Re: is_constant_evaluated

From: John Maddock (jz.maddock_at_[hidden])
Date: 2021-08-04 18:26:29

On 04/08/2021 19:05, Robert Ramey via Boost wrote:
> On 8/3/21 4:29 PM, Robert Ramey via Boost wrote:
>> Any chance of seeing a feature macro for HAS_IS_CONSTANT_EVALUATED 
>> appearing in Boost.Config any time soon?
>
> I guess it's not really necessary as std::is_constant_evaluated is 
> slated for C++20.  And it includes a corresponding feature macro so it 
> shouldn't be necessary for boost.config to include this functionality. 
> I might have preferred the "unified" model of using boost.config for 
> all these but I guess the easiest is just to role with the times.

Actually there is a roll for Boost.Config here: 
std::is_constant_evaluated is valid in C++20 mode only, as is it's 
feature detection macro.  However, the underlying compiler intrinsic 
works just fine all the way back to C++11 mode as long as the compiler 
has implemented it.

Multiprecision has the logic for this already: 
https://github.com/boostorg/multiprecision/blob/dd4d566055ff6eee6006f8f954efd3035bdf442a/include/boost/multiprecision/detail/number_base.hpp#L96-L132 
albeit somewhat complexified because it uses __builtin_constant_p to 
approximate std::is_constant_evaluated on older gcc versions.

John.

-- 
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus