c++ - QDataStream operator << overloading for Abstract class -

i want overload << operator abstract class virtual operator, know how overload operator simple class , code below sample.

  class normalclass     {     public:         int firstfield() const;         void setfirstfield(int firstfield);          int secondfield() const;         void setsecondfield(int secondfield);      private:         int m_firstfield;         int m_secondfield;     };     qdatastream &operator<<(qdatastream &out, const normalclass &obj)     {         out << obj.firstfield();         out << obj.secondfield();         return out;     } 

but lets have abstractbaseclass ,childclass below

class abstractbaseclass { public:     abstractbaseclass() {}      int basefirstfield() const;     void setbasefirstfield(int basefirstfield);      int basesecondfield() const;     void setbasesecondfield(int basesecondfield);  private :     int m_basefirstfield;     int m_basesecondfield;  };   class childclass : public abstractbaseclass { public:     childclass() {}       int childfirstfield() const;     void setchildfirstfield(int childfirstfield);  private:      int m_childfirstfield;  } 

now want overload << operator abstractbaseclass childclass should implement it.

this try

#ifndef abstractbaseclass_h #define abstractbaseclass_h  #include <qdatastream>  class abstractbaseclass { public:     abstractbaseclass();      int basefirstfield() const;     void setbasefirstfield(int basefirstfield);      int basesecondfield() const;     void setbasesecondfield(int basesecondfield);  private :     int m_basefirstfield;     int m_basesecondfield; };  virtual qdatastream &operator<<(qdatastream &out, const abstractbaseclass &obj)=0;    #endif // abstractbaseclass_h 

compiler complains : virtual outside class declaration , have idea?

what have try , compiled , think of method , there better achievements.

this abstract class

.h

#ifndef abstractbaseclass_h #define abstractbaseclass_h  #include <qdatastream>  class abstractbaseclass { public:     abstractbaseclass();      int basefirstfield() const;     void setbasefirstfield(int basefirstfield);      int basesecondfield() const;     void setbasesecondfield(int basesecondfield);      virtual qdatastream &serialize(qdatastream &stream)=0;  private :     int m_basefirstfield;     int m_basesecondfield; };  qdatastream &operator<<(qdatastream &out, abstractbaseclass &obj);  #endif // abstractbaseclass_h 

.cpp

 #include "abstractbaseclass.h"  abstractbaseclass::abstractbaseclass() {     m_basefirstfield = 0;     m_basesecondfield = 0; }  int abstractbaseclass::basefirstfield() const {     return m_basefirstfield; }  void abstractbaseclass::setbasefirstfield(int basefirstfield) {     m_basefirstfield = basefirstfield; }  int abstractbaseclass::basesecondfield() const {     return m_basesecondfield; }  void abstractbaseclass::setbasesecondfield(int basesecondfield) {     m_basesecondfield = basesecondfield; }   qdatastream &operator<<(qdatastream &out, abstractbaseclass &obj) {     return obj.serialize(out); } 

and child class

class childclass : public abstractbaseclass { public:     childclass(){}      int childfirstfield() const     {         return m_childfirstfield;     }     void setchildfirstfield(int childfirstfield)     {         m_childfirstfield = childfirstfield;     }  private:      int m_childfirstfield;      // abstractbaseclass interface public:     qdatastream &serialize(qdatastream &stream) override     {         stream << basefirstfield();         stream << basesecondfield();         stream << childfirstfield();         return stream;     } };