C ++：在父类中创建子类时出现问题

Question

I'm working on a project that involves a class A that contains a few child classes (childB, childC, childD). My overly simplified goal was to have something like this

#include <string>

class A{
public:
   childB cb;                       <<Error C4430: missing type specifier - int assumed
   childC cc;                       <<Error C4430:    "                            "
   childD cd;                       <<Error C4430:    "                            "

   A(){
      cc.setString("xyz");          <<Error C2039: 'cc' is not a member of A
   }
};

class childB : public A{
public:
   int value;
};

class childC : public A{
private:
   std::string str;
public:       
   void setString(std::string s){
      str = s;
   }
   std::string getString(std::string s){
      return str;
   }
};

class childD : public childB{
public:
   double value;
};

#include "classA.h"

int main (){
   A a;
   a.cc.getString();               <<Error C2228: left of '.getString()' must have class/struct/union
}

From this point I would have multiple things in main that would set the values of the different child classes, but when compiling I am getting numerous errors telling me that cb, cc, cd are not members of A. Additional errors are also listed.

Is there any reason why I wouldn't be able to add a child class to the parent class as shown?

Answer 1

I don't think it is possible for a parent class to have an instance of a child class. You can use a child class pointers instead of child class objects for your purpose.

Define child classes in separate header files.

Add forward declarations of child classes in parent class header file.

Use child class pointers instead of child class objects.

Initialize child class pointer members of parent class with corresponding child class objects in a parent method such as init(). Do not initialize them in parent constructor since it will make an infinite recursion. You will have to move the init() method to a cpp file and include child class header files in that.