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Subject: Re: [boost] Pervasive dependency upon mpl::bool_
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2014-06-01 18:21:36

On Sunday 01 June 2014 22:00:28 Jonathan Wakely wrote:
> On 1 June 2014 21:40, Peter Dimov wrote:
> > The problem is, though, that all type traits do derive from mpl::true_ or
> > mpl::false_, and I'm not sure if rewriting blank.hpp in the above manner
> > is
> > actually correct. Should type trait specializations always derive from
> > mpl::true_ or mpl::false_? Will I break something if I specialize a type
> > trait to not derive from mpl::bool_? Or is a nested ::value enough? Who
> > knows. :-)
> Anecdata: I often do tag dispatching on traits and overload a
> function to take either std::true_type or std::false_type, where I
> know the trait is guaranteed to derive from one or the other (which is
> true for all the standard type traits, and others that I write). It
> wouldn't surprise me if other people do that with mpl::true_ and
> mpl::false_ , relying on traits deriving from exactly those types.

I sometimes do this, including outside Boost.

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