How to convert vector to array

There's a fairly simple trick to do so, since the spec now guarantees vectors store their elements contiguously:

std::vector<double> v;
double* a = &v[0];

What for? You need to clarify: Do you need a pointer to the first element of an array, or an array?

If you're calling an API function that expects the former, you can do do_something(&v[0], v.size()), where v is a vector of doubles. The elements of a vector are contiguous.

Otherwise, you just have to copy each element:

double arr[100];
std::copy(v.begin(), v.end(), arr);

Ensure not only thar arr is big enough, but that arr gets filled up, or you have uninitialized values.