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

Question

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.

Answer 1

Yes it is allowed, if the std::vector is not empty. If the vector is empty, vec[0] will evoke Undefined Behavior.

std::vector is required to store elements contiguously.

There is also data() method, but it is C++11 only.

Important:

This will not work on std::vector<bool> (bit-efficient specialization). But it is also not a container, and IMO it should be deprecated.