[cfe-commits] [Patch][Review] constexpr-ification of <limits> and random number generators

[Richard Smith]

On Sun, Apr 1, 2012 at 11:23 AM, Howard Hinnant <hhinnant at apple.com> wrote:

> On Apr 1, 2012, at 3:33 AM, Jonathan Sauer wrote:
>
> > Hello,
> >
> > the attached patch constexpr-ifies <limits> as specified in FDIS as well
> as removes the
> > workarounds in libc++'s random number generators due to missing
> constexpr (the workarounds
> > don't work when using a non-standard PRNG). The corresponding tests are
> modified as well.
> >
> > Please review and, if ok, commit.
> >
> >
> > With many thanks in advance,
> > Jonathan
>
> Thanks Jonathan.  I've been meaning to take care of this.  The current
> libc++ is a mess with respect to constexpr.
>
> One of the things I want to do is to allow libc++ to emulate correct
> behavior even when it is compiled in C++03 mode.  And I don't believe this
> patch will do this.
>
> Another thing that is wrong is that I have an ugly hack in <__config> that
> needs to be fixed:
>
> #ifdef _LIBCPP_HAS_NO_CONSTEXPR
> #define constexpr const
> #endif
>
> This is just wrong, but taking it out is going to take a little more work.
>  I haven't nailed down exactly how we want to support both C++03 and C++11
> mode, but I'm imagining something similar to the way we're handling
> noexcept:
>
> #ifndef _LIBCPP_HAS_NO_CONSTEXPR
> #define _CONSTEXPR_1 constexpr
> #define _CONSTEXPR_0 constexpr
> #else
> #define _CONSTEXPR_1 const
> #define _CONSTEXPR_0
> #endif
>
> (better macro names would be great!)
>
> Like I said, this has been on my to-do list, and I guess now is the time.


How about just:

#ifndef _LIBCPP_HAS_NO_CONSTEXPR
#define _LIBCPP_CONSTEXPR constexpr
#else
#define _LIBCPP_CONSTEXPR
#endif

Then use

_LIBCPP_CONSTEXPR const int n = 5;

for variables, and

struct S {
  _LIBCPP_CONSTEXPR int f() const;
};

for functions.
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