ISO/IEC JTC1 SC22 WG21
Document Number: P0377R0
Audience: Library Evolution Working Group
Matt Calabrese (metaprogrammingtheworld@gmail.com)
2016-05-29
Following EWG's acceptance of P0127, "Declaring non-type template arguments with auto," [1] this paper proposes a form of std::integral_constant that uses the new language feature such that the value type does not need to be explicitly specified by users.
Usage of std::integral_constant can be redundant in that both an explicit value type needs to be specified and a value of that type. In practice, now the value type can be deduced from the value that is specified although there was no way to do this prior to the acceptance of P0127. Already, a standard alias for bool values has been accepted called std::bool_constant [2], and Boost.MPL has long provided convenience wrappers for constants of other various buit-in types, such as int_, char_, and long_ [3]. While a form of std::integral_constant template with a deduced value type would not completely obviate the utility of such named facilities, it would satisfy many uses of them and remove a little bit of redundancy.
There are several ways that such a facility could be provided, each with their own subtleties. The specification that is recommended here is a conservative one where std::constant is an alias of std::integral_constant:
template <auto V> using constant = integral_constant<decltype(V), V>;
The above definition does not change the definition of std::integral_constant, so existing uses of std::integral_constant cannot break. Because std::constant would be an alias of std::integral_constant, developers can safely use it for convenience in most places that std::integral_constant is required.
This formulation of std::constant has a limitation. First, it cannot be used to form a std::integral_constant whose first argument is a reference type. This limitation could be removed by specifying std::constant to have a decltype(auto) parameter instead of an auto parameter, though that would make it possible for users to accidentally specify a reference. It is unclear how unfortunate this limitation may be, as use of std::integral_constant with reference types may be considered by many to be a misuse, and if the type were automatically deduced to be a reference type, it would likely be a programmer error anyway.
The author of the underlying language facility has expressed that in an ideal world, he feels std::constant should be the primary template, with std::integral_constant reformulated to be an alias of std::constant. This option should be considered, although it does risk breaking code, such as code that uses std::integral_constant to create references. If this breakage is considered acceptable, then this option should be reconsidered.
Thanks to James Touton for proposing the underlying language feature and for his feedback on this paper.
[1] James Touton, Mike Spertus: "Declaring non-type template arguments with auto" P0088R0 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0127r1.html
[2] Zhihao Yuan: "Wording for bool_constant" http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4389.html
[3] Aleksey Gurtovoy and David Abrahams: "The MPL Reference Manual" http://www.boost.org/doc/libs/1_61_0/libs/mpl/doc/refmanual/numeric.html