3722. ranges::to reserves the wrong size

Section: 25.5.7.2 [range.utility.conv.to] Status: New Submitter: Hewill Kang Opened: 2022-06-20 Last modified: 2024-01-29

Priority: 4

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Discussion:

In bullet 1.1.4 of ranges::to, if the Container satisfies container-insertable and R models sized_range, it will first construct the Container with args... and then preallocate memory by calling c.reserve().

However, this only makes sense when c is default-initialized. If instead the size of the Container created by args... is not 0, the value passed into c.reserve() will be wrong, for example:

ranges::to<std::string>(std::views::single('o'), "hell");

The size of the string created by "hell" is already 4, whereas the size of R is only 1, which makes c.reserve(1) useless.

[2022-07-08; Reflector poll]

Set priority to 4 after reflector poll. Some suggestions for NAD.

• "The intended use of those additional arguments is for allocators, comparators, max load factors, and the like. Not...this. I can't really bring myself to care about this example - and it adds some (admittedly small) overhead for the by far common case."
• "And it's not the right fix imo. if we wanted to put *some* constraints on Args, we could attempt to do so, although that might be challenging."
• "calling reserve() is just an attempt at an optimization."
• "I disagree with NAD given the fact that we have specifically designed the "reserve" branch of the decision tree."

Proposed resolution:

This wording is relative to N4910.

1. Modify 25.5.7.2 [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- Returns: An object of type C constructed from the elements of r in the following manner:

   1. (1.1) — If convertible_to<range_reference_t<R>, range_value_t<C>> is true:

      1. (1.1.1) — If constructible_from<C, R, Args...> is true:

         C(std::forward<R>(r), std::forward<Args>(args)...)

      2. (1.1.2) — Otherwise, if constructible_from<C, from_range_t, R, Args...> is true:

         C(from_range, std::forward<R>(r), std::forward<Args>(args)...)

      3. (1.1.3) — Otherwise, if

         1. (1.1.3.1) — common_range<R> is true,

         2. (1.1.3.2) — cpp17-input-iterator<iterator_t<R>> is true, and

         3. (1.1.3.3) — constructible_from<C, iterator_t<R>, sentinel_t<R>, Args...> is true:

            C(ranges::begin(r), ranges::end(r), std::forward<Args>(args)...)

      4. (1.1.4) — Otherwise, if

         1. (1.1.4.1) — constructible_from<C, Args...> is true, and

         2. (1.1.4.2) — container-insertable<C, range_reference_t<R>> is true:

            C c(std::forward<Args>(args)...);
            if constexpr (sized_range<R> && reservable-container<C>) {
              using ST = range_size_t<C>;
              using CT = common_type_t<ST, range_size_t<R>>;
              auto sz = ST(CT(ranges::size(c)) + CT(ranges::size(r)));
              c.reserve(szranges::size(r));
            }
            ranges::copy(r, container-inserter<range_reference_t<R>>(c));
            

   2. (1.2) — Otherwise, if input_range<range_reference_t<R>> is true:

      to<C>(r | views::transform([](auto&& elem) {
        return to<range_value_t<C>>(std::forward<decltype(elem)>(elem));
      }), std::forward<Args>(args)...);
      

   3. (1.3) — Otherwise, the program is ill-formed.