Store and retrieve different data types in an object

I want to store and retrieve different data types in one object. This will late be an interface to a database.

Basic types (char, short, int, float, double, ...) should go into a union to save storage space. In addition, I want to store std::string and std::vector. Storage is done by overloading the assignment operator, retrieval is done by conversion operator overloading. The code below is a stripped-down version working with int, double and std::string.

While the int and double parts work fine, the std::string part fails. I'm also not sure if the operator overloading is the most elegant way to get the data in and out. I'm restricted to C++11, so std::any is not an option.

#include <string>

class mydata {
private:

   enum {
      TID_INT,
      TID_DOUBLE,
      TID_STRING
   };

   int m_type_id; // stores one of TID_xxx

   // this object should be able to store int, double, std::string
   union {
      int   m_int;
      double m_double;
   } m;
   std::string m_string;

public:
   // Default constructor
   mydata() { m_type_id = 0; }

   // Overloading the Assignment Operator
   template <typename T>
   const T &operator=(const T &v) {
      if (typeid(v) == typeid(int)) {
         m_type_id = TID_INT;
         m.m_int = v;
      } else if (typeid(v) == typeid(double)) {
         m_type_id = TID_DOUBLE;
         m.m_double = v;
      } else if (typeid(v) == typeid(const char *)) {
         m_type_id = TID_STRING;
         // this fails -->
         //m_string = std::string(v);
      }
      return v;
   }

   operator int() {
      if (m_type_id == TID_INT)
         return m.m_int;
      else if (m_type_id == TID_DOUBLE)
         return (int) m.m_double;
      return 0; // maybe throw an exception here
   }

   operator double() {
      if (m_type_id == TID_INT)
         return (double)m.m_int;
      else if (m_type_id == TID_DOUBLE)
         return m.m_double;
      return 0;
   }

};

/*------------------------------------------------------------------*/

int main() {
   mydata d1, d2, d3;

   d1 = 123;     // store an int
   d2 = 456.789; // store a double

   int i = d1;
   i = d2;

   double d = d1;
   d = d2;

   // this fails --->
   // d3 = "Hello"; // store a string

   return 1;
}