22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.8 Comparisons [comparisons]

22.10.8.1 General [comparisons.general]

The library provides basic function object classes for all of the comparison operators in the language ([expr.rel], [expr.eq]).

For templates less, greater, less_equal, and greater_equal, the specializations for any pointer type yield a result consistent with the implementation-defined strict total order over pointers ([defns.order.ptr]).

[Note 1:

If a ()(a, b), (a > b) == greater()(a, b), and so forth.

— end note]

For template specializations less<void>, greater<void>, less_equal<void>, and greater_equal<void>, if the call operator calls a built-in operator comparing pointers, the call operator yields a result consistent with the implementation-defined strict total order over pointers.

22.10.8.2 Class template equal_to [comparisons.equal.to]

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template<class T = void> struct equal_to {
  constexpr bool operator()(const T& x, const T& y) const;
};

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constexpr bool operator()(const T& x, const T& y) const;

Returns: x == y.

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template<> struct equal_to<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));

  using is_transparent = unspecified;
};

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template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) == std::forward<U>(u));

Returns: std::forward<T>(t) == std::forward(u).

22.10.8.3 Class template not_equal_to [comparisons.not.equal.to]

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template<class T = void> struct not_equal_to {
  constexpr bool operator()(const T& x, const T& y) const;
};

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constexpr bool operator()(const T& x, const T& y) const;

Returns: x != y.

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template<> struct not_equal_to<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));

  using is_transparent = unspecified;
};

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template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) != std::forward<U>(u));

Returns: std::forward<T>(t) != std::forward(u).

22.10.8.4 Class template greater [comparisons.greater]

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template<class T = void> struct greater {
  constexpr bool operator()(const T& x, const T& y) const;
};

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constexpr bool operator()(const T& x, const T& y) const;

Returns: x > y.

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template<> struct greater<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));

  using is_transparent = unspecified;
};

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template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) > std::forward<U>(u));

Returns: std::forward<T>(t) > std::forward(u).

22.10.8.5 Class template less [comparisons.less]

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template<class T = void> struct less {
  constexpr bool operator()(const T& x, const T& y) const;
};

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constexpr bool operator()(const T& x, const T& y) const;

Returns: x < y.

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template<> struct less<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));

  using is_transparent = unspecified;
};

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template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) < std::forward<U>(u));

Returns: std::forward<T>(t) < std::forward(u).

22.10.8.6 Class template greater_equal [comparisons.greater.equal]

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template<class T = void> struct greater_equal {
  constexpr bool operator()(const T& x, const T& y) const;
};

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constexpr bool operator()(const T& x, const T& y) const;

Returns: x >= y.

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template<> struct greater_equal<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));

  using is_transparent = unspecified;
};

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template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) >= std::forward<U>(u));

Returns: std::forward<T>(t) >= std::forward(u).

22.10.8.7 Class template less_equal [comparisons.less.equal]

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template<class T = void> struct less_equal {
  constexpr bool operator()(const T& x, const T& y) const;
};

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constexpr bool operator()(const T& x, const T& y) const;

Returns: x <= y.

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template<> struct less_equal<void> {
  template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));

  using is_transparent = unspecified;
};

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template<class T, class U> constexpr auto operator()(T&& t, U&& u) const
    -> decltype(std::forward<T>(t) <= std::forward<U>(u));

Returns: std::forward<T>(t) <= std::forward(u).

22.10.8.8 Class compare_three_way [comparisons.three.way]

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namespace std { struct compare_three_way { template<class T, class U> constexpr auto operator()(T&& t, U&& u) const; using is_transparent = unspecified; }; }

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template<class T, class U>
  constexpr auto operator()(T&& t, U&& u) const;

Constraints: T and U satisfy three_way_comparable_with .

Preconditions: If the expression std::forward<T>(t) <=> std::forward(u) results in a call to a built-in operator <=> comparing pointers of type P, the conversion sequences from both T and U to P are equality-preserving ([concepts.equality]); otherwise, T and U model three_way_comparable_with .

Effects:

(3.1)
If the expression std::forward<T>(t) <=> std::forward(u) results in a call to a built-in operator <=> comparing pointers of type P, returns strong_ordering::less if (the converted value of) t precedes u in the implementation-defined strict total order over pointers ([defns.order.ptr]), strong_ordering::greater if u precedes t, and otherwise strong_ordering::equal.
(3.2)
Otherwise, equivalent to: return std::forward<T>(t) <=> std::forward(u);