Re: [boost] New dependency report

Subject: Re: [boost] New dependency report
From: Ion Gaztañaga (igaztanaga_at_[hidden])
Date: 2014-06-08 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