How do I write operator<< for std::variant?

Question

I’d like to write operator<< for std::variant. The assumption will be that the operator<< for a particular variant will only be valid if operator<< is valid for all the types that the variant can contain.

Answer 1

//g++ (GCC) 7.2.0
//g++ -std=c++1z -O2 -Wall -pedantic -pthread main.cpp
#include <iostream>
#include <string>
#include <variant>
#include <complex>

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

int main()
{
    using namespace std::complex_literals;
    std::variant<int, std::string, double, std::complex<double>> v = 4;
    std::cout << v << '\n';
    v = "hello";
    std::cout << v << '\n';
    v = 3.14;
    std::cout << v << '\n';
    v = 2. + 3i;
    std::cout << v << '\n';
}

Demo

This relies on passing a generic lambda to std::visit.

See this question for a problem with the previous version of this answer. This answer has been updated to avoid that problem.