< cpp‎ | utility‎ | variant
Defined in header <variant>
inline constexpr std::size_t variant_npos = -1;
(since C++17)

This is a special value equal to the largest value representable by the type std::size_t, used as the return type of index() when valueless_by_exception() is true

See also

returns the zero-based index of the alternative held by the variant
(public member function)
checks if the variant is in the invalid state
(public member function)