c++17

empty_variant.cc

#include <variant>
#include <iostream>
#include <ios>

#define print(x) do { std::cout << #x << ": " << std::boolalpha << (x) << std::endl; } while (0)

struct IsEmptyVisitor
{
    template<typename T>
    bool operator()(const T&) const { return false; }
    bool operator()(std::monostate) const { return true; } // struct nil {} would do as well
};

template<typename T>
bool is_really_empty(const T& _variant)
{
    return _variant.index() == 0;
}

int main()
{
    std::variant<std::monostate, int, long> a;

    const bool is_a_empty = std::visit(IsEmptyVisitor(), a);
    print(is_a_empty);
    const bool is_a_really_empty = is_really_empty(a);
    print(is_a_really_empty);

    std::variant<std::monostate, int, long> b(42);

    const bool is_b_empty = std::visit(IsEmptyVisitor(), b);
    print(is_b_empty);
    const bool is_b_really_empty = is_really_empty(b);
    print(is_b_really_empty);

    std::variant<std::monostate, int, long> c(42L);

    const bool is_c_empty = std::visit(IsEmptyVisitor(), c);
    print(is_c_empty);
    const bool is_c_really_empty = is_really_empty(c);
    print(is_c_really_empty);
}