On 2018-10-08 07:13, Thiago Macieira wrote:
On Sunday, 7 October 2018 15:17:30 PDT Henry Skoglund wrote:
I recommend against changing Qt.
Hi, but isn't C++17's __has_include preprocessor cmd an implicit
endorsement of #pragma once? I mean, they both assume that the file
namespace is stable and idempotent.
No, I don't see how one thing has to do with the other.
__has_include imples that the #include will not fail with "No such file or
directory" and hopefully won't fail either with "Is a directory". Other checks
are welcome, but I wouldn't be too picky if they didn't get done.
#pragma once only applies after the #include and changes nothing about whether
the #include succeeds or not.
The example with ~/src as a bind-mount to ~/dev/src above, reminds me of
the old pointer aliasing problems in C++. But cannot it be solved by
requiring #pragma once to always call realpath() first, i.e. always test
on "the canonicalized absolute pathname" that realpath() returns?
No.
$ realpath ~/dev/src/qt/ ~/src/qt
/home/tjmaciei/dev/src/qt
/home/tjmaciei/src/qt
Bind-mounts are not symlinks. For all intents and purposes they are different
filesystems: you can't link() from one to the other, for example. You may be
able to tell that they are the same because the dev_t is the same:
$ stat -c '%D %i %n' ~/dev/src ~/src
fe03 4456449 /home/tjmaciei/dev/src
fe03 4456449 /home/tjmaciei/src
But to tell that the file is the same you'd need to compare inodes and those
are not guaranteed on network mounts.
Another option is to use name_to_handle_at(), a Linux-specific function, but
that will also fail with bind mounts and multiple mounts of the same
filesystem. The manpage suggests resolving mount IDs to actual disks by
looking for them in /dev/disks/by-uuid.
So you see why this is not a very easy solution.
Yes, I see.
When #pragma once was introduced 20 years ago by Microsoft I thought it
was the greatest thing since sliced bread, but maybe it's time to
reconsider.
So, what about a new preprocessor command:
__has_same_md6_digest
It would require the compiler to store an md6 hash/checksum of the
contents of each of the #include files seen during compilation, but if
most of the compiler's time is spent waiting for file i/o, then
computing those md6 sums wouldn't be so costly.
It would work the same way as the traditional header guards, but more
automated, example:
#if !__has_same_md6_digest
...
...
...
#endif
Rgrds Henry
_______________________________________________
Development mailing list
[email protected]
http://lists.qt-project.org/mailman/listinfo/development
