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

[Howard Hinnant]

On Apr 1, 2012, at 2:38 PM, Richard Smith wrote:

> 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.

Your survey/mapping is helpful.  But what about constexpr constructors?

Howard