c++ - How to use a member variable in a base class function when its size is only known to the derived class -

i want implement function in baseclass uses members of derived classes. each derivedclass, have member of different value. here's example. baseclass has member function foo() uses variable arrstr. content of nul terminated char array found in derived class. how can make baseclass "know" variable arrstr without knowing size? possible?

    class baseclass     {     public:         baseclass();         ~baseclass();      protected:      void foo()     {      prtinf("%s\n", arrstr);     };      };      class derivedclass : public baseclass         {         public:             derivedclass();             ~derivedclass();              protected:              char arrstr[] = "foostring!";      }; 

add (pure) virtual access functions in base class implement in derived class.

#include <iostream> #include <string>  class baseclass { protected:     void foo()     {         std::cout << getstring() << std::endl;     }  private:         virtual std::string getstring() const = 0; };  class derivedclass : public baseclass { private:     virtual std::string getstring() const override     {         return "foostring!";     } }; 

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