Re: [boost] [review] Fusion review begins: iterator equal_to doesn't compare values

From: David Abrahams (dave_at_[hidden])
Date: 2006-05-11 00:18:00

Larry Evans <cppljevans_at_[hidden]> writes:

> On 05/10/2006 04:10 PM, David Abrahams wrote:
>> Larry Evans <cppljevans_at_[hidden]> writes:
> [snip]
>>>OK, maybe I misunderstood.  I thought that since operator* returns a 
>>>value instead of a type (obviously), that two iterators would be equal
>>>only when they were the same length and each value returned by operator*
>>>was equal.  Is that not the case?
>> 
>> 
>> Do you dereference normal STL iterators inside their comparison operators?
>> 
> Nope, you're right.  However, the reason I mentioned operator* was to
> emphasize that the operators, whether * or == applied to values, not
> types.  

Yes, in the STL they apply to values, but not in Fusion.

> The operator== applied to std::vector<I>::iterator and
> std::vector<I>::iterator would probably compare an I* with an  I*
> and not use:
>
>    is_same< std::vector<I>::iterator, std::vector<I>::iterator>::value

Yes, STL has iterators whose position can only be measured at runtime.
Fusion has iterators whose position can be measured at compile-time.
But -- not to put too fine a point on it -- so what?

> I'll admit I shouldn't have mentioned anything about length;
> however, if two stl iterator's do compare equal then the operator*
> would return the same value for the same number of operator++'s,
> wouldn't they?

Assuming the comparison was a well-defined operation in the first
place, yes.  If you're comparing iterators from different sequences,
all bets are off.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com