At 06:24 PM 1/30/2003, Howard Hinnant wrote:
>On Tuesday, January 28, 2003, at 11:19 PM, Greg Colvin wrote:
>
>>My problem with auto_ptr isn't so much the semantics, which
>>have proved useful and are probably the minimum needed to
>>solve the problem that the committee wanted solved. And it
>>isn't so much the "move as copy" syntax that Howard dislikes.
>
>I just read in a newsgroup post today:
>
>>I tried to apply predicate version of std::sort algorithm to vector<
>>auto_ptr<MyClass> > using my predicate. Sometimes, some of auto_ptr loses
>>the ownership while sorting. What is happening?
>
>Now we all know that vector<auto_ptr<MyClass> > won't compile, so this really
>shouldn't be a problem.
>
>But it still IS a problem, because this does compile:
Sigh...
To be clear, I'll be happy to see a better syntax
in the next standard -- auto_ptr was the best we
could do with the syntax we had, but ...
>#include <the proper stuff>
>
>void foo()
>{
> auto_ptr< MyClass > ia[100];
> ... fill ia
> std::sort(ia, ia+100, the_proper_predicate);
>}
>
>This may even do the right thing. Or it may leave an auto_ptr moved into a local
>inside of sort, never to be heard from again. And the reason is that sort thinks it
>can copy an element from the range into a local temporary with:
>
>T partition = *median;
>
>If T is auto_ptr, this does a move instead of a copy, with the result that the range
>to be sorted now no longer contains all of the elements that were supposed to be
>sorted. The committee recognized this danger in 1996, and redesigned auto_ptr so
>that it would not compile if put into a std::container. However, you can still put
>auto_ptr into a raw array, and you can still use std::algorithms on raw arrays.
>auto_ptr still presents a danger because it moves with copy syntax. A better
>solution is to move with syntax other than copy so that code like foo() shown above
>will fail at compile time, instead of at run time (or if it does compile, will run
>correctly).
>
>-Howard
>
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