Re: [boost] Re: [bug]: optional<T> fails on self-assignment

From: Peter Dimov (pdimov_at_[hidden])
Date: 2005-02-15 14:33:59

David Abrahams wrote:
> Jody Hagins <jody-boost-011304_at_[hidden]> writes:
>
>> It seems to me that implementing the
>> canonical assignment operator would work fine...
>>
>>
>> optional & operator=(optional const & rhs)
>> {
>>   optional tmp(rhs);
>>   optional_swap(*this, tmp);
>>   return *this;
>> }
>>
>
> I wish people would stop calling it that.  It's easy, but it's
> overkill.

No, it is not overkill. It is the easiest way to implement a correct 
assignment operator. Forget performance. Subtle bugs in assignment are 
notoriously hard to find. To give the "canonical" example:

optional& operator=( T const & t )
{
    // destroy and recreate
}

Consider what happens when an optional holds a tree node and is assigned a 
child of that node. Or in general, when the 'destroy' step indirectly 
destroys the right hand side.

A correct assignment operator should typically handle self-assignment 
without an explicit test. Adding the test usually doesn't fix the problem, 
only its most obvious manifestation.

Of course if correctness is not a concern, copy+swap is always overkill.