Re: [Boost-users] A forward iterator need not be default-constructible

Subject: Re: [Boost-users] A forward iterator need not be default-constructible
From: Krzysztof Żelechowski (giecrilj_at_[hidden])
Date: 2011-10-01 04:42:54

Andrew Sutton wrote:

>>> template<typename I>
>>> auto f() { I i; assert(*i); } // worst algorithm ever
> 
>> OTOH, if I know that a singular iterator of some type would have no valid
>> operations, and the iterator is only meant to be used by algorithms, the
>> benefit of being unable to create a singular iterator (like when the
>> iterator in question holds a reference to supporting data used in its
>> operation) outweighs the benefit of being able to do so.
> 
> I think I've lost the thread of the argument. Are you saying that you
> should *not* be able to default construct iterators?

What I am saying is that iterators that are not default-constructible are 
sometimes more robust because an attempt to create them out of thin air 
results in a compile-time error.  I am not saying that all iterators are 
like that (I agree with Dave regarding his the interpretation of NULL).  
Moreover, such iterators sometimes come up naturally from standard 
components (and not some shady third-party ones, as has been suggested), as 
evidenced in my code that fails to compile with Boost concepts.

Being a singular iterator is not a concept, it is a run-time property.  The 
compiler cannot check whether an operator is singular, it is equivalent to 
the halting problem.

IMHO,
Chris