[libcxx] Reinstate <string.h> and fix overload sets to be const-correct wherever possible

[Richard Smith via cfe-commits]

Ping.

On Thu, Oct 29, 2015 at 5:21 PM, Richard Smith <richard at metafoo.co.uk>
wrote:

> Hi,
>
> The attached patch undoes the revert of r249929, and adds an extension to
> allow <string.h> (and <wchar.h>) to work properly even in environments such
> as iOS where the underlying libc does not provide C++'s const-correct
> overloads of strchr and friends.
>
> This works as follows:
>
>  * The macro _LIBCPP_PREFERRED_OVERLOAD is, where possible, defined by
> <__config> to an attribute that provides the following semantics:
>    - A function declaration with the attribute declares a different
> function from a function declaration without the attribute.
>    - Overload resolution prefers a function with the attribute over a
> function without.
>  * For each of the functions that has a "broken" signature in C, if we
> don't believe that the C library provided the C++ signatures, and we have a
> _LIBCPP_PREFERRED_OVERLOAD, then we add the C++ declarations and mark them
> as preferred over the C overloads.
>  * The overloads provided in namespace std always exactly match those in
> ::.
>
>
> This results in the following changes in cases where the underlying libc
> provides the C signature not the C++ one, compared to the status quo:
>
>
> <string.h>:
>
>       char *strchr(const char*, int) // #1
>       char *strchr(char*, int) // #2
>       const char *strchr(const char*, int) // #3
>
> We used to provide #1 and #2 in namespace std (in <cstring>) and only #1
> in global namespace (in <string.h>).
>
> For a very old clang or non-clang compiler, we now have only #1 in both
> places (note that #2 is essentially useless). This is unlikely to be a
> visible change in real code, but it's slightly broken either way and we
> can't fix it.
>
> For newer clang (3.6 onwards?), we now have correct signatures (#2 and #3)
> in :: and std (depending on header). Taking address of strchr requires
> ~trunk clang (but it didn't work before either, so this is not really a
> regression).
>
>
> <wchar.h>:
>
>       wchar_t *wcschr(const wchar_t *, wchar_t) // #1
>       const wchar_t *wcschr(const wchar_t *, wchar_t) // #2
>       wchar_t *wcschr(wchar_t *, wchar_t) // #3
>
> We used to provide #1 in global namespace, and #2 and #3 in namespace std.
> This broke code that uses 'using namespace std;'.
>
> For a very old clang or non-clang compiler, we now have #1 in global
> namespace and namespace std. This fixes the ambiguity errors, but decreases
> const-correctness in this case. On the whole, this seems like an
> improvement to me.
>
> For newer clang, we now have correct signatures (#2 and #3) in :: and std
> (depending on header). As above, taking address doesn't work unless you're
> using very recent Clang (this is not a regression in ::, but is a
> regression in namespace std).
>
>
> To summarize, we previously had ad-hoc, inconsistent, slightly broken
> rules for <cstring> and <cwchar>, and with this patch we fix the overload
> set to give the exact C++ semantics where possible (for all recent versions
> of Clang), and otherwise leave the C signatures alone.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20151105/0d07d302/attachment.html>