std::experimental::basic_string_view<CharT,Traits>:: at

From cppreference.com
constexpr const_reference at ( size_type pos ) const ;
(library fundamentals TS)

Returns a reference to the character at specified location pos . Bounds checking is performed, exception of type std::out_of_range will be thrown on invalid access.

Parameters

pos - position of the character to return

Return value

Reference to the requested character.

Exceptions

Throws std::out_of_range if pos >= size ( ) .

Complexity

Constant.

See also

access specified character
(public member function)