...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
Return true if the address is unspecified.
bool is_unspecified() const;
The address 0:0:0:0:0:0:0:0 is called the unspecified address. It indicates the absence of an address.