Decrementing an iterator

Question

I have a normal iterator to a position in a bidirectional data structure (vector in fact). Now I want to perform an operation on the past x elements from my current position. There will always be at least x elements before, although the last x may be the first element of the vector.

I have written this code

vector<Record>::iterator it = itCurrentRecord;
for (unsigned int i = 0; i < length; i++)
{
    (it--)->length = length;
}

Is this safe? I am concerned that when it points to the first element, the final decrement will cause the iterator to point to one before the first element, which is invalid.

If so, how can I rewrite this in a safe way?

Thanks

Answer 1

Use a reverse iterator:

vector<Record>::reverse_iterator it = itCurrentRecord;
for (unsigned int i = 0; i < length; i++)
{
    (it++)->length = length;
}

which allows to semantically point to one-before-begin like a normal iterator is allowed to point one-past-end.

Answer 2

Your concern is valid. Since this is a random access iterator, you can use arithmetic to avoid the undefined behavior:

vector<Record>::iterator it = itCurrentRecord;
for (unsigned int i = 0; i < length; i++)
{
    (it - i)->length = length;
}