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3568. basic_istream_view needs to initialize value_

Section: 25.6.6.2 [range.istream.view] Status: C++23 Submitter: Casey Carter Opened: 2021-06-15 Last modified: 2023-11-22

Priority: Not Prioritized

View all issues with C++23 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 25.6.6.2 [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
     };
   }