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Showing content from https://cplusplus.github.io/LWG/issue3334 below:

basic_osyncstream move assignment and destruction calls basic_syncbuf::emit() twice

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Section: 31.11.3 [syncstream.osyncstream] Status: C++20 Submitter: Tim Song Opened: 2019-11-06 Last modified: 2021-02-25

Priority: 3

View all issues with C++20 status.

Discussion:

These functions are specified to call emit(), which calls emit() on the basic_syncbuf and sets badbit if it fails. Then, the move assignment is specified to move-assign the basic_syncbuf, while the destructor implicitly needs to destroy the basic_syncbuf data member. This calls emit() on the basic_syncbuf again.

[2020-02-13 Tim adds wording after discussion with Peter]

[2020-02 Status to Immediate Thursday afternoon in Prague.]

Proposed resolution:

This wording is relative to N4849.

[Drafting note: There is no need to explicitly call emit at all in these functions; memberwise move-assignment/destruction is sufficient, so we can strike the specification entirely and rely on the wording in 16.3.3.5 [functions.within.classes]. — end drafting note]

1. Edit 31.11.3.2 [syncstream.osyncstream.cons] as indicated:

   ~basic_osyncstream();
   

   -6- Effects: Calls emit(). If an exception is thrown from emit(), that exception is caught and ignored.

2. Strike [syncstream.osyncstream.assign]:

   basic_osyncstream& operator=(basic_osyncstream&& rhs) noexcept;
   

   -1- Effects: First, calls emit(). If an exception is thrown from emit(), that exception is caught and ignored. Move assigns sb from rhs.sb. [ Note: This disassociates rhs from its wrapped stream buffer ensuring destruction of rhs produces no output. — end note ]

   -2- Postconditions: nullptr == rhs.get_­wrapped() is true. get_­wrapped() returns the value previously returned by rhs.get_­wrapped().

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