Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Returns the i-th row or throws an exception.

row_view
at(
    std::size_t i) const;

Strong guranatee. Throws on invalid input.

Type	Thrown On
`std::out_of_range`	`i >= this->size()`

Constant.