23 General utilities library [utilities]

23.14 Function objects [function.objects]

23.14.7 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 strict total order that is consistent among those specializations and is also consistent with the partial order imposed by the built-in operators <, >, <=, >=.
[ Note
:
When a < b is well-defined for pointers a and b of type P, this implies (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 strict total order that is consistent among those specializations and is also consistent with the partial order imposed by those built-in operators.

23.14.7.1 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).

23.14.7.2 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).

23.14.7.3 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).

23.14.7.4 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).

23.14.7.5 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).

23.14.7.6 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).