C++ inheritance question

nuclph · #1 13-Mar-2009, 16:46

Hi all,

Suppose, I have the following classes B, D1 and D2 as defined below.
Also, I have function test which have only pointer to the base class and say
key which will tell D1 or D2 to create.

CPP / C++ / C Code:

void test(B *b, const std::string &key)
{
...
}

I would like to be able inside of test to call fd1 of D1 or fd2 of D2 depending on key.
Based on relationships between B,D1,D2 I cannot do it. Do you have any suggestion how I would do it?

Thank you in advance,
nuclph.

CPP / C++ / C Code:


class B
{
public:
	virtual void print() = 0;
};

class D1 : public B
{
public:
	void print()
	{
		cout << "in class D1" << endl;
	}
	void fd1()
	{
		cout << "class D1 function" << endl;
	}
};

class D2 : public B
{
public:
	void print()
	{
		cout << "in class D2" << endl;
	}
	void fd2()
	{
		cout << "class D2 function" << endl;
	}
};

ocicat · #2 13-Mar-2009, 17:27

Quote:

Originally Posted by nuclph

I would like to be able inside of test to call fd1 of D1 or fd2 of D2 depending on key.
Based on relationships between B,D1,D2 I cannot do it.

Correct. Instances of D1 can only call D1::fd1(). Instances of D2 can only call D2::fd2() by design.

Quote:

Do you have any suggestion how I would do it?

Why can't fd1() & fd2() reside in the same class?

sdinfo · #3 14-Mar-2009, 06:40

I am not so clear about your doubt. Though, I can explain as for as i understood.

The second argument of test() is somewhat unclear to me.
Anyway i just take it as, it informs u about which one among D1 and D2 to choose.

In class B{ }; create two virtual functions fd1() and fd2.

In test() function, create an object with both D1 d1; and D2 d2;

b is a pointer to an object of class B.

Put b = &d1 for fd1() or b = &d2 for fd2().

now b->fd1() or b->fd2() will access D1 :: fd1() or D2 :: fd2() respectively.

Hope this would help you.

ocicat · #4 14-Mar-2009, 07:43

Quote:

Originally Posted by sdinfo

In class B{ }; create two virtual functions fd1() and fd2.

This isn't what was initially described. Originally, B:: print() was the only virtual member function defined in the base class.

Quote:

In test() function, create an object with both D1 d1; and D2 d2;

b is a pointer to an object of class B.

Put b = &d1 for fd1() or b = &d2 for fd2().

now b->fd1() or b->fd2() will access D1 :: fd1() or D2 :: fd2() respectively.

This appears to be a homework problem. While I can implement it, you would learn very little if I were to simply give you the answer. You will learn more if you think about it, toil with the concepts, & implement it yourself.

The purpose of this site is to provide guidance & help students help themselves. Yes, there are sites where code will be written freely or for a fee, but all that is learned is for posters in a bind to go back to them. Very little learning (if any...) occurs. Most of us here choose to spend our time more constructively. If you have specific questions and/or you want to discuss code you have written, then we can have a dialogue. As it is, you have posted a vague description/understanding of what is most likely a homework problem, & until you are clear as to the fundamental issue(s), me writing code for you is counterproductive.

This problem obviously revolves around the concept of the abstraction provided by virtual member functions. I suspect that this second string argument is meant to be a flag which differentiates whether D1::fd1() or D2::fd2() is called. This may be implemented simply as a conditional if-statement or it may be more involved. At this point, you haven't provided sufficient information for me to be any more definitive.

I would also invite you to study the following example which demonstrates how different virtual member functions can be called through a base class pointer. Understanding what instance type is assigned to the pointer is fundamental to understanding the problem you are trying to implement.

CPP / C++ / C Code:

#include <iostream>

using namespace std;

class abstract_base {
public:
  virtual void f() = 0;
};

class derived0 : public abstract_base {
public:
  void f()  { cout << "derived0::f()" << endl; }
};

class derived1 : public abstract_base {
public:
  void f()  { cout << "derived1::f()" << endl; }
};

int
main() {
  derived0 d0;
  derived1 d1;
  abstract_base *p;

  p = &d0;
  p->f();

  p = &d1;
  p->f();

  return 0;
}

So if you want to continue discussion, post what code you may have completed wrapped in [cpp] & [/cpp] tags (according to the posting guidelines...), & we can continue from that point.

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C++ inheritance question

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