# std::variant_npos

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< 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 value of index() when valueless_by_exception() is `true`

Run this code

    #include <iostream>
    #include <stdexcept>
    #include <string>
    #include <variant>

    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

### See also

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

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