variant_test.cpp

#include <iostream>
#include <vector>
#include <boost/variant.hpp>

namespace impl
{
    struct vnum_wrapper;

    using varr = std::vector<vnum_wrapper>; // <- indirection here
    using vnum = boost::variant<int, varr>;

    struct vnum_wrapper
    {
        vnum _v;
        
        template <typename... Ts>
        vnum_wrapper(Ts&&... xs) : _v(std::forward<Ts>(xs)...) { }
    };
}

using vnum = impl::vnum;
using impl::varr;

struct vnum_printer
{
    void operator()(int x) const    
    { 
        std::cout << x << "i\n"; 
    }

    void operator()(const varr& arr) const
    {
        for(const auto& x : arr)
        {
            boost::apply_visitor(*this, x._v);
        }
    }
};

int main()
{
    vnum v0 = varr{vnum{1}, vnum{2}, varr{vnum{3}, vnum{4}}};
    boost::apply_visitor(vnum_printer{}, v0);
}