[tuple.helper]

# 22 General utilities library [[utilities]](./#utilities)

## 22.4 Tuples [[tuple]](tuple#helper)

### 22.4.7 Tuple helper classes [tuple.helper]

[🔗](#lib:tuple_size,in_general)

`template<class T> struct tuple_size;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2761)

Except where specified otherwise,
all specializations of tuple_size meet the[*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements")) with a
base characteristic of integral_constant<size_t, N> for some N[.](#1.sentence-1)

[🔗](#lib:tuple_size)

`template<class... Types>
  struct tuple_size<tuple<Types...>> : integral_constant<size_t, sizeof...(Types)> { };
`

[🔗](#lib:tuple_element)

`template<size_t I, class... Types>
  struct tuple_element<I, tuple<Types...>> {
    using type = TI;
  };
`

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2784)

*Mandates*: I < sizeof...(Types)[.](#2.sentence-1)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2788)

*Type*: TI is the
type of the Ith element of Types,
where indexing is zero-based[.](#3.sentence-1)

[🔗](#lib:tuple_size_)

`template<class T> struct tuple_size<const T>;
`

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2801)

Let TS denote tuple_size<T> of the cv-unqualified type T[.](#4.sentence-1)

If the expression TS​::​value is well-formed
when treated as an [unevaluated operand](expr.context#def:unevaluated_operand "7.2.3 Context dependence [expr.context]"), then
each specialization of the template meets the [*Cpp17UnaryTypeTrait*](meta.rqmts#:Cpp17UnaryTypeTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))
with a base characteristic ofintegral_constant<size_t, TS::value>

Otherwise, it has no member value[.](#4.sentence-3)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2812)

Access checking is performed as if in a context
unrelated to TS and T[.](#5.sentence-1)

Only the validity of the immediate context of the expression is considered[.](#5.sentence-2)

[*Note [1](#note-1)*:

The compilation of the expression can result in side effects
such as the instantiation of class template specializations and
function template specializations, the generation of implicitly-defined functions, and so on[.](#5.sentence-3)

Such side effects are not in the “immediate context” and
can result in the program being ill-formed[.](#5.sentence-4)

— *end note*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2824)

In addition to being available via inclusion of the [<tuple>](tuple.syn#header:%3ctuple%3e "22.4.2 Header <tuple> synopsis [tuple.syn]") header,
the template is available
when any of the headers[<array>](array.syn#header:%3carray%3e "23.3.2 Header <array> synopsis [array.syn]"),[<ranges>](ranges.syn#header:%3cranges%3e "25.2 Header <ranges> synopsis [ranges.syn]"), or[<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") are included[.](#6.sentence-1)

[🔗](#lib:tuple_element_)

`template<size_t I, class T> struct tuple_element<I, const T>;
`

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2840)

Let TE denote tuple_element_t<I, T> of the cv-unqualified type T[.](#7.sentence-1)

Then
each specialization of the template meets the [*Cpp17TransformationTrait*](meta.rqmts#:Cpp17TransformationTrait "21.3.2 Requirements [meta.rqmts]") requirements ([[meta.rqmts]](meta.rqmts "21.3.2 Requirements"))
with a member typedef type that names the type add_const_t<TE>[.](#7.sentence-2)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L2845)

In addition to being available via inclusion of the [<tuple>](tuple.syn#header:%3ctuple%3e "22.4.2 Header <tuple> synopsis [tuple.syn]") header,
the template is available
when any of the headers[<array>](array.syn#header:%3carray%3e "23.3.2 Header <array> synopsis [array.syn]"),[<ranges>](ranges.syn#header:%3cranges%3e "25.2 Header <ranges> synopsis [ranges.syn]"), or[<utility>](utility.syn#header:%3cutility%3e "22.2.1 Header <utility> synopsis [utility.syn]") are included[.](#8.sentence-1)