23 General utilities library [utilities]

23.2 Utility components [utility]

This subclause contains some basic function and class templates that are used throughout the rest of the library.

23.2.1 Header <utility> synopsis [utility.syn]

#include <initializer_list>     // see [initializer_list.syn]

namespace std {
  // [operators], operators
  namespace rel_ops {
    template<class T> bool operator!=(const T&, const T&);
    template<class T> bool operator> (const T&, const T&);
    template<class T> bool operator<=(const T&, const T&);
    template<class T> bool operator>=(const T&, const T&);
  }

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

  // [utility.exchange], exchange
  template <class T, class U = T>
    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 <class T1, class T2> struct tuple_element<0, pair<T1, T2>>;
  template <class T1, class T2> struct tuple_element<1, 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{};

  // floating-point format for primitive numerical conversion
  enum class chars_­format {
    scientific = unspecified,
    fixed = unspecified,
    hex = unspecified,
    general = fixed | scientific
  };



  // [utility.to.chars], primitive numerical output conversion
  struct to_chars_result {
    char* ptr;
    error_code ec;
  };

  to_chars_result to_chars(char* first, char* last, see below value, int base = 10);

  to_chars_result to_chars(char* first, char* last, float value);
  to_chars_result to_chars(char* first, char* last, double value);
  to_chars_result to_chars(char* first, char* last, long double value);

  to_chars_result to_chars(char* first, char* last, float value,
                           chars_format fmt);
  to_chars_result to_chars(char* first, char* last, double value,
                           chars_format fmt);
  to_chars_result to_chars(char* first, char* last, long double value,
                           chars_format fmt);

  to_chars_result to_chars(char* first, char* last, float value,
                           chars_format fmt, int precision);
  to_chars_result to_chars(char* first, char* last, double value,
                           chars_format fmt, int precision);
  to_chars_result to_chars(char* first, char* last, long double value,
                           chars_format fmt, int precision);



  // [utility.from.chars], primitive numerical input conversion
  struct from_chars_result {
    const char* ptr;
    error_code ec;
  };

  from_chars_result from_chars(const char* first, const char* last,
                               see below& value, int base = 10);

  from_chars_result from_chars(const char* first, const char* last, float& value,
                               chars_format fmt = chars_format::general);
  from_chars_result from_chars(const char* first, const char* last, double& value,
                               chars_format fmt = chars_format::general);
  from_chars_result from_chars(const char* first, const char* last, long double& value,
                               chars_format fmt = chars_format::general);
}

The header <utility> defines several types and function templates that are described in this Clause. It also defines the template pair and various function templates that operate on pair objects.

The type chars_­format is a bitmask type with elements scientific, fixed, and hex.

23.2.2 Operators [operators]

To avoid redundant definitions of operator!= out of operator== and operators >, <=, and >= out of operator<, the library provides the following:

template <class T> bool operator!=(const T& x, const T& y);

Requires: Type T is EqualityComparable.

Returns: !(x == y).

template <class T> bool operator>(const T& x, const T& y);

Requires: Type T is LessThanComparable.

Returns: y < x.

template <class T> bool operator<=(const T& x, const T& y);

Requires: Type T is LessThanComparable.

Returns: !(y < x).

template <class T> bool operator>=(const T& x, const T& y);

Requires: Type T is LessThanComparable.

Returns: !(x < y).

In this library, whenever a declaration is provided for an operator!=, operator>, operator>=, or operator<=, and requirements and semantics are not explicitly provided, the requirements and semantics are as specified in this Clause.

23.2.3 swap [utility.swap]

template <class T> void swap(T& a, T& b) noexcept(see below);

Remarks: This function 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>

Requires: Type T shall be MoveConstructible and MoveAssignable.

Effects: Exchanges values stored in two locations.

template <class T, size_t N> void swap(T (&a)[N], T (&b)[N]) noexcept(is_nothrow_swappable_v<T>);

Remarks: This function shall not participate in overload resolution unless is_­swappable_­v<T> is true.

Requires: a[i] shall be swappable with b[i] for all i in the range [0, N).

Effects: As if by swap_­ranges(a, a + N, b).

23.2.4 exchange [utility.exchange]

template <class T, class U = T> T exchange(T& obj, U&& new_val);

Effects: Equivalent to:

T old_val = std::move(obj);
obj = std::forward<U>(new_val);
return old_val;

23.2.5 Forward/move helpers [forward]

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 ([csignal.syn]).

template <class T> constexpr T&& forward(remove_reference_t<T>& t) noexcept; template <class T> constexpr T&& forward(remove_reference_t<T>&& t) noexcept;

Returns: static_­cast<T&&>(t).

Remarks: If the second form is instantiated with an lvalue reference type, the program is ill-formed.

[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;

Returns: static_­cast<remove_­reference_­t<T>&&>(t).

[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;

Returns: std​::​move(x).

23.2.6 Function template as_­const [utility.as_const]

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

Returns: t.

23.2.7 Function template declval [declval]

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

Remarks: If this function is odr-used, the program is ill-formed.

Remarks: The template parameter T of declval may be an incomplete type.

[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]

23.2.8 Primitive numeric output conversion [utility.to.chars]

All functions named to_­chars convert value into a character string by successively filling the range [first, last), where [first, last) is required to be a valid range. If the member ec of the return value is such that the value, when converted to bool, is false, the conversion was successful and the member ptr is the one-past-the-end pointer of the characters written. Otherwise, the member ec has the value errc​::​value_­too_­large, the member ptr has the value last, and the contents of the range [first, last) are unspecified.

The functions that take a floating-point value but not a precision parameter ensure that the string representation consists of the smallest number of characters such that there is at least one digit before the radix point (if present) and parsing the representation using the corresponding from_­chars function recovers value exactly. [Note: This guarantee applies only if to_­chars and from_­chars are executed on the same implementation. end note]

The functions taking a chars_­format parameter determine the conversion specifier for printf as follows: The conversion specifier is f if fmt is chars_­format​::​fixed, e if fmt is chars_­format​::​scientific, a (without leading "0x" in the result) if fmt is chars_­format​::​hex, and g if fmt is chars_­format​::​general.

to_chars_result to_chars(char* first, char* last, see below value, int base = 10);

Requires: base has a value between 2 and 36 (inclusive).

Effects: The value of value is converted to a string of digits in the given base (with no redundant leading zeroes). Digits in the range 10..35 (inclusive) are represented as lowercase characters a..z. If value is less than zero, the representation starts with a minus sign.

Throws: Nothing.

Remarks: The implementation shall provide overloads for all signed and unsigned integer types and char as the type of the parameter value.

to_chars_result to_chars(char* first, char* last, float value); to_chars_result to_chars(char* first, char* last, double value); to_chars_result to_chars(char* first, char* last, long double value);

Effects: value is converted to a string in the style of printf in the "C" locale. The conversion specifier is f or e, chosen according to the requirement for a shortest representation (see above); a tie is resolved in favor of f.

Throws: Nothing.

to_chars_result to_chars(char* first, char* last, float value, chars_format fmt); to_chars_result to_chars(char* first, char* last, double value, chars_format fmt); to_chars_result to_chars(char* first, char* last, long double value, chars_format fmt);

Requires: fmt has the value of one of the enumerators of chars_­format.

Effects: value is converted to a string in the style of printf in the "C" locale.

Throws: Nothing.

to_chars_result to_chars(char* first, char* last, float value, chars_format fmt, int precision); to_chars_result to_chars(char* first, char* last, double value, chars_format fmt, int precision); to_chars_result to_chars(char* first, char* last, long double value, chars_format fmt, int precision);

Requires: fmt has the value of one of the enumerators of chars_­format.

Effects: value is converted to a string in the style of printf in the "C" locale with the given precision.

Throws: Nothing.

See also: ISO C 7.21.6.1

23.2.9 Primitive numeric input conversion [utility.from.chars]

All functions named from_­chars analyze the string [first, last) for a pattern, where [first, last) is required to be a valid range. If no characters match the pattern, value is unmodified, the member ptr of the return value is first and the member ec is equal to errc​::​invalid_­argument. Otherwise, the characters matching the pattern are interpreted as a representation of a value of the type of value. The member ptr of the return value points to the first character not matching the pattern, or has the value last if all characters match. If the parsed value is not in the range representable by the type of value, value is unmodified and the member ec of the return value is equal to errc​::​result_­out_­of_­range. Otherwise, value is set to the parsed value and the member ec is set such that the conversion to bool yields false.

from_chars_result from_chars(const char* first, const char* last, see below& value, int base = 10);

Requires: base has a value between 2 and 36 (inclusive).

Effects: The pattern is the expected form of the subject sequence in the "C" locale for the given nonzero base, as described for strtol, except that no "0x" or "0X" prefix shall appear if the value of base is 16, and except that a minus sign is the only sign that may appear, and only if value has a signed type.

Throws: Nothing.

Remarks: The implementation shall provide overloads for all signed and unsigned integer types and char as the referenced type of the parameter value.

from_chars_result from_chars(const char* first, const char* last, float& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, double& value, chars_format fmt = chars_format::general); from_chars_result from_chars(const char* first, const char* last, long double& value, chars_format fmt = chars_format::general);

Requires: fmt has the value of one of the enumerators of chars_­format.

Effects: The pattern is the expected form of the subject sequence in the "C" locale, as described for strtod, except that

  • the only sign that may appear is a minus sign;

  • if fmt has chars_­format​::​scientific set but not chars_­format​::​fixed, the otherwise optional exponent part shall appear;

  • if fmt has chars_­format​::​fixed set but not chars_­format​::​scientific, the optional exponent part shall not appear; and

  • if fmt is chars_­format​::​hex, the prefix "0x" or "0X" is assumed. [Example: The string 0x123 is parsed to have the value 0 with remaining characters x123. end example]

In any case, the resulting value is one of at most two floating-point values closest to the value of the string matching the pattern.

Throws: Nothing.

See also: ISO C 7.22.1.3, 7.22.1.4