Any, variant and lambda visitor

variant_any_cast_example.cpp

#include <boost/variant.hpp>
#include <boost/spirit/home/support/detail/hold_any.hpp>
#include <iostream>
#include <type_traits>

using Any = boost::spirit::hold_any;

template<typename ...> struct VariantAnyCast;

template<class T, class ... Rest>
struct VariantAnyCast<T, Rest...>
{
    template<typename ... V>
    void operator()(boost::variant<V ...>& v, Any const& any) const
    {
        any.type() == typeid(T) ? (v = boost::spirit::any_cast<T>(any), 0) 
                                : (VariantAnyCast<Rest...>{}(v, any), 0);     
    }
};

template<>
struct VariantAnyCast<>
{
    template<typename ...T>
    void operator()(T&& ...) const 
    {
        //throw bad any cast
        throw std::runtime_error{"bad variant cast"};
    }
};

template<typename ...T>
auto variant_any_cast(Any const& any)
{
    boost::variant<T...> result{};
    VariantAnyCast<T...>{}(result, any);
    return result;
}

int main()
{
    auto v = variant_any_cast<int,double>(Any{1.2});
    boost::apply_visitor([](auto const& x){
        std::cout << x << std::endl;    
    }, v);
}