Document Number: D2335R0 Date: 2021-03-11 Author: David Stone ([email protected]) Audience: Library Evolution Working Group (LEWG)
This is not a proposal for a particular feature. Instead, this is a proposal for a new LEWG design policy that future feature proposals would follow.
Type traits in the standard library that produce a value follow a particular pattern: there is one version of the trait that is a struct that has a static value data member, and then another version that is a variable template that has the same name as the struct version of the trait but has _v appened to its name. For example: std::is_same<T, U>::value and std::is_same_v<T, U>. This paper proposes that for all new proposals, we do not define the struct version at all and thus dispense with the _v suffix on the variable template.
The current design came about as an accident of timing. The type_traits header was proposed for C++11 in 2003, and borrowed heavily from Boost. Variable templates were added to C++14 in 2013. The functionality simply did not exist when we standardized the majority of the traits.
- Almost all uses of these traits just want the boolean value, so we should give the better name to the thing people actually want to do
- Instantiating a class template takes longer to compile than instantiating a variable template
- Inconsistency with the existing traits
- Requires writing a wrapper to make instantiating a trait lazy