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2212. tuple_size for const pair request <tuple> header

Section: 22.2 [utility] Status: C++17 Submitter: Alisdair Meredith Opened: 2012-11-09 Last modified: 2017-07-30

Priority: 3

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Discussion:

The <utility> header declares sufficient of the tuple API to specialize the necessary templates for pair, notably tuple_size and tuple_element. However, it does not make available the partial specializations that support cv-qualified template arguments, so while I can write the following after including only <utility>:

#include <utility>

using TestType = std::pair<int, int>;
static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements");
std::tuple_element<0, TestType>::type var{1};

the following may fail to compile unless I also include <tuple>:

#include <utility>

using TestType = const std::pair<int, int>;
static_assert(2 == std::tuple_size<TestType>(), "Pairs have two elements");
std::tuple_element<0, TestType>::type var{1};

Note, however, that the latter may compile with some standard library implementations but not others, leading to subtle portability issues.

[2013-03-15 Issues Teleconference]

Moved to Open.

Howard notes that we have the same issue with array, so any resolution should apply to that header too.

[2013-10-18 Daniel provides wording]

The suggested wording uses a similar approach as we already have in 24.7 [iterator.range] to ensure that the range access templates are available when at least one of an enumerated list of header files is included.

I also think that the restricted focus on tuple_size of this issue is too narrow and should be extended to the similar partial template specializations of tuple_element as well. Therefore the suggested wording ensures this as well.

[2014-03-27 Library reflector vote]

The issue has been identified as Tentatively Ready based on eight votes in favour.

Proposed resolution:

This wording is relative to N3936.

1. Change 22.4.7 [tuple.helper] as indicated:

   template <class T> class tuple_size<const T>;
   template <class T> class tuple_size<volatile T>;
   template <class T> class tuple_size<const volatile T>;
   

   -3- Let TS denote tuple_size<T> of the cv-unqualified type T. Then each of the three templates shall meet the UnaryTypeTrait requirements (20.10.1) with a BaseCharacteristic of

   integral_constant<size_t, TS::value>
   

   -?- In addition to being available via inclusion of the <tuple> header, each of the three templates are available when any of the headers <array> or <utility> are included.

   template <size_t I, class T> class tuple_element<I, const T>;
   template <size_t I, class T> class tuple_element<I, volatile T>;
   template <size_t I, class T> class tuple_element<I, const volatile T>;
   

   -?- Let TE denote tuple_element<I, T> of the cv-unqualified type T. Then each of the three templates shall meet the TransformationTrait requirements (20.10.1) with a member typedef type that names the following type:

   • for the first specialization, add_const<TE::type>::type,

   • for the second specialization, add_volatile<TE::type>::type, and

   • for the third specialization, add_cv<TE::type>::type.

   -?- In addition to being available via inclusion of the <tuple> header, each of the three templates are available when any of the headers <array> or <utility> are included.