Re: Testing for the beginning of a vector
peter koch wrote:
On 8 Apr., 21:53, Victor Bazarov <v.Abaza...@comAcast.net> wrote:
mzdude wrote:
On Apr 7, 5:20 pm, Comp1...@yahoo.co.uk wrote:
Suppose I have a function with a parameter of type
vector<int>::iterator
How can I test whether that iterator is at the beginning of a vector?
If i denotes the vector<int>::iterator parameter, I could be tempted
to say if (--i == 0) However I believe it's an error to
decrement an iterator before the beginning of a vector.
There's probably a solution involving copying the iterator to a
reverse_iterator but I'm probably missing something really simple.
assuming
std::vector<int> v;
std::vector<int>::iterator i;
....
if( i == v.begin() )
// i is at the start of the vector
vector<int>::difference_type dif = std::distance( v.begin(), i );
if( dif == 0 )
// i is a the start of the vector
// otherwise it tells how far from the start i really is.
The problem, however, can present itself if 'i' is not an iterator in
'v' at all, because then comparing 'i' with any iterator in 'v' (like
the one returned by 'begin()') or passing them to 'std::distance', might
actually be undefined behavior. IIRC.
You are right. What could be done is comparing &(*i) with &v[0]. But I
understand the original posters question as if you can determine
whether an iterator points to the first element of any given vector in
your program. This certainly is impossible.
I wasn't contesting that. Totally agree.
Question remains, however, if the two pointers (since you proposed to
compare addresses instead of iterators) do not belong to the same object
(array, etc.) doesn't calling 'std::distance' for those have undefined
behaviour? I understand that comparing for equality should be OK, but
calculating the distance between two objects which could reside in
completely different segments of virtual memory? Thanks.
V
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