constexpr reference operator[](size_type idx) const;
Preconditions:
idx < size() is
true. Returns:
*(data() + idx). constexpr reference at(size_type idx) const;
Returns:
*(data() + idx). Throws:
out_of_range if
idx >= size() is
true. constexpr reference front() const;
Preconditions:
empty() is
false. constexpr reference back() const;
Preconditions:
empty() is
false. Returns:
*(data() + (size() - 1)). constexpr pointer data() const noexcept;