
Boost : 
Subject: Re: [boost] New dependency report
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 20140608 17:17:01
El 07/06/2014 17:37, Vicente J. Botet Escriba escribió:
>>
>> To be standard conforming, we would need to add
>> "std::integral_constant<bool, false>" as "type" member. But IMHO this
>> seems a bit redundant.
>>
> Well, currently something more
>
> template <class T, T v>
> struct integral_constant {
>
> static constexpr T value = v;
> typedef T value_type;
> typedef integral_constant<T,v> type;
> constexpr operator value_type() const { return value; }
> constexpr value_type operator()() const { return value; }
>
> };
>
> Vicente C++ International Standard
Yes. The issue is not about deriving from integral_constant if it's a
very simple header with no dependencies. Currently if deriving from
integral_constant:
1) We need to depend on another Boost library (type_traits and its
dependencies, boost::integral_constant derives from mpl::integral_c so
we start adding dependencies)
2) In C++11 compilers we need to choose between boost::integral_constant
or std::integral_constant. A standard conforming trait is not
convertible to boost::true_type.
On the other hand:
static const bool value = true/false
seems very lightweight and portable.
An alternative is to inject std::integral_constant into the boost
namespace or derive boost::integral_constant from std::integral_constant
if it adds more operations.
If integral_constant is needed to define a simple trait, then
integral_constant should be:
 compatible with C++11.
 extremely simple, without any further dependencies, and live in core
(not in type_traits, mpl or any other library).
Just my 2 cents,
Ion
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