how do i define a class that is to be publicly derived from two other classes

I have two classes and I am going to have another, but I want to have this new class derived from the first two.

Here are my classes each in  there own header and at the bottom is what I'm proposing.

#ifndef DATE
   #define DATE
class date
{
  private:
      long  month,
            day,
            year;
  public:

      date(){ }
      date(long m, long d, long y)
          { month = m;
            day = d;
                year = y;
          }
      void get_date(void);
      void set_date(long m,long d,long y);
      long last_day(long m,long y);
      long show_day();
      long show_month();
      long show_year();
      bool leap_year(long y);
      date operator +(long number_days);
      date operator -(long number_days);
      long operator  -(date other_date);
      long days_since(long m,long d,long y);
      long f(long m,long y);
      long g(long m);
      bool incorrect_date(long m,long d,long y);
      bool operator <(date a_date);
      bool operator <=(date a_date);
      bool operator ==(date a_date);
        bool operator !=(date a_date);
      bool operator >(date a_date);
      bool operator >=(date a_date);
      friend ostream &operator << (ostream &stream, date ab)
      {
         stream << ab.month << "/";
         stream << ab.day << "/";
         stream << ab.year <<" ";
         return stream;
      }
   

      friend istream &operator >> (istream &stream, date &ab)
      {
         char slash;
 
         stream >> ab.month >> slash >> ab.day >> slash >> ab.year;
 
         if(ab.year< 100)
              if(ab.year >10)
                  ab.year += 1900;
              else
                  ab.year += 2000;

         return stream;
     }
};



#endif

AND

#ifndef LUMBER
#define LUMBER

const short LEN = 40;       // maximum length of strings

class Type                // type of lumber
{
   private:
      char dimensions[LEN];
      char grade[LEN];
   public:
      Type();                     // constructor (no args)
      
      Type(char di[], char gr[]);  // constructor (two args)
      
      void gettype();              // get type from user

      void showtype();             // display type

      char * show_grade();         // displays the grade

      char * show_dimensions();    // displays the dimensions
};

class Distance                    // English Distance class
{
   private:
      short feet;
      double inches;
   public:
      Distance();                  // constructor (no args)
      
      Distance(short ft, double in);  // constructor (two args)

      void getdist();              // get length from user

      int show_feet();             // shows the feet

      double show_inches();        // shows the inches

      void showdist();             // display distance      
};

class Lumber : public Type, public Distance
{
   private:
      short quantity;                      // number of pieces
      double price;                       // price of each piece
   public:
      Lumber(); // constructor (no args)  
                                     // constructor (6 args)
      Lumber( char di[], char gr[],      // args for Type
            short ft, double in,          // args for Distance
              short qu, double prc );

      void getlumber();                    // gets from the keyboard the lumber values
      
      void showlumber();                   // displays on the screen the values

      int show_quantity();                 // diplays on the screen the quantity

      double show_price();                 // diplays on the screen the  price
};


#endif



WOULD THE NEW CLASS LOOK LIKE THIS?

class inventory : public date, public lumber
{


Would I need virtual classes? I do not want to modify my existing code, I just wnat this in a hearder and have it linked to the other classess.