Specifying -std when compiling both C and C++ code

Question

Suppose I have a codebase consisting of both C and C++ code, like so:

t.c:

int derp(void)
{
    return 42;
}

t.cpp:

#include <iostream>

extern "C" int derp(void);

int main(void)
{
    std::cout << derp() << std::endl;
    return 0;
}

With clang(++), I can both compile them in one go, like so:

clang++ -o t -xc++ t.cpp -xc t.c

However, if I now want to use non-standard features from e.g. gnu++14 and invoke the compiler like so:

clang++ -o t -xc++ -std=gnu++14 t.cpp -xc t.c

I am greeted with an error:

error: invalid argument '-std=gnu++14' not allowed with 'C/ObjC'

Unlike -x, -std does not seem to work based on a file level, but to be a global option, because adding -std=c11 like so:

clang++ -o t -xc++ -std=gnu++14 t.cpp -xc -std=c11 t.c

Simply gives me the "inverse" error, so to speak:

error: invalid argument '-std=c11' not allowed with 'C++/ObjC++'

And I know that I can compile each source file into an .o file separately and link them together afterwards (and yes, I'm automating the whole process anyway), but I cannot help but think that compiling with different standards per language should be possible. After all, clang supports compiling files in different languages together, and when doing that it would need separate values for the standards already, or not?

So, is there a way with clang(++) to manually specify standards when compiling both C and C++ code?

Answer 1

You can compile C, and you can compile C++.

You cannot compile as both C and C++.

Compile your C source files with C-appropriate flags.

Then compile your C++ source files with C++-appropriate flags (e.g. -std=gnu14).

Then link the results together to get an executable.

clang   -o a.o t.c
clang++ -o b.o t.cpp -std=gnu++14
clang++ -o t   a.o b.o

You cannot use the shorthand you've attempted to do both at once. It is designed as a shortcut for when all the flags and all the whatevers are the same. That's despite the magic of -x which sort of gets you close-ish.