[kazinator]

Is it trivial to implement?

The detailed semantics of #pragma once is murky. If #pragma once appears in a file, will that preclude an identical copy of the file from being included? Do we go by content, or just some filesystem identifier like device and inode number? Or the absolute path? Will #pragma once preclude that same file from being included again through a symbolic or hard link? Is the answer the same for all compilers?

The semantics of the #ifndef/#define/#endif trick is crystal clear; we can predict what it will do in any given situation. The identity of the file, for the purpose of suppressing duplicates, is tied to a made-up symbol which the program explicitly specifies, and that's that. It may go wrong (e.g. due to clashes on the identifier between unrelated files), but in a way that is understandable.

#pragma once seems like a bad trade-off for the sake of saving two lines of preprocessing.

[loeg]

> Is it trivial to implement?

Yes.

https://github.com/llvm-mirror/clang/blob/799b6c6d7/lib/Lex/...

https://github.com/llvm-mirror/clang/blob/799b6c6d7/include/...

[kazinator]

I agree that it's trivial to invoke a member function called MarkFileIncludeOnce on some object in response to encountering a #pragma once.

I don't think there is any good way to specify how this should behave for all the corner cases; but a given specification isn't hard to implement.