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Subject: Re: [boost] [config] consexpr workaround
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-11-15 07:29:50

vicente.botet wrote:
> "John Maddock" <boost.regex_at_[hidden]> wrote:
> >>For this case the existing macro BOOST_STATIC_CONSTANT
> >>could be modified to use BOOST_CONSTEXPR_OR_CONST when
> >>static const was used.
> >
> > I don't see how that can work:
> >
> > * If the type being initialized is known to be an integer
> > type then we can use BOOST_STATIC_CONSTANT and we wouldn't
> > gain anything from using constexp?
> I think I'm missing something. Do you mean that we don't need
> to use constexpr for the variable value?

The purpose of constexpr is to mark something as a compile time constant such that it can be used in other compile time expressions, including types that weren't allowed as compile time constants before. It allows for constant-expression functions, data, and even constructors. Constant-expression data disallows dynamic initialization which const objects can undergo. Without constexpr support, neither constant-expression functions nor constructors are supported, nor is floating point constant-expression data. Thus, const is inappropriate in those cases, so BOOST_CONSTEXPR_OR_CONST cannot be used.

For non-floating point constant-expression data, const could work fairly well, but it permits dynamic initialization, so for platforms supporting constexpr, code would fail to compile that mistakenly compiled on platforms without constexpr support. Thus, BOOST_CONSTEXPR_OR_CONST would lead to maintenance issues. That means library code using BOOST_CONSTEXPR_OR_CONST and client code using that library code must either assume that BOOST_CONSTEXPR_OR_CONST merely means "const" or it must be conditionally compiled to account for either "const" or "constexpr." In that case, BOOST_CONSTEXPR_OR_CONST does nothing to help.

I don't think BOOST_CONSTEXPR_OR_CONST is helpful.

Rob Stewart robert.stewart_at_[hidden]
Software Engineer, Core Software using std::disclaimer;
Susquehanna International Group, LLP

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