Is the std::array bit compatible with the old C array?

This doesn’t directly answer your question, but you should simply use std::copy:

T c[N];
std::array<T, N> cpp;

// from C to C++
std::copy(std::begin(c), std::end(c), std::begin(cpp));

// from C++ to C
std::copy(std::begin(cpp), std::end(cpp), std::begin(c));

If T is a trivially copyable type, this’ll compile down to memcpy. If it’s not, then this’ll do element-wise copy assignment and be correct. Either way, this does the Right Thing and is quite readable. No manual byte arithmetic necessary.