I have some questions about the way to assign values to the std::array

Double-braces required in C++11 prior to the CWG 1270 (not needed in C++11 after the revision and in C++14 and beyond):

// construction uses aggregate initialization
std::array<int, 5> a{ {1, 2, 3, 4, 5} }; // double-braces required in C++11 prior to the CWG 1270 revision
std::array<int, 5> a{1, 2, 3, 4, 5}; // not needed in C++11 after the revision and in C++14 and beyond
std::array<int, 5> a = {1, 2, 3, 4, 5};  // never required after =

std::array reference