Re: [boost] Modules

23 Aug 2020

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Gesendet: Sonntag, 23. August 2020 um 19:09 Uhr
Von: "Zach Laine via Boost" 
An: "Boost mailing list" 
Cc: "Zach Laine" 
Betreff: Re: [boost] Modules
On Sun, Aug 23, 2020 at 2:13 AM Mike via Boost  wrote:
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Gesendet: Sonntag, 23. August 2020 um 00:40 Uhr
Von: "Zach Laine via Boost" 
An: "Boost mailing list" 
Cc: "Zach Laine" 
Betreff: Re: [boost] Modules
On Fri, Aug 21, 2020 at 1:54 PM Damian Vicino via Boost
 wrote:
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Hi,
Are there any guidelines on the use of modules inside boost?
Are we planning any steps for adoption of them?
Modules are pretty hard for most Boost libs to adopt.  Consider how
many times we have code like this:
#if BOOST_LIBNAME_DO_WHATEVER
some_code;
#else
come_other_code;
#endif
That cannot exist inside a module.  The net result is that your code
has to be all the same lexically -- for all compilers and platforms --
if it is in a module.
Are you sure? I've never hear of such a restriction.
Yes, although it may not be clear what I mean.  You can only have
preprocessor directives before the keyword module:
#ifdef SOMETHING // ok
#include "foo.h" // ok
#endif // ok
module foo;
#ifdef SOMETHING // error
#include "bar.h" // error
#endif // error
Could you maybe point me to the relevant location in the standard? A quick test with
compiler explorer (https://godbolt.org/z/EP18s1) turns not up any errors,
but of course I doubt any compiler implements modules completely to spec yet.
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However,  you should be able to do the same as other legacy libraries: Make sure your headers can be used as  header units and/or provide a wrapper module for each library.
Right, it's just that if you have lots of code in common, but just
want it to vary a bit, you have to put that code in the module prefix
and then export it from the module.  It's not the end of the world,
but it is anti-idiomatic to all teaching you're likely to see about
how to write modules.
Not sure to which of the two suggestions you are replying, but I really
don't understand why it should require any work for most of boost
(except maybe the PP library) to be consumable as a header unit.
And for a wrapper module, what I have in mind is something like:

module;
#include 
export module Boost.Variant;

namespace boost {
    export using boost::variant;
}

But I'm absolutely no modules expert.

Mike
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Zach
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