[c++] C++ STL Vectors: Get iterator from index?

So, I wrote a bunch of code that accesses elements in an stl vector by index[], but now I need to copy just a chunk of the vector. It looks like vector.insert(pos, first, last) is the function I want... except I only have first and last as ints. Is there any nice way I can get an iterator to these values?

way mentioned by @dirkgently ( v.begin() + index ) nice and fast for vectors

but std::advance( v.begin(), index ) most generic way and for random access iterators works constant time too.

EDIT
differences in usage:

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

or

std::vector<>::iterator it = v.begin();
std::advance( it, index );

added after @litb notes.

Also; auto it = std::next(v.begin(), index);

Update: Needs a C++11x compliant compiler

Actutally std::vector are meant to be used as C tab when needed. (C++ standard requests that for vector implementation , as far as I know - replacement for array in Wikipedia) For instance it is perfectly legal to do this folowing, according to me:

int main()
{

void foo(const char *);

sdt::vector<char> vec;
vec.push_back('h');
vec.push_back('e');
vec.push_back('l');
vec.push_back('l');
vec.push_back('o');
vec.push_back('/0');

foo(&vec[0]);
}

Of course, either foo must not copy the address passed as a parameter and store it somewhere, or you should ensure in your program to never push any new item in vec, or requesting to change its capacity. Or risk segmentation fault...

Therefore in your exemple it leads to

vector.insert(pos, &vec[first_index], &vec[last_index]);

Or you can use std::advance

vector<int>::iterator i = L.begin();
advance(i, 2);