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3568. basic_istream_view needs to initialize value_

Section: 24.6.5.2 [range.istream.view] Status: WP Submitter: Casey Carter Opened: 2021-06-15 Last modified: 2021-10-17

Priority: Not Prioritized

View all issues with WP status.

Discussion:

P2325R3 removes the default member initializers for basic_istream_view's exposition-only stream_ and value_ members. Consequently, copying a basic_istream_view before the first call to begin may produce undefined behavior since doing so necessarily copies the uninitialized value_. We should restore value_'s default member initializer and remove this particular footgun.

[2021-06-23; Reflector poll]

Set status to Tentatively Ready after seven votes in favour during reflector poll.

[2021-10-14 Approved at October 2021 virtual plenary. Status changed: Voting → WP.]

Proposed resolution:

Wording relative to the post-202106 working draft (as near as possible). This PR is currently being implemented in MSVC.

  1. Modify 24.6.5.2 [range.istream.view] as indicated:

    namespace std::ranges {
      […]
      template<movable Val, class CharT, class Traits>
        requires default_initializable<Val> &&
                 stream-extractable<Val, CharT, Traits>
      class basic_istream_view : public view_interface<basic_istream_view<Val, CharT, Traits>> {
        […]
        Val value_ = Val(); // exposition only
      };
    }