ranges::to should reserve when sized_sentinel_for is satisfied
Hewill Kang

Created on 2024-04-13.00:00:00 last changed 2 months ago


Date: 2024-04-16.14:56:03

Proposed resolution:

This wording is relative to N4981.

  1. Modify [range.utility.conv.to] as indicated:

    template<class C, input_range R, class... Args> requires (!view<C>)
      constexpr C to(R&& r, Args&&... args);

    -1- Mandates: C is a cv-unqualified class type.

    -2- Returns: An object of type C constructed from the elements of r in the following manner:

    1. (2.1) — […]

      1. (2.1.1) — […]

      2. (2.1.2) — […]

      3. (2.1.3) — […]

      4. (2.1.4) — Otherwise, if

        1. ( — constructible_from<C, Args...> is true, and

        2. ( — container-appendable<C, range_reference_t<R>> is true:

          C c(std::forward<Args>(args)...);
          subrange s{r};
          if constexpr (sized_range<R>requires { s.size(); } && reservable-container<C>)
          ranges::for_each(rs, container-append(c));
Date: 2024-04-13.00:00:00

ranges::to currently only reserves when r satisfies sized_range. However, we can also extract its size when r is an input_range that its sentinel-iterator pair satisfies sized_sentinel_for.

Given that we have specifically designed the decision tree, I see no reason not to reserve in this case, since we'd be calling ranges::begin(r) anyway.

Date User Action Args
2024-04-14 08:49:20adminsetmessages: + msg14058
2024-04-13 00:00:00admincreate