std::ranges::SizedRange, std::ranges::disable_sized_range

< cpp‎ | ranges
Defined in header <ranges>
template< class T >

concept SizedRange = Range<T> &&
  ! disable_sized_range<std::remove_cvref_t<T>> &&
  requires(T& t) {

inline constexpr bool disable_sized_range = false;
1) The SizedRange concept specifies the requirements of a Range type that knows its size in constant time with the size function.

Formally, given an lvalue t of type std::remove_reference_t<T>, T models SizedRange only if

  • ranges::size(t)
  • has non-amortized constant-time complexity,
  • does not modify t, and
  • is equal to ranges::distance(t), and
  • if iterator_t<T> models ForwardIterator, ranges::size(t) is well-defined regardless of the evaluation of ranges::begin(t) (in other words, a single-pass sized range may support a call to size only before the first call to begin, but a forward range must support size at all times)
2) disable_sized_range exists to allows use of range types that satisfy but do not in fact model SizedRange.