Is &vec[0] defined behavior for a std::vector vec?

I see this a lot:

std::vector<Something> vec;
do_something_with_vec(vec);
Something *arr=&vec[0];
do_something_that_needs_carray(arr);

I mean, a vector will probably use an array internally so I see why this works, I'm just wondering whether or not this is defined behavior (like, is an implementor allowed to run an implementation of std::vector with which this would break).

If there are conflicts between the standards, I'm interested in what the C++11 standard says.