Question

If you have an STL vector which has been resized, is it safe to take the address of element 0 and assume the rest of the vector will follow in memory?

e.g.

vector<char> vc(100); // do some stuff with vc vc.resize(200); char* p = &vc[0]; // do stuff with *p 

Answer 1

Yes, that is a valid assumption (*).

From the C++03 standard (23.2.4.1):

The elements of a vector are stored contiguously, meaning that if v is a vector where T is some type other than bool, then it obeys the identity &v[n] == &v[0] + n for all 0 <= n < v.size().

(*) … but watch out for the array being reallocated (invalidating any pointers and iterators) after adding elements to it.