23 General utilities library [utilities]

23.5 Tuples [tuple]

23.5.3 Class template tuple [tuple.tuple]

namespace std {
  template <class... Types>
    class tuple  {
    public:
      // [tuple.cnstr], tuple construction
      EXPLICIT constexpr tuple();
      EXPLICIT constexpr tuple(const Types&...);         // only if sizeof...(Types) >= 1
      template <class... UTypes>
        EXPLICIT constexpr tuple(UTypes&&...);           // only if sizeof...(Types) >= 1

      tuple(const tuple&) = default;
      tuple(tuple&&) = default;

      template <class... UTypes>
        EXPLICIT constexpr tuple(const tuple<UTypes...>&);
      template <class... UTypes>
        EXPLICIT constexpr tuple(tuple<UTypes...>&&);

      template <class U1, class U2>
        EXPLICIT constexpr tuple(const pair<U1, U2>&);   // only if sizeof...(Types) == 2
      template <class U1, class U2>
        EXPLICIT constexpr tuple(pair<U1, U2>&&);        // only if sizeof...(Types) == 2

      // allocator-extended constructors
      template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a);
      template <class Alloc>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, const Types&...);
      template <class Alloc, class... UTypes>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, UTypes&&...);
      template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a, const tuple&);
      template <class Alloc>
        tuple(allocator_arg_t, const Alloc& a, tuple&&);
      template <class Alloc, class... UTypes>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&);
      template <class Alloc, class... UTypes>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&);
      template <class Alloc, class U1, class U2>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&);
      template <class Alloc, class U1, class U2>
        EXPLICIT tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

      // [tuple.assign], tuple assignment
      tuple& operator=(const tuple&);
      tuple& operator=(tuple&&) noexcept(see below);

      template <class... UTypes>
        tuple& operator=(const tuple<UTypes...>&);
      template <class... UTypes>
        tuple& operator=(tuple<UTypes...>&&);

      template <class U1, class U2>
        tuple& operator=(const pair<U1, U2>&);              // only if sizeof...(Types) == 2
      template <class U1, class U2>
        tuple& operator=(pair<U1, U2>&&);                   // only if sizeof...(Types) == 2

      // [tuple.swap], tuple swap
      void swap(tuple&) noexcept(see below);
    };

  template<class... UTypes>
    tuple(UTypes...) -> tuple<UTypes...>;
  template<class T1, class T2>
    tuple(pair<T1, T2>) -> tuple<T1, T2>;
  template<class Alloc, class... UTypes>
    tuple(allocator_arg_t, Alloc, UTypes...) -> tuple<UTypes...>;
  template<class Alloc, class T1, class T2>
    tuple(allocator_arg_t, Alloc, pair<T1, T2>) -> tuple<T1, T2>;
  template<class Alloc, class... UTypes>
    tuple(allocator_arg_t, Alloc, tuple<UTypes...>) -> tuple<UTypes...>;
}

23.5.3.1 Construction [tuple.cnstr]

For each tuple constructor, an exception is thrown only if the construction of one of the types in Types throws an exception.

The defaulted move and copy constructor, respectively, of tuple shall be a constexpr function if and only if all required element-wise initializations for copy and move, respectively, would satisfy the requirements for a constexpr function. The defaulted move and copy constructor of tuple<> shall be constexpr functions.

The destructor of tuple shall be a trivial destructor if (is_­trivially_­destructible_­v<Types> && ...) is true.

In the constructor descriptions that follow, let i be in the range [0, sizeof...(Types)) in order, be the type in Types, and be the type in a template parameter pack named UTypes, where indexing is zero-based.

EXPLICIT constexpr tuple();

Effects: Value-initializes each element.

Remarks: This constructor shall not participate in overload resolution unless is_­default_­constructible_­v<> is true for all i. [Note: This behavior can be implemented by a constructor template with default template arguments. end note] The constructor is explicit if and only if is not implicitly default-constructible for at least one i. [Note: This behavior can be implemented with a trait that checks whether a const & can be initialized with {}. end note]

EXPLICIT constexpr tuple(const Types&...);

Effects: Initializes each element with the value of the corresponding parameter.

Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) >= 1 and is_­copy_­constructible_­v<> is true for all i. The constructor is explicit if and only if is_­convertible_­v<const &, > is false for at least one i.

template <class... UTypes> EXPLICIT constexpr tuple(UTypes&&... u);

Effects: Initializes the elements in the tuple with the corresponding value in std​::​forward<UTypes>(u).

Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and sizeof...(Types) >= 1 and is_­constructible_­v<, &&> is true for all i. The constructor is explicit if and only if is_­convertible_­v<&&, > is false for at least one i.

tuple(const tuple& u) = default;

Requires: is_­copy_­constructible_­v<> is true for all i.

Effects: Initializes each element of *this with the corresponding element of u.

tuple(tuple&& u) = default;

Requires: is_­move_­constructible_­v<> is true for all i.

Effects: For all i, initializes the element of *this with std​::​forward<>(get<i>(u)).

template <class... UTypes> EXPLICIT constexpr tuple(const tuple<UTypes...>& u);

Effects: Initializes each element of *this with the corresponding element of u.

Remarks: This constructor shall not participate in overload resolution unless

  • sizeof...(Types) == sizeof...(UTypes) and

  • is_­constructible_­v<, const &> is true for all i, and

  • sizeof...(Types) != 1, or (when Types... expands to T and UTypes... expands to U)
    !is_­convertible_­v<const tuple<U>&, T> && !is_­constructible_­v<T, const tuple<U>&>
    && !is_­same_­v<T, U>
    is true.

The constructor is explicit if and only if is_­convertible_­v<const &, > is false for at least one i.

template <class... UTypes> EXPLICIT constexpr tuple(tuple<UTypes...>&& u);

Effects: For all i, initializes the element of *this with std​::​forward<>(get<i>(u)).

Remarks: This constructor shall not participate in overload resolution unless

  • sizeof...(Types) == sizeof...(UTypes), and

  • is_­constructible_­v<, &&> is true for all i, and

  • sizeof...(Types) != 1, or (when Types... expands to T and UTypes... expands to U)
    !is_­convertible_­v<tuple<U>, T> && !is_­constructible_­v<T, tuple<U>> &&
    !is_­same_­v<T, U>
    is true.

The constructor is explicit if and only if is_­convertible_­v<&&, > is false for at least one i.

template <class U1, class U2> EXPLICIT constexpr tuple(const pair<U1, U2>& u);

Effects: Initializes the first element with u.first and the second element with u.second.

Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == 2, is_­constructible_­v<, const U1&> is true and is_­constructible_­v<, const U2&> is true.

The constructor is explicit if and only if is_­convertible_­v<const U1&, > is false or is_­convertible_­v<const U2&, > is false.

template <class U1, class U2> EXPLICIT constexpr tuple(pair<U1, U2>&& u);

Effects: Initializes the first element with std​::​forward<U1>(u.first) and the second element with std​::​forward<U2>(u.second).

Remarks: This constructor shall not participate in overload resolution unless sizeof...(Types) == 2, is_­constructible_­v<, U1&&> is true and is_­constructible_­v<, U2&&> is true.

The constructor is explicit if and only if is_­convertible_­v<U1&&, > is false or is_­convertible_­v<U2&&, > is false.

template <class Alloc> tuple(allocator_arg_t, const Alloc& a); template <class Alloc> EXPLICIT tuple(allocator_arg_t, const Alloc& a, const Types&...); template <class Alloc, class... UTypes> EXPLICIT tuple(allocator_arg_t, const Alloc& a, UTypes&&...); template <class Alloc> tuple(allocator_arg_t, const Alloc& a, const tuple&); template <class Alloc> tuple(allocator_arg_t, const Alloc& a, tuple&&); template <class Alloc, class... UTypes> EXPLICIT tuple(allocator_arg_t, const Alloc& a, const tuple<UTypes...>&); template <class Alloc, class... UTypes> EXPLICIT tuple(allocator_arg_t, const Alloc& a, tuple<UTypes...>&&); template <class Alloc, class U1, class U2> EXPLICIT tuple(allocator_arg_t, const Alloc& a, const pair<U1, U2>&); template <class Alloc, class U1, class U2> EXPLICIT tuple(allocator_arg_t, const Alloc& a, pair<U1, U2>&&);

Requires: Alloc shall meet the requirements for an Allocator.

Effects: Equivalent to the preceding constructors except that each element is constructed with uses-allocator construction.

23.5.3.2 Assignment [tuple.assign]

For each tuple assignment operator, an exception is thrown only if the assignment of one of the types in Types throws an exception. In the function descriptions that follow, let i be in the range [0, sizeof...​(Types)) in order, be the type in Types, and be the type in a template parameter pack named UTypes, where indexing is zero-based.

tuple& operator=(const tuple& u);

Effects: Assigns each element of u to the corresponding element of *this.

Remarks: This operator shall be defined as deleted unless is_­copy_­assignable_­v<> is true for all i.

Returns: *this.

tuple& operator=(tuple&& u) noexcept(see below);

Effects: For all i, assigns std​::​forward<>(get<i>(u)) to get<i>(*this).

Remarks: This operator shall be defined as deleted unless is_­move_­assignable_­v<> is true for all i.

Remarks: The expression inside noexcept is equivalent to the logical and of the following expressions:

is_nothrow_move_assignable_v<>

where is the type in Types.

Returns: *this.

template <class... UTypes> tuple& operator=(const tuple<UTypes...>& u);

Effects: Assigns each element of u to the corresponding element of *this.

Remarks: This operator shall not participate in overload resolution unless sizeof...(Types) == sizeof...(UTypes) and is_­assignable_­v<&, const &> is true for all i.

Returns: *this.

template <class... UTypes> tuple& operator=(tuple<UTypes...>&& u);

Effects: For all i, assigns std​::​forward<>(get<i>(u)) to get<i>(*this).

Remarks: This operator shall not participate in overload resolution unless is_­assignable_­v<&, &&> == true for all i and sizeof...(Types) == sizeof...(UTypes).

Returns: *this.

template <class U1, class U2> tuple& operator=(const pair<U1, U2>& u);

Effects: Assigns u.first to the first element of *this and u.second to the second element of *this.

Remarks: This operator shall not participate in overload resolution unless sizeof...(Types) == 2 and is_­assignable_­v<&, const U1&> is true for the first type in Types and is_­assignable_­v<&, const U2&> is true for the second type in Types.

Returns: *this.

template <class U1, class U2> tuple& operator=(pair<U1, U2>&& u);

Effects: Assigns std​::​forward<U1>(u.first) to the first element of *this and
std​::​forward<U2>(u.second) to the second element of *this.

Remarks: This operator shall not participate in overload resolution unless sizeof...(Types) == 2 and is_­assignable_­v<&, U1&&> is true for the first type in Types and is_­assignable_­v<&, U2&&> is true for the second type in Types.

Returns: *this.

23.5.3.3 swap [tuple.swap]

void swap(tuple& rhs) noexcept(see below);

Requires: Each element in *this shall be swappable with ([swappable.requirements]) the corresponding element in rhs.

Effects: Calls swap for each element in *this and its corresponding element in rhs.

Remarks: The expression inside noexcept is equivalent to the logical and of the following expressions:

is_nothrow_swappable_v<>

where is the type in Types.

Throws: Nothing unless one of the element-wise swap calls throws an exception.

23.5.3.4 Tuple creation functions [tuple.creation]

In the function descriptions that follow, the members of a parameter pack XTypes are denoted by X for i in [0, sizeof...(XTypes)) in order, where indexing is zero-based.

template<class... TTypes> constexpr tuple<VTypes...> make_tuple(TTypes&&... t);

The pack VTypes is defined as follows. Let U be decay_­t<T> for each T in TTypes. If U is a specialization of reference_­wrapper, then V in VTypes is U​::​type&, otherwise V is U.

Returns: tuple<VTypes...>(std​::​forward<TTypes>(t)...).

[Example:

int i; float j;
make_tuple(1, ref(i), cref(j))

creates a tuple of type tuple<int, int&, const float&>. end example]

template<class... TTypes> constexpr tuple<TTypes&&...> forward_as_tuple(TTypes&&... t) noexcept;

Effects: Constructs a tuple of references to the arguments in t suitable for forwarding as arguments to a function. Because the result may contain references to temporary variables, a program shall ensure that the return value of this function does not outlive any of its arguments (e.g., the program should typically not store the result in a named variable).

Returns: tuple<TTypes&&...>(std​::​forward<TTypes>(t)...).

template<class... TTypes> constexpr tuple<TTypes&...> tie(TTypes&... t) noexcept;

Returns: tuple<TTypes&...>(t...). When an argument in t is ignore, assigning any value to the corresponding tuple element has no effect.

[Example: tie functions allow one to create tuples that unpack tuples into variables. ignore can be used for elements that are not needed:

int i; std::string s;
tie(i, ignore, s) = make_tuple(42, 3.14, "C++");
// i == 42, s == "C++"

end example]

template <class... Tuples> constexpr tuple<CTypes...> tuple_cat(Tuples&&... tpls);

In the following paragraphs, let be the type in Tuples, be remove_­reference_­t<T>, and be the parameter in the function parameter pack tpls, where all indexing is zero-based.

Requires: For all i, shall be the type tuple<...>, where is the (possibly empty) cv-qualifier-seq and is the parameter pack representing the element types in . Let be the type in . For all the following requirements shall be satisfied:

  • If is deduced as an lvalue reference type, then is_­constructible_­v<, &> == true, otherwise

  • is_­constructible_­v<, &&> == true.

Remarks: The types in CTypes shall be equal to the ordered sequence of the extended types ..., ..., , ..., where n is equal to sizeof...(Tuples). Let ... be the ordered sequence of tuple elements of the resulting tuple object corresponding to the type sequence .

Returns: A tuple object constructed by initializing the type element in ... with

get<>(std::forward<>())

for each valid and each group in order.

[Note: An implementation may support additional types in the parameter pack Tuples that support the tuple-like protocol, such as pair and array. end note]

23.5.3.5 Calling a function with a tuple of arguments [tuple.apply]

template <class F, class Tuple> constexpr decltype(auto) apply(F&& f, Tuple&& t);

Effects: Given the exposition-only function:

template <class F, class Tuple, size_t... I>
constexpr decltype(auto)
    apply_impl(F&& f, Tuple&& t, index_sequence<I...>) {                // exposition only
  return INVOKE(std::forward<F>(f), std::get<I>(std::forward<Tuple>(t))...);
}

Equivalent to:

return apply_impl(std::forward<F>(f), std::forward<Tuple>(t),
                  make_index_sequence<tuple_size_v<decay_t<Tuple>>>{});

template <class T, class Tuple> constexpr T make_from_tuple(Tuple&& t);

Effects: Given the exposition-only function:

template <class T, class Tuple, size_t... I>
constexpr T make_from_tuple_impl(Tuple&& t, index_sequence<I...>) {     // exposition only
  return T(get<I>(std::forward<Tuple>(t))...);
}

Equivalent to:

return make_from_tuple_impl<T>(forward<Tuple>(t),
                               make_index_sequence<tuple_size_v<decay_t<Tuple>>>{});

[Note: The type of T must be supplied as an explicit template parameter, as it cannot be deduced from the argument list. end note]

23.5.3.6 Tuple helper classes [tuple.helper]

template <class T> struct tuple_size;

Remarks: All specializations of tuple_­size shall meet the UnaryTypeTrait requirements with a base characteristic of integral_­constant<size_­t, N> for some N.

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

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

Requires: I < sizeof...(Types). The program is ill-formed if I is out of bounds.

Type: TI is the type of the Ith element of Types, where indexing is zero-based.

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>;

Let TS denote tuple_­size<T> of the cv-unqualified type T. If the expression TS​::​value is well-formed when treated as an unevaluated operand, then each of the three templates shall meet the UnaryTypeTrait requirements with a base characteristic of

integral_constant<size_t, TS::value>

Otherwise, they shall have no member value.

Access checking is performed as if in a context unrelated to TS and T. Only the validity of the immediate context of the expression is considered. [Note: 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. Such side effects are not in the “immediate context” and can result in the program being ill-formed. end note]

In addition to being available via inclusion of the <tuple> header, the three templates are available when either 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_­t<I, T> of the cv-unqualified type T. Then each of the three templates shall meet the TransformationTrait requirements with a member typedef type that names the following type:

  • for the first specialization, add_­const_­t<TE>,

  • for the second specialization, add_­volatile_­t<TE>, and

  • for the third specialization, add_­cv_­t<TE>.

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

23.5.3.7 Element access [tuple.elem]

template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>& get(tuple<Types...>& t) noexcept; template <size_t I, class... Types> constexpr tuple_element_t<I, tuple<Types...>>&& get(tuple<Types...>&& t) noexcept; // Note A template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...>>& get(const tuple<Types...>& t) noexcept; // Note B template <size_t I, class... Types> constexpr const tuple_element_t<I, tuple<Types...>>&& get(const tuple<Types...>&& t) noexcept;

Requires: I < sizeof...(Types). The program is ill-formed if I is out of bounds.

Returns: A reference to the Ith element of t, where indexing is zero-based.

[Note A: If a T in Types is some reference type X&, the return type is X&, not X&&. However, if the element type is a non-reference type T, the return type is T&&. end note]

[Note B: Constness is shallow. If a T in Types is some reference type X&, the return type is X&, not const X&. However, if the element type is a non-reference type T, the return type is const T&. This is consistent with how constness is defined to work for member variables of reference type. end note]

template <class T, class... Types> constexpr T& get(tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr T&& get(tuple<Types...>&& t) noexcept; template <class T, class... Types> constexpr const T& get(const tuple<Types...>& t) noexcept; template <class T, class... Types> constexpr const T&& get(const tuple<Types...>&& t) noexcept;

Requires: The type T occurs exactly once in Types.... Otherwise, the program is ill-formed.

Returns: A reference to the element of t corresponding to the type T in Types....

[Example:

  const tuple<int, const int, double, double> t(1, 2, 3.4, 5.6);
  const int& i1 = get<int>(t);        // OK. Not ambiguous. i1 == 1
  const int& i2 = get<const int>(t);  // OK. Not ambiguous. i2 == 2
  const double& d = get<double>(t);   // ERROR. ill-formed

end example]

[Note: The reason get is a non-member function is that if this functionality had been provided as a member function, code where the type depended on a template parameter would have required using the template keyword. end note]

23.5.3.8 Relational operators [tuple.rel]

template<class... TTypes, class... UTypes> constexpr bool operator==(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Requires: For all i, where 0 <= i and i < sizeof...(TTypes), get<i>(t) == get<i>(u) is a valid expression returning a type that is convertible to bool. sizeof...(TTypes) == sizeof...(UTypes).

Returns: true if get<i>(t) == get<i>(u) for all i, otherwise false. For any two zero-length tuples e and f, e == f returns true.

Effects: The elementary comparisons are performed in order from the zeroth index upwards. No comparisons or element accesses are performed after the first equality comparison that evaluates to false.

template<class... TTypes, class... UTypes> constexpr bool operator<(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Requires: For all i, where 0 <= i and i < sizeof...(TTypes), both get<i>(t) < get<i>(u) and get<i>(u) < get<i>(t) are valid expressions returning types that are convertible to bool. sizeof...(TTypes) == sizeof...(UTypes).

Returns: The result of a lexicographical comparison between t and u. The result is defined as: (bool)(get<0>(t) < get<0>(u)) || (!(bool)(get<0>(u) < get<0>(t)) && t < u), where r for some tuple r is a tuple containing all but the first element of r. For any two zero-length tuples e and f, e < f returns false.

template<class... TTypes, class... UTypes> constexpr bool operator!=(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Returns: !(t == u).

template<class... TTypes, class... UTypes> constexpr bool operator>(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Returns: u < t.

template<class... TTypes, class... UTypes> constexpr bool operator<=(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Returns: !(u < t).

template<class... TTypes, class... UTypes> constexpr bool operator>=(const tuple<TTypes...>& t, const tuple<UTypes...>& u);

Returns: !(t < u).

[Note: The above definitions for comparison functions do not require t (or u) to be constructed. It may not even be possible, as t and u are not required to be copy constructible. Also, all comparison functions are short circuited; they do not perform element accesses beyond what is required to determine the result of the comparison. end note]

23.5.3.9 Tuple traits [tuple.traits]

template <class... Types, class Alloc> struct uses_allocator<tuple<Types...>, Alloc> : true_type { };

Requires: Alloc shall be an Allocator.

[Note: Specialization of this trait informs other library components that tuple can be constructed with an allocator, even though it does not have a nested allocator_­type. end note]

23.5.3.10 Tuple specialized algorithms [tuple.special]

template <class... Types> void swap(tuple<Types...>& x, tuple<Types...>& y) noexcept(see below);

Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<> is true for all i, where . The expression inside noexcept is equivalent to:

noexcept(x.swap(y))

Effects: As if by x.swap(y).