std::ranges::less
| 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 |
||
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 comparing pointers | only semantic requirements relaxed |
See also
| function object implementing x < y (class template) |