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

Retrieves the underlying value as a double or throws an exception.

constexpr double
as_double() const;

Strong guarantee. Throws on type mismatch.

Type	Thrown On
`bad_field_access`	If `!this->is_double()`