question on invalidation of iterators on vector<T>

[subramanian100in]

Suppose I have

vector<int> c;
for (int i = 0; i < 10; ++i)
c.push_back(i);

vector<int>::iterator it = find(c.begin(), c.end(), 5);

If I do,
c.insert(c.begin(), 10);

then it can cause increase in container size in which case
reallocation happens and so the insertion operation can POTENTIALLY
invalidate all iterators into the vector.

How to know whether all the iterators into a vector are invalidated
after insertion ? Or should we safely always assume that all iterators
into a vector will become invalid after insertion ?

Now consider
c.erase(it--);

Can reallocation happen due to the shrinkage of vector size by means
of erasure ? If this is true, then erase also can invalidate all
iterators into a vector. Am I correct ?

Also, is it correct to say 'it--' in the argument passed (ie postfix
decrement on the iterator argument passed) to erase function as done
above ? Will it be a valid iterator after erase ?

Or equivalently, should I say,

vector<int>::iterator nit = c.erase(it);
--nit;

Kindly clarify

Thanks
V.Subramanian

 

[joseph cook]

Suppose I have

How to know whether all the iterators into a vector are invalidated
after insertion ? Or should we safely always assume that all iterators
into a vector will become invalid after insertion ?

This is generally a good idea to assume such, but if vector.size()
does not exceed the vector.capacity() before you did the insert, then
the iterator will still be valid. (You can ensure the capacity() is
at least a certain size, by calling vector.reserve(size) up front
somewhee.

Now consider
c.erase(it--);

Can reallocation happen due to the shrinkage of vector size by means
of erasure ? If this is true, then erase also can invalidate all
iterators into a vector. Am I correct ?

No, erase will only invalidate all iterators after 'it'.

Also, is it correct to say 'it--' in the argument passed (ie postfix
decrement on the iterator argument passed) to erase function as done
above ? Will it be a valid iterator after erase ?

Assuming "it" didn't point to begin()

Or equivalently, should I say,

vector<int>::iterator nit = c.erase(it);
--nit;

Yes, this is better, although be sure "it" doesn't point to begin here
too. Why do you want to point to the element just before the one that
was deleted? Generally, you just want "nit = c.erase(it)" to get you
to the next element. Id you are trying to walk through a vector in
reverse order, consider a reverse_iterator

Joe Cook