std::ranges::less(3)

NAME
std::ranges::less - std::ranges::less

Synopsis
   Defined in header <functional>
   struct less; (since C++20)
   Function object for performing comparisons. Deduces the parameter types of the
   function call operator from the arguments (but not the return type).
  Implementation-defined strict total order over pointers
   The function call operator yields the implementation-defined strict total order over
   pointers if the < operator between arguments invokes a built-in comparison operator
   for a pointer, even if the built-in < operator does not.
   The implementation-defined strict total order is consistent with the partial order
   imposed by built-in comparison operators (<=>, <, >, <=, and >=), and consistent
   among following standard function objects:
     * std::less, std::greater, std::less_equal, and std::greater_equal, when the
       template argument is a pointer type or void
     * std::ranges::equal_to, std::ranges::not_equal_to, std::ranges::less,
       std::ranges::greater, std::ranges::less_equal, std::ranges::greater_equal, and
       std::compare_three_way

Member types
   Member type Definition
   is_transparent /* unspecified */

Member functions
   operator() checks if the first argument is less than the second
              (public member function) std::ranges::less::operator()
   template< class T, class U >
   requires std::totally_ordered_with<T, U> // with different semantic requirements
   constexpr bool operator()(T&& t, U&& u) const;
   Compares t and u, equivalent to return std::forward<T>(t) < std::forward<U>(u);,
   except when that expression resolves to a call to a built-in operator< comparing
   pointers.
   When a call would not invoke a built-in operator comparing pointers, the behavior is
   undefined if std::totally_ordered_with<T, U> is not modeled.
   When a call would invoke a built-in operator comparing pointers of type P, the
   result is instead determined as follows:
     * Returns true if the (possibly converted) value of the first argument precedes
       the (possibly converted) value of the second argument in the
       implementation-defined strict total ordering over all pointer values of type P.
       This strict total ordering is consistent with the partial order imposed by the
       built-in operators <, >, <=, and >=.
     * Otherwise, returns false.
   The behavior is undefined unless the conversion sequences from both T and U to P are
   equality-preserving (see below).
  Equality preservation
   An expression is equality preserving if it results in equal outputs given equal
   inputs.
     * The inputs to an expression consist of its operands.
     * The outputs of an expression consist of its result and all operands modified by
       the expression (if any).
   In specification of standard concepts, operands are defined as the largest
   subexpressions that include only:
     * an id-expression, and
     * invocations of std::move, std::forward, and std::declval.
   The cv-qualification and value category of each operand is determined by assuming
   that each template type parameter denotes a cv-unqualified complete non-array object
   type.
   Every expression required to be equality preserving is further required to be
   stable: two evaluations of such an expression with the same input objects must have
   equal outputs absent any explicit intervening modification of those input objects.

Notes
   Unlike std::less, std::ranges::less requires all six comparison operators <, <=, >,
   >=, == and != to be valid (via the totally_ordered_with constraint).

Example
    This section is incomplete
    Reason: no example
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3530 C++20 syntactic checks were relaxed while only semantic requirements
                       comparing pointers relaxed

See also
   less function object implementing x < y
        (class template)