[cfe-dev] RFC: A mechanism for importing a declaration only when it exists

[Richard Smith via cfe-dev]

On Thu, 25 Jun 2020, 21:05 Louis Dionne via cfe-dev, <cfe-dev at lists.llvm.org>
wrote:

> Hi,
>
> The C++ Standard Library has multiple headers of the form <cfoobar>, which
> are
> basically importing declarations from the corresponding <foobar.h> header
> into
> namespace std. For example, <cstdio> basically imports all the
> declarations from
> <stdio.h> into namespace std:
>
>     namespace std {
>         using ::FILE;
>         using ::fpos_t;
>         using ::size_t;
>         ...
>     }
>
> When porting libc++ to new platforms, especially more embedded-ish
> platforms,
> it is very common that some declarations are not provided by the underlying
> C Standard Library. This leads to having to detect what platform we're on,
> and then to conditionally exclude some declarations:
>
>     namespace std {
>     #if !defined(SOME_WEIRD_PLATFORM)
>         using ::FILE;
>         using ::fpos_t;
>     #endif
>         using ::size_t;
>     }
>
> Unfortunately, different platforms often offer slightly different subsets,
> so
> these #ifs quickly become difficult to maintain. Trying to find common
> themes
> for excluding declarations (e.g. #if !defined(_LIBCPP_HAS_NO_FILE)) is
> vain,
> because the subset provided by a platform is often arbitrary. For example,
> I've
> seen platforms where malloc() and free() were not provided, however
> operator new
> was -- so trying to carve out something like _LIBCPP_HAS_NO_ALLOCATION
> wouldn't
> really make sense.
>
> Given the difficulty of manually excluding such using declarations, I came
> to
> the conclusion that what we wanted was often something like (pseudocode):
>
>     namespace std {
>     #if __has_declaration(::FILE)
>         using ::FILE;
>     #endif
>
>     #if __has_declaration(::fpos_t)
>         using ::fpos_t;
>     #endif
>
>     #if __has_declaration(::size_t)
>         using ::size_t;
>     #endif
>
>         ...
>     }
>
> Basically, we want to import each declaration into namespace std only if
> the
> global namespace has such a declaration.
>
> Now, I understand this raises several questions. Of course, this couldn't
> be
> done at the preprocessor level because I don't think we can know whether a
> declaration exists at that time. However, I was curious to ask on this list
> whether someone could think of a reasonable way to solve this problem.
>
> Having some mechanism for doing this would be a huge life improvement for
> libc++.
>

Do you always want to guard a namespace & scope using-declaration? We could
probably support an attribute on such declarations that would result in
them having no effect if no names are found -- or perhaps being invalid to
reference if no names are found. (Otherwise, Clang already has some support
for MSVC's __if_exists, but it's got some deep semantic problems, and I
don't think we should recommend its use.)

Cheers,
> Louis
>
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