std::variant_npos - cppreference.com (original) (raw)

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

#include #include #include #include   struct Demon { Demon(int) {} Demon(const Demon&) { throw std::domain_error("copy ctor"); } Demon& operator= (const Demon&) = default; };   int main() { std::variant<int, Demon> var{42}; std::cout << std::boolalpha << "index == npos: " << (var.index() == std::variant_npos) << '\n';   try { var = Demon{666}; } catch (const std::domain_error& ex) { std::cout << "Exception: " << ex.what() << '\n' << "index == npos: " << (var.index() == std::variant_npos) << '\n' << "valueless: " << var.valueless_by_exception() << '\n'; } }

Possible output:

index == npos: false Exception: copy ctor index == npos: true valueless: true

[edit] See also

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