Re: Copying singular iterators
mhilferink@objectvision.nl (MathGis) wrote (abridged):
in my code I have been indicating empty ranges of T with default
constructed vector<T>::iterators and I was disappointed when moving to
VC8 (iterator debug mode) to find that the standard indeed prohibits
this. I still wonder why.
I think the main issue is what should happen when you compare such
iterators. Currently you are not supposed to compare iterators from
different containers. If default constructed iterators are supposed to act
like NULL pointers, then they should be comparable to any iterator of the
same type. Would you expect these asserts to pass?
std::vector<int> v1;
std::vector<int> v2;
std::vector<int>::iterator i();
assert( i != v1.begin() );
assert( i != v1.end() );
assert( v1.begin() != v2.begin() );
v1.push_back( 0 );
assert( i != v1.begin() );
assert( i != v1.end() );
assert( v1.begin() != v2.begin() );
With some implementations, the first block would fail and the second block
pass, because vector::begin() can return a NULL pointer if its reserve is
0.
I think it would be reasonable to say that a default-constructed iterator
must not compare equal to any dereferenceable iterator, and must compare
equal to other default constructed iterators of the same type, with other
cases implementation-defined.
This would allow implementations to use default-constructed iterators for
end(), but not for begin() unless begin() == end(). I don't think that
would place onerous new restrictions on the implementation, but you can
see it is getting a bit scary; you have to try to guess what future
implementations and containers might want to do.
-- Dave Harris, Nottingham, UK.
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