TITLE: using auto_ptr in containers

(Newsgroups: comp.lang.c++.moderated, 2 Mar 2000)

NAGLE: John Nagle
 
> Did the use of collections with auto_ptr elements ever get fixed,
> or is that still broken?  

CLAMAGE:   Steve Clamage <stephen.clamage@sun.com>

The intended design of auto_ptr is to have ownership semantics.
Copying an auto_ptr passes ownership of the object it points to.
If auto_ptrs don't have ownership semantics, passing them
to and from functions is not safe. You don't know whether to
destroy the object when the auto_ptr is destroyed.

The C++ Committee considered alternative designs, such as no automatic
transfer of ownership plus a transfer method. These other designs
were judged inferior overall.

Copying an auto_ptr therefore must modify the auto_ptr itself.
Since standard containers require that elements can be freely copied,
you can't put a standard auto_ptr in a standard container.

You can call that "broken" if you want, but the simple fact is that
you cannot have both free copying of container elements and ownership
semantics for auto_ptr, and still allow containers of auto_ptr.

If you don't need ownership semantics, you can create your own pointer
class and use it in containers.

The various complaints about auto_ptr all boil down to having
conflicting sets of requirements. They can't all be satisfied
at once. I think a future C++ standard could have a collection of
safe-pointer classes, each with different semantics.



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