Re: [boost] Dereferencing End Iterators (Was: Performance Tuning?)

Subject: Re: [boost] Dereferencing End Iterators (Was: Performance Tuning?)
From: Edward Grace (ej.grace_at_[hidden])
Date: 2009-07-22 13:14:29

>> I assert that the problem is that the bounds checking in MSVC is
>> wrong - it should only be checking for the de-referencing of invalid
>> iterators not their creation.
>
> It is not valid according to the standard.  The operational  
> equivalent for v.begin() + n, in Table 76 (random access iterator  
> requirements) is:
>
>    vector::iterator tmp(v.begin());
>    tmp += n;
>
> The operational semantics for tmp += n, same table, is:
>
>    iterator_traits<vector::iterator>::difference_type m(n);
>    if (m >= 0) while (m--) ++tmp;
>    else while (m++) --tmp;
>
> Finally, the precondition for ++tmp, from Table 74 (forward  
> iterator requirements), and for --tmp, from Table 75 (bidirectional  
> iterator requirements), requires that tmp be dereferenceable.

Presumably the above requirement for the random access iterator is a  
result of it being an extension of a forward iterator?

> Once n >= v.size(), the resulting iterator is no longer  
> dereferenceable and the MSVC checking is valid.

Fair enough.  It is what it is.  As long as I know what to believe  
I'm happy.

-ed
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