Question

I want to insert an int value in a vector object at position -1, i.e. :

vector<int> v;
int p = 12;
vector<int>::iterator it = -1;
v.insert(it,1,p);

is it possible in c++ ? or it must always be positive ?

Answer 1

Insertion using the insert method occurs always before the specified iterator, so to insert at the first position (i.e. * before* the current first element) use:

v.insert(v.begin(), p);

(you can omit the count parameter, if you only insert a single element).

Insertion at the last position can be done using the end() iterator like

v.insert(v.end(), p);

or, in case of vectors, using push_back() like

v.push_back(p);

For accessing vectors, you can only use non-negative indices, i.e. v[-1] is invalid.

Also you cannot assign an integer to an iterator, mainly because the iterator encapsulates both, the position and the container it is refering to. If you want to ‘construct’ an iterator for a specified position, use additions like:

std::vector<int> it = v.begin() + index;

(Note, that addition to iterators only works for so called random access iterators, thus it depends on the container type.)