gcc precompiled header: pragma once in main file

Question

I created a header file. Something simple as follows.

#pragma once

#include <iostream>

template<typename T>
void say(T t) {
    std::cout << t << std::endl;
}

and then use g++ to create the gch pre-compiled header with g++ hello.h. It gives me this warning ->

pch.h:2:9: warning: #pragma once in main file
    2 | #pragma once
      |         ^~~~

But the gch file created and the pre-compiled header works fine. This error goes away if I use header guards.

Am I doing something wrong here?

Answer 1

You're not doing anything wrong; this is a quality of implementation issue that has been mentioned on the issue tracker before (but, to my knowledge, there are currently no plans to change the behaviour).

In Clang, you could turn off the warning for that particular compiler invocation (with -Wno-pragma-once-outside-header); in GCC you'll just have to grin and bear it for now.