string::at (1 of 2 overloads)

Return a character with bounds checking.

char&
at(
    std::size_t pos,
    boost::source_location const& loc = BOOST_CURRENT_LOCATION);

Returns a reference to the character specified at location pos.

Constant.

Strong guarantee.

Name	Description
`pos`	A zero-based index to access.
`loc`	`source_location` to use in thrown exception; the source location of the call site by default.

Type	Thrown On
boost::system::system_error	`pos >= size()`.