C++ and inline templates

A while back I wrote an article on using inline templates. It's a bit of a niche article as I would generally advise people to write in C/C++, and tune the compiler flags and source code until the compiler generates the code that they want to see.

However, one thing that I didn't mention in the article, it's implied but not stated, is that inline templates are defined as C functions. When used from C++ they need to be declared as extern "C", otherwise you get linker errors. Here's an example template:

.inline nothing
  nop
.end

And here's some code that calls it:

void nothing();

int main()
{
  nothing();
}

The code works when compiled as C, but not as C++:

$ cc i.c i.il
$ ./a.out
$ CC i.c i.il
Undefined                       first referenced
 symbol                             in file
void nothing()                   i.o
ld: fatal: Symbol referencing errors. No output written to a.out

To fix this, and make the code compilable with both C and C++ we use the __cplusplus feature test macro and conditionally include extern "C". Here's the modified source:

#ifdef __cplusplus
  extern "C"
  {
#endif
    void nothing();
#ifdef __cplusplus
  }
#endif

int main()
{
  nothing();
}