formatter.cpp

template <class T>
class MyFormatting;

typedef boost::variant<bool, int, std::string> var_t;

template <>
class MyFormatting<int>
{
private:
    const int& nucleus;

public:
    MyFormatting(const int& n) : nucleus(n) {}

    friend std::ostream& operator << (std::ostream& os, const MyFormatting& w)
    {
        return os << "int:" << w.nucleus;
    }
};
//... specilization for bool and std::string

template <class... T>
using Variant_Formatting_t = boost::variant < MyFormatting<T>... > ;

Variant_Formatting_t<bool, int, std::string> 
FMT(const var_t& A__)
{
 return Variant_Formatting_t<bool, int, std::string>{A__}; 
}

int main()
{
    var_t var_int = 3;
    var_t var_str = "123";
    std::cout << FMT(var_int)<< FMT(var_str) <<  std::endl;
}