std::variant_npos (3) Linux Manual Page

std::variant_npos – std::variant_npos

Synopsis

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
index (public member function)
                       checks if the variant is in the invalid state
valueless_by_exception (public member function)