Re: [boost] enabling constexpr with Visual studio 2015 Update 3 (RC)

Subject: Re: [boost] enabling constexpr with Visual studio 2015 Update 3 (RC)
From: Paul A. Bristow (pbristow_at_[hidden])
Date: 2016-06-24 04:38:55

> -----Original Message-----
> From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Billy O'Neal (VC LIBS)
> Sent: 23 June 2016 18:57
> To: boost_at_[hidden]
> Subject: Re: [boost] enabling constexpr with Visual studio 2015 Update 3 (RC)
> 
> >I think it may even be a C++17 feature and I have tried /std:c++14 and  /std:c++latest to try to enable this.
> 
> 
> Extended constexpr was a C++14 feature. /std:c++14 and /std:c++latest don't turn this on because C1XX does not have
> support for it. We implement only C++11 constexpr at this time, which means constexpr functions must be of the form
> "return expression;".

:-(        (especially when /std:c++14 or /std:c++latest imply this should be supported).

I look forward to update 4 (or 5...)   ;-)

Paul

 ________________________________
> From: Boost <boost-bounces_at_[hidden]> on behalf of Paul A. Bristow <pbristow_at_[hidden]>
> Sent: Thursday, June 23, 2016 8:46:30 AM
> To: boost_at_[hidden]
> Subject: Re: [boost] enabling constexpr with Visual studio 2015 Update 3 (RC)
> 
> > -----Original Message-----
> > From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Billy O'Neal (VC LIBS)
> > Sent: 23 June 2016 15:29
> > To: boost_at_[hidden]; boost_at_[hidden]
> > Subject: Re: [boost] enabling constexpr with Visual studio 2015 Update 3 (RC)
> >
> > That is C++14 constexpr; C1XX only supports C++11 constexpr.
> 
> I think it may even be a C++17 feature and I have tried /std:c++14 and  /std:c++latest to try to enable this.
> 
> https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fen.cppreference.com%2fw%2fcpp%2flanguage%2fcon
> stexpr&data=01%7c01%7cbion%40microsoft.com%7ca35d14a8cc9b416013fd08d39b7d9c7d%7c72f988bf86f141af91ab2d7c
> d011db47%7c1&sdata=z1Qv6kRkG8d7dCFKX4Q8DwB2a4wI3S6w9pftYjeDZdI%3d says
> 
> the function body must be either deleted or defaulted
> 
> * or contain any statements except:*
> 
> an asm declaration.  Not this
> a goto statement.  No goto.
> a statement with a label other than case and default. NO labels.
> a try-block . No trying.
> a definition of a variable of non-literal type.  Is literal (int, float ...)
> a definition of a variable of static or thread storage duration.  No static.
> a definition of a variable for which no initialization is performed.  (variable IS initialized)
> 
> But maybe the language lawyers can advise if I am mistaken.
> 
> either way, it's a showstopper for taking existing code and making constexpr functions.
> 
> But there are more hurdles:
> template<typename FloatingPointType>
> ...
> BOOST_CONSTEXPR_OR_CONST bool is_neg = (f < FloatingPointType(0));
> 
> and
> 
> // Extract the mantissa and exponent.
>       int exp2 = 0; // Initialize to make constexpr possible.
>       BOOST_CONSTEXPR_OR_CONST FloatingPointType fp(frexp((!is_neg) ? f : -f, &exp2));
> 
> both fail to compile on VS 2015 update 3 RC and GCC 5.8.0.
> 
> So there are hoops that look out of reach for now.
> 
> Paul
> 
> > On Thu, Jun 23, 2016 at 7:23 AM -0700, "Paul A. Bristow" <pbristow_at_[hidden]<mailto:pbristow_at_hetp.u-
> net.com>>
> > wrote:
> >
> > > -----Original Message-----
> > > From: Boost [mailto:boost-bounces_at_[hidden]] On Behalf Of Beman Dawes
> > > Sent: 11 June 2016 17:06
> > > To: Boost Developers List
> > > Subject: Re: [boost] enabling constexpr with Visual studio 2015 Update 3 (RC)
> > >
> > > You might want to read the thread "[config] Changes needed for VC++ 2015
> > > Update 3?" if you have not already done so.
> > >
> > > Please file a MSVC bug report if you hit any C++11 constexpr bugs in the
> > > Update 3 RC. They giving very high priority to C++11 bugs affecting Boost
> > > code. They would like us to be able to turn BOOST_NO_CXX11_CONSTEXPR off
> > > for Update 3.
> >
> > Unless I am doing something very silly using VS 2015 Update 3 RC,
> > it is not possible to declare and initialization any constexpr intermediate variables within a constexpr function.
> >
> > constexpr int tripled(int n)
> > {
> >   constexpr int f = 3;  // C3250: 'f': declaration is not allowed in 'constexpr' function body
> >   // constexpr_variable_in_function.cpp(35) : error C3249 : illegal statement or sub - expression for 'constexpr' function
> >
> >   return n * f;
> > }
> > even using /std:c++latest
> >
> > Reported as
> >
> >
> https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fconnect.microsoft.com%2fVisualStudio%2ffeedback%
> >
> 2fdetails%2f2849367&data=01%7c01%7cbion%40microsoft.com%7cff91fe2ba4ea4628db3908d39b71f2e8%7c72f988bf86f1
> > 41af91ab2d7cd011db47%7c1&sdata=s%2fuVgTFXoqGoAfgDEOSv%2bgo5ZkyDQ%2b03m8B6of9icHw%3d
> >
> > My reading of C++17 draft is that is now an added feature, and this work with GCC 5.3.0 (not even the most recent
> > version).
> >
> > Lack of this is a showstopper when trying to define any (mathy) function where it is impossible/impractical to provide a
> > single
> > return statement.
> >
> > Paul
> >
> > ---
> > Paul A. Bristow
> > Prizet Farmhouse
> > Kendal UK LA8 8AB
> > [phone]
> >
> >
> >
> >
> >
> >
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