Re: [boost] [optional] problems with swap()

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2008-04-10 17:36:11

Frank Mori Hess skrev:
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> On Thursday 10 April 2008 16:02 pm, Thorsten Ottosen wrote:
>>> Your approach requires the T to be default constructible which is not
>>> always the case.
>> Why is this better than the far more expensive copy-construction?
> 
> The documentation indicates optional<T> requires T to be copy constructible.  
> It explicitly does not require T to be default constructible.

Well, not all operations requries T to be copy-constructible, just as 
not all operations would require it to be default constructible.

>>> Besides, to my mind, the current implementation is more
>>> obvious for the user.
>> No user expects swap to throw or invalidate iterators or references like
>> the current implementation does.
> 
> Personally, I consider any preservation of iterators/references/pointers to 
> the internals of a swapped object to be purely a side-effect of a swap 
> specialization, whose raise d'etre is as a speed optimization.

If speed is an issue, then the current implementation is also worse than 
what i suggested:

   copy construction + destruction

is *far more expensive* than

   default construction + specialized swap + destruction

for non-trivial classes with heap-based memory. The latter typically 
requires zero heap allocations.

-Thorsten