Re: Copying singular iterators
pete@versatilecoding.com (Pete Becker) wrote (abridged):
I haven't seen a convincing case for it. All the suggestions I've seen
for having such a thing have come from misunderstanding how STL is
designed. Iterators aren't pointers, and they don't need to act like
them. Applications that need pointer semantics should use pointers.
If you want pointer semantics and you are also using standard containers,
this incompatibility between the two can create a problem. You can't
replace pointers with iterators because iterators have no way of
representing NULL, and you can't replace iterators with pointers because
the containers don't work with pointers. (Eg there is no efficient way to
convert a pointer into an iterator that you can pass to std::list.)
However, it is not wrong to want to use NULL, nor is it wrong to want to
use standard containers. I don't see why it should be a design error to
want both at the same time. It's just a limitation of the STL.
Most of the time you can just use c.end(), but that's not always
convenient because it needs access to the container. One workaround is to
use a pair<bool,iterator> where the bool is set to false to represent
NULL. Another workaround is to create a container especially, eg:
struct ContainerView {
typedef std::list<int> Container;
typedef Container::iterator iterator;
Container *c;
static Container ifNone;
iterator begin() {
return c ? c->begin() : ifNone.begin();
}
iterator end() {
return c ? c->end() : ifNone.end();
}
// ...
};
where if c is NULL we want begin/end to return an empty sequence.
-- Dave Harris, Nottingham, UK.
---
[ comp.std.c++ is moderated. To submit articles, try just posting with ]
[ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ]
[ --- Please see the FAQ before posting. --- ]
[ FAQ: http://www.comeaucomputing.com/csc/faq.html ]