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

Access an element, with bounds checking.

value&
at(
    std::size_t pos);

This function is used to access elements of the underlying array, or throw an exception if the value is not an array.

Constant.

Strong guarantee.

Name	Description
`pos`	A zero-based array index.

this->as_array().at( pos ).