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

From: Daniela Engert <dani_at_[hidden]>
Date: Sat, 16 Apr 2022 14:06:03 +0200
Hi Nico,

having read all the messages on the respective reflectors, our private
conversations and social media, I'm still having trouble to see what
you're after.

Neither msvc (the compiler) nor MSBuild (the build system) nor
VisualStudio (the IDE with its project system) requires any particular
file extension whatsoever to work with modules. In fact, in the
beginning when I started seriously investigating this modules thing in
2018, I was uniformly using the .cpp extension with msvc and clang. A
year later during my modularization of the {fmt} library I've switched
to .cc for the module interface unit because it is the coding convention
for all C++ TUs in {fmt}. And both msvc and cmake were happy with it
(and still are). And since we have modules in production and active
development at my workplace, we conventionally settled on .ixx for the
PMIU and kept .cpp for everything else. This turned out to be the most
convenient and practical convention *in that particular environment*
using *the given toolset* and with cross-platform considerations totally
being a non-issue. This .ixx extension for the PMIUs is in no way
mandated by any component involved there, I could use .cpp just as well
as I did in the beginning. But I appreciate .ixx for the module
interfaces simply because they are so pivotal in understanding a module
and thus better stand out of the sea of other TUs on the first glance
just due to the different shape of the glyphs.

 From my understanding of other compilers, this is just the same with
them. The real issue is with the build tools that drive the whole
compilation process. As a user, I don't actually care about the
incantation ceremonies required by compilers, versions, modes etc. This
is what MSBuild and the module dependency scanner are taking care of in
my development environment. Or CMake might provide in the future. Or any
other tool for that matter.

But my biggest beef with the state of the ecosystem is the total lack of
understanding of the module semantics by tools like static code
analysers and such. My daily experience with them is like I'm talking
Plankalkül to them instead of post-post-post modern C++. 🙂

So Nico, what is it what you are looking for? It can't be only the file
extension given the many of them that are already in existence and use
during the past couple of decades.


Am 16.04.2022 um 12:53 schrieb Nico Josuttis via Ext:
> Hi Roger,
> I agree.
> However, cmake and other tools use the command line. So, they would
> have these problems solved.
> Even for Visual Studio, if there is support for arbitrary file
> extensions and no need for specific command line options, code coming
> from different compiler can just be used as it is.
> So, programmers could have all files with suffix .cpp (as is fine for
> gcc) and just add them to the Visual Studio project without further
> action.
> While of course VC++ still can reommend special suffixes like .ixx
> (still I don't know the suffix for internal partitions), the other
> compilers would not have to adopt their convention and the community
> will decide which suffix "wins" (as it happened with ".cpp" which came
> from a Microsoft, although strangly it is not used by them in a
> modified form for modules).
> I will today publish a script that fix the problems Visual Studio has
> with module files for the command line. That way, programmers can
> write their first portable module programm... (portable in the
> practical not formal sense, Gaby).
> As usual, correct me if I miss something.
> Am 16. April 2022 12:10:03 MESZ schrieb Roger Orr via Ext
> <ext_at_[hidden]>:
> Hello Nico,
> I am slightly puzzled by the use of 'command line' and 'command' in your email.
> In my own experience I suspect few of the C++ programmers I work with ever compile with a command line; they either work within an IDE or run a build script, likely using cmake. It us unclear to me what is the benefit of seeking a unified command line that most programmers are not even aware of.
> Regards,
> Roger.
> -----Original Message-----
> From: Ext [mailto:ext-bounces_at_[hidden]] On Behalf Of Nico Josuttis via Ext
> Sent: 16 April 2022 07:43
> To: Gabriel Dos Reis;ext_at_[hidden];sg15_at_[hidden]
> Cc: Nico Josuttis; Nathan Sidwell
> Subject: Re: [isocpp-ext] [SG15] Can we expect that all C++ source files can have the same suffix?
> Gaby,
> you want me to tell programmers that we have portable examples, for which unfortunately there is not defined HOW to deal with them?
> And selling this as "you can use modules in practice"? 🤔
> Of course for programmers a portable program implies that
> a) I don't have to rename files
> b) I don't have to use different conmand-line options for files having the same suffix
> c) I can compile all code with a single command
> Once we have that, we can teach content and programmers will use it. So far, the clear conclusion is that you cannot use modules in practice.
> It would be absolutely no problem to provide that for Visual C++. Ideally ignoring file extensions and analyze C++ file content.
> Therefore, I wonder why you do not WANT that (and at the same time tell that you are interesting that modules become widely used).
> And, BTW, I am desperately looking for the file extension Visual C++ expect for internal patition units
> (to skip /internalPartition).
> PLEASE just tell us.
> Thanks
> Am 15. April 2022 21:46:13 MESZ schrieb Gabriel Dos Reis<gdr_at_[hidden]>:
> Fortunately, there IS a portable C++ example of “hello world”
> program, today. *How* to compile a given source file has
> always depended on compilers, their environment of
> invocations, and supporting toolsets. That won’t change. And
> Modules don’t have a goal of changing that. Requiring that the
> set of satellite files that a compiler has to produce depends
> solely on the contents of the source file, and not on the
> invocation command lines, isn’t going to work in real world,
> production environments. That is what build systems are for,
> to abstract over the details. My hope is that professional
> teaching of programming with modules direct C++ programmers to
> relying on their build systems. We are having a conversation
> in SG15 about common ways of describing to build systems what
> are the needs of a program and library, and let the build
> system make the build happens. And that is not restricted to
> modules, even though they make the conversation urgent. --
> Gaby From: Nico Josuttis <nico_at_[hidden]> Sent: Friday,
> April 15, 2022 12:05 PM To: ext_at_[hidden]; Gabriel Dos
> Reis via Ext <ext_at_[hidden]>; ext_at_[hidden];
> sg15_at_[hidden] Cc: Gabriel Dos Reis
> <gdr_at_[hidden]>; Nathan Sidwell <nathan_at_[hidden]> Subject:
> Re: [isocpp-ext] [SG15] Can we expect that all C++ source
> files can have the same suffix? haha, I REALLY would like to
> have the first portable "hello module" example. Currently,
> there is simply no way to have it. That's really a shame. Am
> 15. April 2022 20:46:59 MESZ schrieb Gabriel Dos Reis via Ext
> <ext_at_[hidden]<mailto:ext_at_[hidden]>>: Nathan -
> you recount is all correct. 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. I am not seeing any movement to make filename
> suffixes significant in the linguistic interpretation, by the
> compiler, of the content of source file. Maybe I am not
> looking right; but I would definitely recommend against such
> move. There are extra linguistic considerations that might
> force a toolset (not just a compiler) to require certain
> suffixes, but those suffixes do not determine the meaning a
> C++ program -- this is not different from compilers like GCC
> or MSVC refusing to compile in default mode files ending with
> ".h" or similar because they generally have other
> connotations. For some reasons, the topic of the "right"
> suffix seems to generate more passion than the topic of what
> can we do with modules, so maybe we are already doing a lot
> with modules 😝 -- Gaby -----Original Message----- From: Ext
> <ext-bounces_at_[hidden]<mailto:ext-bounces_at_[hidden]>>
> On Behalf Of Nathan Sidwell via Ext Sent: Friday, April 15,
> 2022 11:21 AM To:
> sg15_at_[hidden]<mailto:sg15_at_[hidden]>;
> ext_at_[hidden]<mailto:ext_at_[hidden]>; WG21
> Tooling Study Group SG15
> <tooling_at_[hidden]<mailto:tooling_at_[hidden]>> Cc: Nathan
> Sidwell <nathan_at_[hidden]<mailto:nathan_at_[hidden]>> Subject: Re:
> [isocpp-ext] [SG15] Can we expect that all C++ source files
> can have the same suffix? On 4/13/22 17:10, Nico Josuttis via
> SG15 wrote: I should add that the fact that we need module; 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. nathan 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 that). 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
> modules. I can still have my own suffixes and use -xc++
> though. - *VC++* currently requires different suffixes or
> different command-line arguments. 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 handling. (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. Thanks Nico

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Received on 2022-04-16 12:06:10