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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