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3568. basic_istream_view needs to initialize value_

Section: [range.istream.view] Status: C++23 Submitter: Casey Carter Opened: 2021-06-15 Last modified: 2023-11-22

Priority: Not Prioritized

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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 [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