table of contents
std::variant_npos(3) | C++ Standard Libary | std::variant_npos(3) |
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)
2022.07.31 | http://cppreference.com |