< cpp‎ | experimental‎ | ranges
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template< class T = void >

    requires StrictTotallyOrdered<T> ||
             Same<T, void> ||
             /* < on two const T lvalues invokes a built-in operator comparing pointers */

struct less;
(ranges TS)
struct less<void>;
(ranges TS)

Function object for performing comparisons. The primary template invokes operator< on const lvalues of type T. The specialization less<void> deduces the parameter types of the function call operator from the arguments (but not the return type).

All specializations of less are Semiregular.


[edit] Member types

Member type Definition
is_transparent (member only of less<void> specialization) /* unspecified */

[edit] Member functions

checks if the first argument is less than the second
(public member function)


constexpr bool operator()(const T& x, const T& y) const;
(1) (member only of primary less<T> template)
template< class T, class U >

    requires StrictTotallyOrderedWith<T, U> ||
             /* std::declval<T>() < std::declval<U>() resolves to
                a built-in operator comparing pointers */

constexpr bool operator()(T&& t, U&& u) const;
(2) (member only of less<void> specialization)
1) Compares x and y. Equivalent to return ranges::less<>{}(x, y);.
2) 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 builtin operator< comparing pointers.

When a call to (1) or (2) 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 builtin 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).

[edit] 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).

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.

[edit] Notes

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

[edit] Example

[edit] See also

function object implementing x < y
(class template) [edit]