler (possibly even a specific version of the compiler). The correct way to do it is extern "C" int template_int_function_name(int param1) { return function_name(param1); } and then the C calls template_int_function_name(). This gives you a C-callable wrapper for the template function. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 18, 2016 at 13:37
The application may suddenly turn out to be written in C++ ;-) (if you decide to compile it with a C++ compiler). Chances are that you need minor adaptions, in particular if the code follows unwind's advice to not cast the return value of malloc(), but the edits are usually fairly minor. — Peter - Reinstate Monica
– Peter - Reinstate Monica, Commented Nov 18, 2016 at 13:39
@JonathanLeffler Re "call template function directly through mangled name": The instantiation may not even exist (and lead to a link-time error) if no C++ code, i.e. here no code inside the library, calls the function with an int argument. If the function is a recent addition to a library that is rather likely. — Peter - Reinstate Monica
– Peter - Reinstate Monica, Commented Nov 18, 2016 at 13:41
@PeterA.Schneider: Yes; that's another possible defect with trying to call the mangled name. Unlikely, but possible. And just another nail in the coffin of a thoroughly dead idea — it is absolutely not the correct way to go. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 18, 2016 at 13:46
Yes, you would need wrappers for every type that needs to be used by the C code. It would be sensible not to use the template functions from C. Rewriting those functions in C++ may be better, but idiomatic modern C++ is very different from C. — Jonathan Leffler
– Jonathan Leffler, Commented Nov 19, 2016 at 14:08

unwind · Accepted Answer · 2016-11-18 13:33:34Z

9

No, of course not, template functions don't exist in C and you can't parse a template declaration with a C compiler.

You're going to have to add a C wrapper on the C++ side for the int instance of the template, and call that (e.g. something like function_int()).

answered Nov 18, 2016 at 13:33

unwind

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Peter - Reinstate Monica · Accepted Answer · 2016-11-18 13:48:24Z

1

Since you mix C and C++ already (and therefore must have taken care of the possible issues), another solution is to continue developing the application in C++. That way new code can use all the nice features of the C++ library, like templates ;-), overloaded functions etc., and has access to C++'s standard library which can greatly enhance productivity.

I don't see principal obstacles to using C++ for new source files in the application; whether it's easily possible and worthwhile to switch existing C files to C++ upon edits is another question.

answered Nov 18, 2016 at 13:48

Peter - Reinstate Monica

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Kaeraz · Accepted Answer · 2016-11-18 22:10:55Z

0

Since C does not support templates, you can consider to use Type-Generic Functions - a macro-based method of defining generic expressions.

If this is not a case, I suggest to stay with C++ compiler.

answered Nov 18, 2016 at 22:10

Kaeraz

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1 Comment

Jonathan Leffler Over a year ago

If you mean _Generic, it is not a macro-based solution but is a full operator in the language — it is described in the standard in section §6.5.1.1 Generic selection, which is part of section §6.5.1 Primary expressions. Section 6.10 describes the preprocessor.

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