std::array<T,N>::at

reference at( size_type pos );		(until C++17)
constexpr reference at( size_type pos );		(since C++17)
const_reference at( size_type pos ) const;		(until C++14)
constexpr const_reference at( size_type pos ) const;		(since C++14)

Returns a reference to the element at specified location pos, with bounds checking.

If pos is not within the range of the container, an exception of type std::out_of_range is thrown.

pos

position of the element to return

Reference to the requested element.

std::out_of_range if !(pos < size()).

Constant.