Rule of 3 for a simple class...

[JustSomeGuy]

I am developing a class. It's a pretty simple one.
The class has one data member which is a std::string.
The purpose of the class is simply to make sure that the string ends up having an even length.
The class is called a uid.
I'm a little confused.. should this just inherit from std::string ie
class uid:public std::string ?
I'm not sure I'm handling the copy constructors properly.

class uid 
{ 
private: 
   std::string id; 
public: 
   uid(std::string _id) 
   { 
      id = _id; 
      if (id.size() & 1) // is the length odd? 
         id.append(" ");  // yes. Now the length should be even 
   } 
   uid& operator=(std::string& _id) // Is this method necessary?
   { 
      id = _id; 
      if (id.size() & 1) // is the length odd? 
         id.append(" "); 
   } 
   uid& operator=(uid& _id) 
   { 
      id = _id; 
   } 
   ~uid(void) 
   { 
      // Nothing to do really. 
   }
   int getSize(void)
   {
      return id.size();
   }
   const void *getData(void)
   {
      return id.c_str();
   }
};