Get the current value of an std::variant at runtime in C++

variant.cpp

// the "secret sauce" here is that everything has to happen at compile-time

// step 3) this gets called for every possible index at compile time
template <std::size_t index, class... Args> void visitor_impl(const std::variant<Args...>& variant)
{
    // this code path narrows us down to the type that's currently in the variant
    if (variant.index() == index)
    {
        // we now have the current value in a fully generic way
        auto         value = std::get<index>(variant);
        // do what you need to do with your variant underlying value here...
    }
}

// step 2) the helper uses a fold sequence to iterate over every possible type index at compile-time
template <std::size_t... IndexSequence, class... Args>
void visitor_helper(std::index_sequence<IndexSequence...>, const std::variant<Args...>& variant)
{
    (visitor_impl<IndexSequence>(variant), ...);
}

// step 1) visitor creates a compile time integer sequence, and feeds it to the helper
template <class... Args> void visitor(const std::variant<Args...>& variant)
{
    visitor_helper(std::make_index_sequence<sizeof...(Args)>{}, variant);
}