20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.7 Comparisons [comparisons]

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The library provides basic function object classes for all of the comparison operators in the language ([expr.rel], [expr.eq]).
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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
:
If a < b is well-defined for pointers a and b of type P, then (a < b) == less<P>()(a, b), (a > b) == greater<P>()(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.

20.14.7.1 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;
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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));
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Returns: std​::​forward<T>(t) == std​::​forward<U>(u).

20.14.7.2 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;
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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));
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Returns: std​::​forward<T>(t) != std​::​forward<U>(u).

20.14.7.3 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;
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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));
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Returns: std​::​forward<T>(t) > std​::​forward<U>(u).

20.14.7.4 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;
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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));
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Returns: std​::​forward<T>(t) < std​::​forward<U>(u).

20.14.7.5 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;
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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));
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Returns: std​::​forward<T>(t) >= std​::​forward<U>(u).

20.14.7.6 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;
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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));
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Returns: std​::​forward<T>(t) <= std​::​forward<U>(u).

20.14.7.7 Class compare_­three_­way [comparisons.three.way]

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In this subclause, BUILTIN-PTR-THREE-WAY(T, U) for types T and U is a boolean constant expression.
BUILTIN-PTR-THREE-WAY(T, U) is true if and only if <=> in the expression 
declval<T>() <=> declval<U>()
resolves to a built-in operator comparing pointers.
struct compare_three_way {
  template<class T, class U>
    requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U)
  constexpr auto operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};
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template<class T, class U> requires three_way_comparable_with<T, U> || BUILTIN-PTR-THREE-WAY(T, U) constexpr auto operator()(T&& t, U&& u) const;
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Preconditions: If the expression std​::​forward<T>(t) <=> std​::​forward<U>(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]).
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Effects:
  • (3.1)
    If the expression std​::​forward<T>(t) <=> std​::​forward<U>(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>(u);