# std::ranges::less

From cppreference.com
< cpp‎ | utility‎ | functional

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.

### Equality preservation

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

### 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

less | function object implementing x < y (class template)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/functional/ranges/less&oldid=148591"