20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.9 Concept-constrained comparisons [range.cmp]

In this subclause, BUILTIN-PTR-CMP(T, op, U) for types T and U and where op is an equality ([expr.eq]) or relational operator ([expr.rel]) is a boolean constant expression.

BUILTIN-PTR-CMP(T, op, U) is true if and only if op in the expression declval<T>() op declval() resolves to a built-in operator comparing pointers.

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struct ranges::equal_to {
  template<class T, class U>
    requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

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]).

Effects:

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

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struct ranges::not_equal_to {
  template<class T, class U>
    requires equality_comparable_with<T, U> || BUILTIN-PTR-CMP(T, ==, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to: return !ranges::equal_to{}(std::forward<T>(t), std::forward(u));

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struct ranges::greater {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(U, <, T)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to: return ranges::less{}(std::forward(u), std::forward<T>(t));

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struct ranges::less {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

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]).

For any expressions ET and EU such that decltype((ET)) is T and decltype((EU)) is U, exactly one of ranges::less{}(ET, EU), ranges::less{}(EU, ET), or ranges::equal_to{}(ET, EU) is true.

Effects:

(7.1)
If the expression std::forward<T>(t) < std::forward(u) results in a call to a built-in operator < comparing pointers: returns true if (the converted value of) t precedes u in the implementation-defined strict total order over pointers ([defns.order.ptr]) and otherwise false.
(7.2)
Otherwise, equivalent to: return std::forward<T>(t) < std::forward(u);

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struct ranges::greater_equal {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(T, <, U)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to: return !ranges::less{}(std::forward<T>(t), std::forward(u));

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struct ranges::less_equal {
  template<class T, class U>
    requires totally_ordered_with<T, U> || BUILTIN-PTR-CMP(U, <, T)
  constexpr bool operator()(T&& t, U&& u) const;

  using is_transparent = unspecified;
};

operator() has effects equivalent to: return !ranges::less{}(std::forward(u), std::forward<T>(t));