Ping. On Mon, Nov 23, 2015 at 6:55 PM, Richard Smith <rich...@metafoo.co.uk> wrote:
> 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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