19 General utilities library [utilities]

19.2 Utility components [utility]

19.2.1 Header <utility> synopsis [utility.syn]

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The header <utility> contains some basic function and class templates that are used throughout the rest of the library.
#include <initializer_list>     // see [initializer_list.syn]

namespace std {
  // [utility.swap], swap
  template<class T>
    constexpr void swap(T& a, T& b) noexcept(see below);
  template<class T, size_t N>
    constexpr void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);

  // [utility.exchange], exchange
  template<class T, class U = T>
    constexpr T exchange(T& obj, U&& new_val);

  // [forward], forward/move
  template<class T>
    constexpr T&& forward(remove_reference_t<T>& t) noexcept;
  template<class T>
    constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
  template<class T>
    constexpr remove_reference_t<T>&& move(T&&) noexcept;
  template<class T>
    constexpr conditional_t<
        !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&>
      move_if_noexcept(T& x) noexcept;

  // [utility.as_const], as_­const
  template<class T>
    constexpr add_const_t<T>& as_const(T& t) noexcept;
  template<class T>
    void as_const(const T&&) = delete;

  // [declval], declval
  template<class T>
    add_rvalue_reference_t<T> declval() noexcept;  // as unevaluated operand

  // [intseq], Compile-time integer sequences
  template<class T, T...>
    struct integer_sequence;
  template<size_t... I>
    using index_­sequence = integer_sequence<size_t, I...>;

  template<class T, T N>
    using make_integer_sequence = integer_sequence<T, see below>;
  template<size_t N>
    using make_­index_­sequence = make_integer_sequence<size_t, N>;

  template<class... T>
    using index_­sequence_­for = make_index_sequence<sizeof...(T)>;

  // [pairs], class template pair
  template<class T1, class T2>
    struct pair;

  // [pairs.spec], pair specialized algorithms
  template<class T1, class T2>
    constexpr bool operator==(const pair<T1, T2>&, const pair<T1, T2>&);
  template<class T1, class T2>
    constexpr bool operator!=(const pair<T1, T2>&, const pair<T1, T2>&);
  template<class T1, class T2>
    constexpr bool operator< (const pair<T1, T2>&, const pair<T1, T2>&);
  template<class T1, class T2>
    constexpr bool operator> (const pair<T1, T2>&, const pair<T1, T2>&);
  template<class T1, class T2>
    constexpr bool operator<=(const pair<T1, T2>&, const pair<T1, T2>&);
  template<class T1, class T2>
    constexpr bool operator>=(const pair<T1, T2>&, const pair<T1, T2>&);

  template<class T1, class T2>
    void swap(pair<T1, T2>& x, pair<T1, T2>& y) noexcept(noexcept(x.swap(y)));

  template<class T1, class T2>
    constexpr see below make_pair(T1&&, T2&&);

  // [pair.astuple], tuple-like access to pair
  template<class T> class tuple_size;
  template<size_t I, class T> class tuple_element;

  template<class T1, class T2> struct tuple_size<pair<T1, T2>>;
  template<size_t I, class T1, class T2> struct tuple_element<I, pair<T1, T2>>;

  template<size_t I, class T1, class T2>
    constexpr tuple_element_t<I, pair<T1, T2>>& get(pair<T1, T2>&) noexcept;
  template<size_t I, class T1, class T2>
    constexpr tuple_element_t<I, pair<T1, T2>>&& get(pair<T1, T2>&&) noexcept;
  template<size_t I, class T1, class T2>
    constexpr const tuple_element_t<I, pair<T1, T2>>& get(const pair<T1, T2>&) noexcept;
  template<size_t I, class T1, class T2>
    constexpr const tuple_element_t<I, pair<T1, T2>>&& get(const pair<T1, T2>&&) noexcept;
  template<class T1, class T2>
    constexpr T1& get(pair<T1, T2>& p) noexcept;
  template<class T1, class T2>
    constexpr const T1& get(const pair<T1, T2>& p) noexcept;
  template<class T1, class T2>
    constexpr T1&& get(pair<T1, T2>&& p) noexcept;
  template<class T1, class T2>
    constexpr const T1&& get(const pair<T1, T2>&& p) noexcept;
  template<class T2, class T1>
    constexpr T2& get(pair<T1, T2>& p) noexcept;
  template<class T2, class T1>
    constexpr const T2& get(const pair<T1, T2>& p) noexcept;
  template<class T2, class T1>
    constexpr T2&& get(pair<T1, T2>&& p) noexcept;
  template<class T2, class T1>
    constexpr const T2&& get(const pair<T1, T2>&& p) noexcept;

  // [pair.piecewise], pair piecewise construction
  struct piecewise_construct_t {
    explicit piecewise_construct_t() = default;
  };
  inline constexpr piecewise_construct_t piecewise_construct{};
  template<class... Types> class tuple;        // defined in <tuple>

  // in-place construction
  struct in_place_t {
    explicit in_place_t() = default;
  };
  inline constexpr in_place_t in_place{};
  template<class T>
    struct in_place_type_t {
      explicit in_place_type_t() = default;
    };
  template<class T> inline constexpr in_place_type_t<T> in_place_type{};
  template<size_t I>
    struct in_place_index_t {
      explicit in_place_index_t() = default;
    };
  template<size_t I> inline constexpr in_place_index_t<I> in_place_index{};
}

19.2.2 swap [utility.swap]

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template<class T> constexpr void swap(T& a, T& b) noexcept(see below);
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Remarks: This function is a designated customization point ([namespace.std]) and shall not participate in overload resolution unless is_­move_­constructible_­v<T> is true and is_­move_­assignable_­v<T> is true.
The expression inside noexcept is equivalent to:
is_nothrow_move_constructible_v<T> && is_nothrow_move_assignable_v<T>
2
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Requires: Type T shall be Cpp17MoveConstructible (Table 25) and Cpp17MoveAssignable (Table 27).
3
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Effects: Exchanges values stored in two locations.
🔗
template<class T, size_t N> constexpr void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);
4
#
Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<T> is true.
5
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Requires: a[i] shall be swappable with b[i] for all i in the range [0, N).
6
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Effects: As if by swap_­ranges(a, a + N, b).

19.2.3 exchange [utility.exchange]

🔗
template<class T, class U = T> constexpr T exchange(T& obj, U&& new_val);
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Effects: Equivalent to: 
T old_val = std::move(obj);
obj = std::forward<U>(new_val);
return old_val;

19.2.4 Forward/move helpers [forward]

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The library provides templated helper functions to simplify applying move semantics to an lvalue and to simplify the implementation of forwarding functions.
All functions specified in this subclause are signal-safe.
🔗
template<class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept; template<class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;
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Returns: static_­cast<T&&>(t).
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Remarks: If the second form is instantiated with an lvalue reference type, the program is ill-formed.
4
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[ Example
: 
template<class T, class A1, class A2>
shared_ptr<T> factory(A1&& a1, A2&& a2) {
  return shared_ptr<T>(new T(std::forward<A1>(a1), std::forward<A2>(a2)));
}

struct A {
  A(int&, const double&);
};

void g() {
  shared_ptr<A> sp1 = factory<A>(2, 1.414); // error: 2 will not bind to int&
  int i = 2;
  shared_ptr<A> sp2 = factory<A>(i, 1.414); // OK
}
In the first call to factory, A1 is deduced as int, so 2 is forwarded to A's constructor as an rvalue.
In the second call to factory, A1 is deduced as int&, so i is forwarded to A's constructor as an lvalue.
In both cases, A2 is deduced as double, so 1.414 is forwarded to A's constructor as an rvalue.
— end example
 ]
🔗
template<class T> constexpr remove_reference_t<T>&& move(T&& t) noexcept;
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Returns: static_­cast<remove_­reference_­t<T>&&>(t).
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[ Example
: 
template<class T, class A1>
shared_ptr<T> factory(A1&& a1) {
  return shared_ptr<T>(new T(std::forward<A1>(a1)));
}

struct A {
  A();
  A(const A&);      // copies from lvalues
  A(A&&);           // moves from rvalues
};

void g() {
  A a;
  shared_ptr<A> sp1 = factory<A>(a);                // “a” binds to A(const A&)
  shared_ptr<A> sp1 = factory<A>(std::move(a));     // “a” binds to A(A&&)
}
In the first call to factory, A1 is deduced as A&, so a is forwarded as a non-const lvalue.
This binds to the constructor A(const A&), which copies the value from a.
In the second call to factory, because of the call std​::​move(a), A1 is deduced as A, so a is forwarded as an rvalue.
This binds to the constructor A(A&&), which moves the value from a.
— end example
 ]
🔗
template<class T> constexpr conditional_t< !is_nothrow_move_constructible_v<T> && is_copy_constructible_v<T>, const T&, T&&> move_if_noexcept(T& x) noexcept;
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#
Returns: std​::​move(x).

19.2.5 Function template as_­const [utility.as_const]

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template<class T> constexpr add_const_t<T>& as_const(T& t) noexcept;
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Returns: t.

19.2.6 Function template declval [declval]

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The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands.
🔗
template<class T> add_rvalue_reference_t<T> declval() noexcept; // as unevaluated operand
2
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Remarks: If this function is odr-used, the program is ill-formed.
3
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Remarks: The template parameter T of declval may be an incomplete type.
4
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[ Example
: 
template<class To, class From> decltype(static_cast<To>(declval<From>())) convert(From&&);
declares a function template convert which only participates in overloading if the type From can be explicitly converted to type To.
For another example see class template common_­type ([meta.trans.other]).
— end example
 ]