Re: Testing for the beginning of a vector
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.
V
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