C++ Logo


Advanced search

Re: [EXTERNAL] Re: [isocpp-ext] Can we expect that all C++ source files can have the same suffix?

From: Cameron DaCamara <Cameron.DaCamara_at_[hidden]>
Date: Fri, 15 Apr 2022 18:54:36 +0000
  * While convention for file suffixes help, every compiler should be able to process C++ translation and module units just based on their content.
That gives BY FAR the best flexibility.

I am not convinced that this can be done in every scenario though. For example, most compilers have a way of force including a file at the beginning of a TU, but what is the 'beginning' mean? If you want correct semantics for force included files the compiler really needs to try and make the synthesized directive appear as the first set of pp-tokens in the TU so it does not have the opportunity to scan for 'module;' or 'export module x;'. If offering flexibility is on the table then options like '/FI' and '-include' should be useable in modules as well.

From: SG15 <sg15-bounces_at_[hidden]> On Behalf Of Nico Josuttis via SG15
Sent: Friday, April 15, 2022 11:45 AM
To: Nathan Sidwell <nathan_at_[hidden]>; sg15_at_[hidden]; ext_at_[hidden]; WG21 Tooling Study Group SG15 <tooling_at_[hidden]>
Cc: Nico Josuttis <nico_at_[hidden]>
Subject: [EXTERNAL] Re: [SG15] [isocpp-ext] Can we expect that all C++ source files can have the same suffix?

Thanks Nathan.
And as I see it now I agree.

While convention for file suffixes help, every compiler should be able to process C++ translation and module units just based on their content.
That gives BY FAR the best flexibility.

I opened a bug for Visual C++ to enable that.
I'd assume there is no problem to fulfill that request, because you only have to find the module declaration to know which option to turn on.
And with "module;" at the beginning you know whether such a declaration will come.

Once Visual C++ has the fix, suddenly all the suffix and options discussion is gone.
(ok, clang could then follow).

Am 15. April 2022 20:20:45 MESZ schrieb Nathan Sidwell <nathan_at_[hidden]<mailto:nathan_at_[hidden]>>:

On 4/13/22 17:10, Nico Josuttis via SG15 wrote:

I should add that the fact that we need
at the beginning of the global module fragment was only introduced to let a file identify itself as module file.
If we would require different suffixes, that would not have been necessary.

But correct me if I am wrong.

I shall correct you :)

Here's the history (as I recall, all persons mentioned are real, and not to be confused with ficticious characters)

* prior to me doing things with gcc, there was only 'module FOO;' as a module declaration at-most once within a TU. MSVC (the only compiler with module smarts at the time), had a flag to tell it 'this is an interface' vs 'this is an implementation'.

* I found this unsatisfying, as it meant that there was something outside the source tokens that told you how to interpret them. In effect we had two languages.

* IIRC, Gaby, Jason (Merrill) and I came up with the 'export module FOO;' vs 'module foo;' distinction. But still this could be anywhere in the source stream. I was able to implement this functionality to a working system.

* Daveed proposed an early signifier of 'hey, this is gonna be a module', should the actual module declaration not be first. Hence 'module;' was born. (My understanding was that this was driven by implementors, as they had difficulty entering a module-like mode not at start of compilation, and indeed it was a little tricky to do that. I do not know if this was also a user request.)

* post p1103, the requirement that everything between 'module;' and the module decl come from #include came to be.

Hope that helps.

I know I've mentioned it more than once, but I find it unsettling, given there was great opposition to there being a (two way?) mapping between file names and module names, that there is a move in the direction of making file names 'significant'. ISTM that the desire for bob.$REGULARSUFFIX and alice.$MODULESUFFIX is taking us all the way back to the first objection above about having two languages.


Am 13. April 2022 22:58:13 MESZ schrieb Nicolai Josuttis via Ext <ext_at_[hidden]<mailto:ext_at_[hidden]>>:

    What I teach about modules is compelling. Programmers like and want to use it.
    However, they ask how they should organize module files in practice.

    So far I cannot recommend a specific suffix (and I might never be able to do
    However there is one important question that IMO the standard should answer:
    *Do we **/need /**different suffixes?*

    I understand that a suffix discussion is only of practical value.
    But IMO the standard has to give an answer here (which has nothing to do
    with which suffixes are used).

    Let me elaborate that in detail:

    Not having a standard suffix has interesting consequences.
    So far we have header files and translation units.
    But once we know what a C++ translation unit is, we can just compile them
    all with the same compiler options or commands. Because in practice we have
    different suffixes for header and source files, we can set-up generic rules
    to compile our code.

    This works for any suffix, provided you know the way to tell the compiler
    that we have a C++ file here:
    (use /Tp with VC++ and -xc++ with gcc and you are done).

    Is this still true with modules?
    That is: Can we expect that identifying a file as C++ file is enough to be
    able to (pre) compile it as C++ file?

    Current compilers give different answers (AFAIK):

    - *gcc *says the same suffix is possible. There is not special option for
    I can still have my own suffixes and use -xc++ though.

    - *VC++* currently requires different suffixes or different command-line
    Identifying a file as C++ file is not enough.
    For example
       - This is not enough: /Tp mymod.cppm
       - You need: /interface /Tp mymod.cppm

    I wonder whether the behavior of VC++ is standard conforming.

    I see no place in the C++ standard saying that there has to be different
    treatment of C++ source files to make them work.
    Or do we require this somewhere?
    We do not require different treatment just because we have templates,
    namespaces, or exceptions used inside.
    Therefore, I would expect that also using modules does not require special
    (This is independent from the question whether different suffixes help to
    deal with these files).

    If I am right, VC++ is not standard conforming.

    In any case it would help a lot to clarify:
    Can all C++ source files expect that treating them the same way works fine?

    If not, we obviously need different suffixes. But then we should clearly say
    so (without necessarily saying which suffix it is).

    I hope this questions brings us a bit forward to be able teach the first
    *portable *"hello, modules" example.



    -- ---
    Nicolai M. Josuttis
    +49 (0)531 / 129 88 86
    +49 (0)700 / JOSUTTIS


Nico Josuttis
(sent from my mobile phone)
SG15 mailing list
Nathan Sidwell
Nico Josuttis
(sent from my mobile phone)

Received on 2022-04-15 18:54:40