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 an iterator pointing to the first resultset that this object contains.

iterator
begin() const;

this->has_value() == true

No-throw guarantee.

The returned iterator and any reference obtained from it are valid as long as *this is alive. Move operations invalidate iterators.

Constant.