900. Stream move-assignment

Section: 30.9.3.2 [ifstream.assign] Status: C++11 Submitter: Niels Dekker Opened: 2008-09-20 Last modified: 2016-02-10

Priority: Not Prioritized

View all issues with C++11 status.

Discussion:

It appears that we have an issue similar to issue 675 regarding the move-assignment of stream types. For example, when assigning to an std::ifstream, ifstream1, it seems preferable to close the file originally held by ifstream1:

ifstream1 = std::move(ifstream2); 

The current Draft (N2723) specifies that the move-assignment of stream types like ifstream has the same effect as a swap:

Assign and swap 30.9.3.2 [ifstream.assign]

basic_ifstream& operator=(basic_ifstream&& rhs); 

Effects: swap(rhs).

[ Batavia (2009-05): ]

Howard agrees with the analysis and the direction proposed.

Move to Open pending specific wording to be supplied by Howard.

[ 2009-07 Frankfurt: ]

Howard is going to write wording.

[ 2009-07-26 Howard provided wording. ]

[ 2009-09-13 Niels adds: ]

Note: The proposed change of 30.9.2.2 [filebuf.assign] p1 depends on the resolution of LWG 1204, which allows implementations to assume that *this and rhs refer to different objects.

[ 2009 Santa Cruz: ]

Leave as Open. Too closely related to 911 to move on at this time.

[ 2010 Pittsburgh: ]

Moved to Ready for Pittsburgh.

Proposed resolution:

Change 30.8.2.2 [stringbuf.assign]/1:

basic_stringbuf& operator=(basic_stringbuf&& rhs);

-1- Effects: swap(rhs). After the move assignment *this reflects the same observable state it would have if it had been move constructed from rhs (30.8.2.1 [stringbuf.cons]).

Change 30.8.3.2 [istringstream.assign]/1:

basic_istringstream& operator=(basic_istringstream&& rhs);

-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 30.8.4.2 [ostringstream.assign]/1:

basic_ostringstream& operator=(basic_ostringstream&& rhs);

-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 30.8.5.2 [stringstream.assign]/1:

basic_stringstream& operator=(basic_stringstream&& rhs);

-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 30.9.2.2 [filebuf.assign]/1:

basic_filebuf& operator=(basic_filebuf&& rhs);

-1- Effects: swap(rhs). Begins by calling this->close(). After the move assignment *this reflects the same observable state it would have if it had been move constructed from rhs (30.9.2.1 [filebuf.cons]).

Change 30.9.3.2 [ifstream.assign]/1:

basic_ifstream& operator=(basic_ifstream&& rhs);

-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 30.9.4.2 [ofstream.assign]/1:

basic_ofstream& operator=(basic_ofstream&& rhs);

-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.

Change 30.9.5.2 [fstream.assign]/1:

basic_fstream& operator=(basic_fstream&& rhs);

-1- Effects: swap(rhs). Move assigns the base and members of *this with the respective base and members of rhs.