std:: variant_npos

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::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

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)
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