N4115, Parameter Pack Searching (original) (raw)

Searching for Types in Parameter Packs

Bill Seymour

Stephan T. Lavavej
2014-07-04

Abstract:

This paper proposes that a mechanism for finding types in parameter packs be added to the standard library.

The basic idea:

This paper proposes three new class templates for <type_traits>. One is just a holder of a parameter pack; the other two report at compile time whether a parameter pack contains one or more types.

Possible implementations of the last two are shown as proof of concept. They both compile to either std::true_type or std::false_type.

And in the spirit ofN3854, this paper also adds the “_v” values.

A class template that just holds a parameter pack:

template <class... T> struct packer { };

This is inspired by std::tuple, but it lacks members, so it could serve as an empty base class, and an object of the ultimate type could always be instantiated (even if the parameter pack contains voidor some type that lacks a default constructor).

Whether type T is in parameter pack P:

template <class T, class... P> struct is_contained_in;

template <class T, class... P>
  constexpr bool is_contained_in_v = is_contained_in<T,P>::value;

One possible implementation:

template <class T> struct is_contained_in<T> : false_type { };

template <class First, class... Rest>
  struct is_contained_in<First, First, Rest...> : true_type { };

template <class T, class First, class... Rest>
  struct is_contained_in<T, First, Rest...>
    : is_contained_in<T, Rest...> { };

Whether a packer, T, has a parameter pack that’s a superset of the pack of some minimum packer, U:

template <class T, class U> struct contains_types;

template <class T, class U>
  constexpr bool contains_types_v = contains_types<T,U>::value;

One possible implementation:

template <class... TPack>
  struct contains_types<packer<TPack...>, packer<>> : true_type { };

template <class... TPack, class UFirst, class... URest>
  struct contains_types<packer<TPack...>, packer<UFirst, URest...>>
    : integral_constant<bool,
        is_contained_in<UFirst, TPack...>::value &&
        contains_types<packer<TPack...>, packer<URest...>>::value> { };

Standardese relative toN3936:

In 20.10.2 Header <type_traits> synopsis [meta.type.synop]:

• Change “// 20.10.3, _helper class:_”
  • Make the heading plural:

    // 20.10.3, helper classes:

  • Add at the end:
    template<class... T> struct packer;
• Add to “// 20.10.6, _type relations:_”
  template <class T, class... P> struct is_contained_in;
  template <class T, class U> struct contains_types;
  template <class T, class... P>
  constexpr bool is_contained_in_v = is_contained_in<T,P>::value;
  template <class T, class U>
  constexpr bool contains_types_v = contains_types<T,U>::value;

In 20.10.3 Helper classes [meta.help]:

• Add just before the closing brace of namespace std:
  template<class... T> struct packer { };
• Add a new paragraph after paragraph 1:

  The class template packer simply holds a parameter pack.

In 20.10.6 Relationships between types [meta.rel], add to the end of Table 51:

template <class T, class... P>struct is_contained_in;	Type T is containedin parameter pack P
template <class T, class U>struct contains_types;	T’s parameter pack containsat least one instanceof each of the typesinU’s parameter pack	T and U are packers (20.10.3).

Reply to: stdbill.h@pobox.com