Re: Inheritance: Can the base class access a function in the class which is inheriting from it?

From:
ngoonee@gmail.com
Newsgroups:
comp.lang.c++
Date:
Fri, 14 Mar 2008 01:00:51 -0700 (PDT)
Message-ID:
<412ddf5c-b845-41a8-b3aa-029d9b762b32@s13g2000prd.googlegroups.com>
On Mar 14, 1:41 pm, red floyd <no.s...@here.dude> wrote:

ngoo...@gmail.com wrote:

On Mar 14, 1:23 pm, red floyd <no.s...@here.dude> wrote:

ngoo...@gmail.com wrote:

Hi all,
My intent is to code some algorith which repeatedly applies a certain
comparison function in C++. The algorithm itself works fine, but I'm
searching for easy ways to select the comparison function to be
applied, preferably at run-time (ie. no preprocessor defines).
A very simplified example of the idea I had:-
class baseclass {
private:
  virtual int compare(int a,int b);
public:
  int do_something(int a, int b) {
    return compare(a,b);
  }
};
class class2:public baseclass {
private:
  int compare(int a,int b) {
    return (b-a);
  }
public:
};
class class1:public baseclass {
private:
  int compare(int a,int b) {
    return (a-b);
  }
public:
};
Hence, if I create an instance of class1 and call the function
do_something, I'd like it to access the compare() function within
class1. Obviously, however, it doesn't work as is, meaning the method
I'm trying is not legal.
1. Could anyone suggest a legal way of doing what I'm trying to
accomplish, besides simply copy-pasting the whole code of
do_something() into a seperate class?
2. Is there any other way besides class inheritance to accomplish the
above?

What doesn't work?
The only problem that I can see is that you haven't implemented
baseclass::compare. Either provide an implmeentation or declare it as
pure virtual.


Perhaps I wasn't being clear enough, apologies. I don't want to (in
any situation) use baseclass::compare. I will not be creating any
objects of class baseclass, only of class1 and class2. I would like
class1 and class2 to be able to use baseclass's do_something()
function, but using their own unique versions of compare().


Then declare it pure virtual...
Note the "= 0" in the class below.

What you have created is a well known design pattern called "Template"
(not to be confused with C++ templates).

class baseclass {
private:
   virtual int compare(int a,int b) = 0;
public:
   int do_something(int a, int b) {
     return compare(a,b);
   }

};


Thank you. I tried this but in itself it did not solve my problem. The
issue was with me, however, and my incomplete understanding of
polymorphism. It turns out that I shouldn't create an instance of
class1 or class2 per se, instead I should have done the below:-

// code fragment

baseclass * bclass_ptr = new class1();
int result = (*bclass_ptr).do_something(1,5);

// end code fragment

Thank you for your help, red floyd sir. Any further advise on the
topic of polymorphism would be appreciated, though my immediate
problem has been solved.

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