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Subject: Re: [boost] Pervasive dependency upon mpl::bool_
From: Jonathan Wakely (jwakely.boost_at_[hidden])
Date: 2014-06-01 17:00:28

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.

> But really, if a header, whose entire purpose is to define an empty struct,
> can't get by without including type traits and mpl, we've lost the
> dependency game before it's even started.

That pervasive dependency and the hundreds of lines that get included
are why I immediately replaced all boost type traits with the C++11
equivalents when a couple of projects moved to a C++11 compiler.

Would it be possible to define true_ and false_ with a lot less code?

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