Re: [boost] implementation of moveconstructor (was [shared_ptr] dangerous implementation of moveconstructor)

From: Joe Gottman (jgottman_at_[hidden])
Date: 2007-04-17 19:51:25

Sohail Somani wrote:
> Still, for generic code, I don't think you should expect to be able to
> reuse a moved object except for destructing it. Specific classes, maybe,
> but not as a general rule.
> 
> Even still, I can't think of a case off-hand where assignment requires
> some previous state so maybe this is a moot point. Maybe for some really
> crazy overloading of operator=.

    At a bare minimum you need to be able to destruct or assign to a 
moved object.  For example, consider the code for inserting a value into 
the middle of an vector.  If the vector doesn't need to be reallocated 
you would use the following algorithm:

1)  Use the move constructor to move the last element of the vector to 
where end() currently points to.  Note that this leaves a "hole" in the 
vector in what is now the second-to-last position.
2) Iterating backward, use move assignment to move the element 
immediately in front of the hole to the position of the hole, thus 
moving the hole back one.  Continue doing this until the hole is
in the position where you want to do your insertion.
3) Assign your new value to the position of the hole, thus filling the 
hole.  Assuming no exceptions were thrown every element of the vector 
now has a determined value.

Note that steps 2 and 3 above required assigning to an element that had 
previously been moved from.

Joe Gottman