Reaching inherited variables with base class pointers

Question

I learnt the work of virtual functions: if the inherited classes inherit a function from the base class, and it is custom for each ones, I can call these functions with pointers that point to the base class, this way:

BaseClass* PointerName = &InheritedClassObject;

But what about variables? I found this question on the site that tells: I can't create virtual variables in C++. My experience proves it: for variables, Visual C++ says: 'virtual' is not allowed.

Then, what is the way to reach the value of a(n inherited) variable that belongs to an inherited class by using base class pointers?

How comes your base class knows that a variable is going to be added in *every* class derived from it? (And this variable will have the same type for each derived class etc.) — dyp, Jul 20 '13 at 22:03
I want to call variables that are inherited from the base class, and consequently all the inherited class have them. — Zoltán Schmidt, Jul 20 '13 at 22:05
Could you provide an example? It might be the case that there's a very simple solution to your problem. — dyp, Jul 20 '13 at 22:07
@ZoltánSchmidt See my answer. I think there may have been some confusion with the wording of the question. if not, please tell me and I will delete my response. — Steve P., Jul 20 '13 at 22:11
@DyP My exact problem is that an expression like this: `PointerArray[Object.X][Object.Y] = &Object;` (where `Object` is an inherited class object and `X` and `Y` are inherited from base class) does nothing. The value of `PointerArray[Object.X][Object.Y]` is still the same, most certainly because neither of `X` or `Y` is called. @SteveP. Okay! — Zoltán Schmidt, Jul 20 '13 at 22:13

score 1 · Answer 1 · answered Jul 20 '13 at 22:00

1

You don't. Virtual functions use them, do whatever needs to be done and return result if needed.

answered Jul 20 '13 at 22:00

Tomek

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Steve P. · Accepted Answer · 2013-07-20T22:53:43.357

1

Based off your comment, I think what you are trying to ask if how do child classes access their parent's variables. Consider this example:

class Parent 
{
public:
  Parent(): x(0) {}
  virtual ~Parent() {}

protected:
  int x;
};

class Child: public Parent
{
public:
  Child(): Parent(), num(0) {}

private:
  int num;
};

void Child::foo() 
{
  num = x; //Gets Parent's x, 
}

NB:
If you define an x in Child, that masks the x in Parent. So, if you want to get the x in Parent, you would need: Parent::x. To simply get x from a Child c, you use c.x if x is public or use a getter if x is protected or private:

int Child::getNum()
{
     return num;
}

edited Jul 20 '13 at 22:53

answered Jul 20 '13 at 22:08

Steve P.

14,489
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True, I haven't programmed in C++ in quite some time. Thanks. – Steve P. Jul 20 '13 at 22:15
In my program, the parent is necessary only for declaring variables that every inherited class owns, so it's not a problem. However, your code is not exactly clear for me: `parent()` is obviously a function, but what is `x(0) {}`? A function with the name of a variable, and a zero input? Anyway, for my exact problem, see the comment below the main post. – Zoltán Schmidt Jul 20 '13 at 22:28
1

@ZoltánSchmidt it just initializes `x` to be 0. I made an edit. It should help. – Steve P. Jul 20 '13 at 22:29
2

@ZoltánSchmidt For the `: x(0)`, see [What is this weird colon-member syntax in the constructor?](http://stackoverflow.com/questions/1711990/what-is-this-weird-colon-member-syntax-in-the-constructor) – dyp Jul 20 '13 at 22:45

score 1 · Answer 3 · answered Jul 20 '13 at 22:10

You can't use any function, data member of an inherited class if it's casted back to base class. However, you can alter those variables with virtual functions. Example:

#include <iostream>

class BaseClass {
public:
  BaseClass() {}
  virtual void do_smth() = 0;

private:
};

class InheritedClass: public BaseClass {
public:
  InheritedClass(): a(1) {}
  virtual void do_smth() { std::cout << ++a << std::endl; }
private:
  int a;
};

int main() {
  BaseClass* ptr = new InheritedClass();
  ptr->do_smth();
  return 0;
}

In this piece of code, virtual function did alteration of variable belongs to InheritedClass.

Reaching inherited variables with base class pointers

3 Answers3