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

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2008-04-10 16:02:16

Andrey Semashev skrev:
> Thorsten Ottosen wrote:
>> Hi Fernando,
>>
>> I have some comments on the way optional<T>::swap() is implemented. I 
>> looks like this:
>>
>> // optional's swap:
>> // If both are initialized, calls swap(T&, T&). If this swap throws, 
>> //both will remain initialized but their values are now unspecified.
>> // If only one is initialized, calls U.reset(*I), THEN I.reset().
>> // If U.reset(*I) throws, both are left UNCHANGED (U is kept 
>> //uinitialized and I is never reset)
>> // If both are uninitialized, do nothing (no-throw)
> 
> [snip]
> 
>> The problem here is that the two first cases do not delegate to the 
>> version of swap found by ADL. That only happens in cases where both 
>> objects are initialized. This is, AFAICT, a major problem because 
>> copy-construction has quite different semantics than swap() when it 
>> comes to invalidation of iterators or pointers. It works ok for ints, 
>> but creates havoc when using a container with internal memory.
>>
>> I suggest we do something along these lines:
> 
> [snip]
> 
>> Comments?
> 
> 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?

> 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.

-Thorsten