NAME

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