Virtual Functions

This is a discussion on Virtual Functions within the C++ Programming forums, part of the General Programming Boards category; Hi! what the difference between overriding a function and virtual functions?! im really stuck! for example i have this: PHP ...

  1. #1
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    Virtual Functions

    Hi!
    what the difference between overriding a function and virtual functions?! im really stuck!
    for example i have this:
    PHP Code:
    #include <iostream.h>
    class one{
    public:
    one(){}
    virtual void override(){
    cout <<"virtual function one"<<endl;
    }
    };
    class 
    two: public one{
    public:
    two(){}
    virtual void override(){
    cout <<"virtual function two"<<endl;
    }
    };
    int main ()
    {  
    one test;
       
    two test2;

       
    test.override();
       
    test2.override();
       
    system ("PAUSE");
      return 
    0;
       } 

    and this:

    PHP Code:
    #include <iostream.h>
    class one{
    public:
    one(){
    }
    void override(){
    cout <<"virtual function one"<<endl;
    }
    };
    class 
    two: public one{
    public:
    two(){
    }
    void override(){
    cout <<"virtual function two"<<endl;
    }
    };
    int main ()
    {  
    one test;
       
    two test2;

       
    test.override();
       
    test2.override();
       
    system ("PAUSE");
      return 
    0;
       } 
    what the difference between them?! they both show me the same output!
    thanks!

  2. #2
    Green Member Cshot's Avatar
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    892
    First off, you need to get rid of the virtual identifier in class two. It only needs to be defined in the parent class.

    Both examples here should output the same thing but there's some differences.

    Let's say you declared in your main something like this:
    Code:
    int main()
    {
       one *A;
       A = new one;
       A->override();
       delete A;
    
       A = new two;
       A->override();
       delete A;
    
       return 0;
    }
    After fixing your virtual function as I stated above, what does the output give you for your first case? How about the second case?

    Here's a good tutorial too:
    http://byerley.cs.waikato.ac.nz/c++/chap10.html
    Last edited by Cshot; 10-03-2002 at 02:36 PM.
    Try not.
    Do or do not.
    There is no try.

    - Master Yoda

  3. #3
    S Sang-drax's Avatar
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    Göteborg, Sweden
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    2,072
    Originally posted by Cshot
    First off, you need to get rid of the virtual identifier in class two. It only needs to be defined in the parent class.
    He doesn't 'need' to do this. Perhaps it's good to explicity state that a function is virtual?
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  4. #4
    Green Member Cshot's Avatar
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    Sorry, poor choice of wording. I still prefer that way though. What do you guys use?
    Try not.
    Do or do not.
    There is no try.

    - Master Yoda

  5. #5
    Seeking motivation... endo's Avatar
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    537
    If its virtual I type it, if I dont then I'll forget
    Couldn't think of anything interesting, cool or funny - sorry.

  6. #6
    Peace
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    1,510
    >>If its virtual I type it, if I dont then I'll forget

    Dito. Since I dont use virtual functions to any large extent, it is good to easily and instantly note any case where I have. AFAIK theres no harm to it. Doesnt sound like bad coding practice either.
    "There's always another way"
    -lightatdawn (lightatdawn.cprogramming.com)

  7. #7
    &TH of undefined behavior Fordy's Avatar
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    Originally posted by lightatdawn
    >>If its virtual I type it, if I dont then I'll forget

    Dito. Since I dont use virtual functions to any large extent, it is good to easily and instantly note any case where I have. AFAIK theres no harm to it. Doesnt sound like bad coding practice either.
    You dont have to keep defining virtual in every derived class...if its virtual in the base....its virtual all the way up....

    Personally I think its good to keep redefining it virtual, as you may find that someone who reuses your class may be doing so directly from a derived class...and not pay attention to the definition of your base class where you actually defined a member function virtual.....for all they know, your function is not virtual....just gives them a little extra info IMO

  8. #8
    Registered User
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    thank you all, i think i got it!

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