Can't stream std::endl with overloaded operator<<() for std::variant

Question

This answer describes how to stream a standalone std::variant. However, it doesn\'t seem to work when std::variant is stored in a std::unorde

Answer 1

For some reason, your code (which looks correct to me) is trying to instantiate std::variant<> (empty alternatives) both in clang and gcc.

The workaround I found is to make a template for a specifically non-empty variant. Since std::variant cannot be empty anyway, I think it is usually good to write generic functions for non-empty variants.

template
std::ostream& operator<<(std::ostream& os, const std::variant& v)
{
    std::visit([&os](auto&& arg) {
        os << arg;
    }, v);
    return os;
}

With this change, your code works for me.

---

I also figured out that if std::variant had a specialization of std::variant<> without a single-argument constructor, this problem would not have happened in the first place. See the first lines in https://godbolt.org/z/VGih_4 and how it makes it work.

namespace std{
   template<> struct variant<>{ ... no single-argument constructor, optionally add static assert code ... };
}

I am doing this just to illustrate the point, I don't necessarely recommend doing this.