Ping. On Thu, Nov 5, 2015 at 6:32 PM, Richard Smith <rich...@metafoo.co.uk> wrote:
> Ping. > > On Thu, Oct 29, 2015 at 5:21 PM, Richard Smith <rich...@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. >> > >
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