如何从父类调用子类函数？

Question

I'm new to C++ language and I'm doing library Management System. i have a classes like this. eg:

class mainLibrary{
  void library(){
  cout<<"Welcome to Our Library. Please choose following options"<<endl;
  cout<<"1. Member Section, 2. Lending Section"<<endl;
  if(userinput==1){
   memberclassfunction();//Error : use of undeclared identifier 'memberclassfunction'.
  }else{
  lendingclassfunction();//Error : use of undeclared identifier 'lendingclassfunction'.
 }
}
};

class Member:public mainLibrary{
 void memberclassfunction(){
 //do something
 }
};

class lending:public Member{
 void lendingclassfunction(){
 //do something
}
};

class mainSystem:public lending{
 //this is empty and inherit all. 
};
void main{
 mainSystem s1;
 s1.library();
}

i did something like this. but in mainLibrary Class if i call memberclassfunction(); program show me error. use of undeclared identifier 'memberclassfunction'.

can anyone help me how to do that. i think i have to do something with pointers right?

Answer 1

memberclassfunction必须是mainLibrary中的虚函数，这意味着mainLibrary将成为一个接口，您将无法实例化它。

Answer 2

Neither memberclassfunction nor lendingclassfunction are members of class mainLibrary , that's why the compiler is giving you these errors. I think your intention was to make these functions virtual .

Whenever class B inherit from class A , the former gets the latter's members, not the other way around.

Answer 3

Generally it's not a good idea to have a base class depend on methods in its sub-classes. However if you MUST do this, you could always cast this to the subclass and if the cast succeeds then you can call the method.

lending *lendingThis = dynamic_cast<lending *>(this);
if (lendingThis) {
    lendingThis->lendingclassfunction();
}