Date
2009-07-26.00:00:00
Message id
4329

Content

[ 2009-07-26 Howard adds: ]

I started out thinking I would recommend NAD for this one. I've turned around to agree with the proposed resolution (which I've updated to the current draft). I did not fully understand Ganesh's rationale, and attempt to describe my improved understanding below.

The move constructor, move assignment operator, and swap function are different for basic_istream, basic_ostream and basic_iostream than other classes. A timely conversation with Daniel reminded me of this long forgotten fact. These members are sufficiently different that they would be extremely confusing to use in general, but they are very much needed for derived clients.

  • The move constructor moves everything but the rdbuf pointer.
  • The move assignment operator moves everything but the rdbuf pointer.
  • The swap function swaps everything but the rdbuf pointer.

The reason for this behavior is that for the std-derived classes (stringstreams, filestreams), the rdbuf pointer points back into the class itself (self referencing). It can't be swapped or moved. But this fact isn't born out at the stream level. Rather it is born out at the fstream/sstream level. And the lower levels just need to deal with that fact by not messing around with the rdbuf pointer which is stored down at the lower levels.

In a nutshell, it is very confusing for all of those who are not so intimately related with streams that they've implemented them. And it is even fairly confusing for some of those who have (including myself). I do not think it is safe to swap or move istreams or ostreams because this will (by necessary design) separate stream state from streambuffer state. Derived classes (such as fstream and stringstream must be used to keep the stream state and stream buffer consistently packaged as one unit during a move or swap.

I've implemented this proposal and am living with it day to day.