Re: [PATCH] Enable libstdc++ numeric conversions on Cygwin

[Jonathan Wakely <jwakely at redhat dot com>]

On 18/09/15 12:01 -0400, Jennifer Yao wrote:
> Forgot to include the patch.
>
> On Fri, Sep 18, 2015 at 11:17 AM, Jennifer Yao
> <jenny.hyphen.fa@gmail.com> wrote:
> > A number of functions in libstdc++ are guarded by the _GLIBCXX_USE_C99
> > preprocessor macro, which is only defined on systems that pass all of
> > the checks for a large set of C99 functions. Consequently, on systems
> > which lack any of the required C99 facilities (e.g. Cygwin, which
> > lacks some C99 complex math functions), the numeric conversion
> > functions (std::stoi(), std::stol(), std::to_string(), etc.) are not
> > definedâa rather silly outcome, as none of the numeric conversion
> > functions are implemented using C99 math functions.
> >
> > This patch enables numeric conversion functions on the aforementioned
> > systems by splitting the checks for C99 support and defining several
> > new macros (_GLIBCXX_USE_C99_STDIO, _GLIBCXX_USE_C99_STDLIB, and
> > _GLIBCXX_USE_C99_WCHAR), which replace the use of _GLIBCXX_USE_C99 in
> > #if conditionals where appropriate.

(Coming back to this now that Jennifer's copyright assignment is
complete...)

Splitting the _GLIBCXX_USE_C99 macro into more fine-grained macros for
separate features is definitely the right direction.

However your patch also changes the configure tests to use -std=c++0x
(which should be -std=c++11, but that's a minor point). On an OS that
only makes the C99 library available conditionally that will mean that
configure determines that C99 library features are supported, but we
will get errors if we try to use those features in C++03 parts of the
library.

I think a more complete solution is to have two sets of configure
tests and two sets of macros, so that we define _GLIBCXX_USE_C99_STDIO
when C99 stdio is available unconditionally, and define
_GLIBCXX11_USE_C99_STDIO when it's available with -std=c++11.

Then in the library code we can check _GLIBCXX_USE_C99_STDIO if we
want to use C99 features in C++03 code, and check
_GLIBCXX11_USE_C99_STDIO if we want to use the features in C++11 code.

That should still solve the problem for the numeric conversion
functions, because they are defined in C++11 and so would check
_GLIBCXX11_USE_C99_STDIO, which will be defined for newlib.

Other pieces of the library, such as locales, will use
_GLIBCXX_USE_C99_STDIO and that might still be false for newlib (and
for other strict C libraries like the Solaris and FreeBSD libc).

I will make the changes to acinclude.m4 to duplicate the tests, so we
test once with -std=c++98 and once with -std=c++11, and then change
the library to check either _GLIBCXX_xxx or _GLIBCXX11_xxx as
appropriate.