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 <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[edit] See also returns the zero-based index of the alternative held by the
variant
variant is in the invalid state
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