Different question about inheritance

This is a discussion on Different question about inheritance within the C++ Programming forums, part of the General Programming Boards category; Here is what I'm trying to do: Create a Vector of a parent class, and fill it up with different ...

  1. #1
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    Different question about inheritance

    Here is what I'm trying to do:
    Create a Vector of a parent class, and fill it up with different children of that class, then make a loop which cicles through the vector and calls a method from each class. The problem is that even if the child class has overriden the method, it keeps calling the parent's method. The method is declared as virtual in both the parent and the child class isn't that how it should be?

    I'm absolutely sure the method has the same signature: return type, name and parameters (in this case none), this is correct, right?

    Here is the section I'm having trouble with
    Code:
    void CSDL::render(std::vector<Entity> entities)
    {    
        Uint32 color;
        
        color = SDL_MapRGB(m_screen->format, 0, 0, 0);
        SDL_FillRect(m_screen, NULL, color); 
        
        for (int i = 0; i < entities.size(); i++)
        {
            drawSprite(entities[i].getX(), entities[i].getY(),
                       entities[i].getSprite());
            entities[i].update(); // <--- This is the method it should call from the child class, but is calling from the parent.
        }
    
        SDL_Flip(m_screen);
    }
    Why drink and drive when you can smoke and fly?

  2. #2
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    The method is declared as virtual in both the parent and the child class isn't that how it should be?
    Yes, but that is not sufficient. You should learn to use virtual functions in a simpler setting. Open your book to the chapter on virtual functions and polymorphism and write a small program with a simple base class that has one virtual function and a derived class that overrides the virtual function and get that to work.

  3. #3
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    The problem is that you are holding Base class objects in your vector instead of pointers or references. That means that when you add a derived class object to the vector, its derived-ness gets sliced off and only the Base class part of the object is stored in the vector.

    Since you cannot hold references in a vector, the solution is to hold pointers. This means that you have to allocate each derived object with new and save the pointer in the vector. Then whenever you erase an element from the vector or whenever you clear() the vector (or when the vector is destructed) you have to remember to call delete on the pointers.

    Note that the destructor of your base class must be virtual, which should be done anyways.

    There are other more elegant solutions than storing the raw pointer in the vector, which include storing a boost::shared_ptr of the base class, or using a boost ptr_container.

    Also note that this is not a problem specific to the vector, it would happen with a C array or any other container that stores your objects.

  4. #4
    Call me AirBronto
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    exactly as what daved said, have a pointer to the base class that equals a pointer to the derived class on the stack

    BaseClass* Test = new DerivedClass;

  5. #5
    semi-colon generator ChaosEngine's Avatar
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    Quote Originally Posted by Daved
    There are other more elegant solutions than storing the raw pointer in the vector, which include storing a boost::shared_ptr of the base class, or using a boost ptr_container.
    hey man, that's not cool. You're stealing my position as forum boost \/\/h0re...

    [edit]awww it would'nt let me say \/\/h0re!!
    Last edited by ChaosEngine; 03-02-2006 at 02:50 PM.
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  6. #6
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    Thanks a bunch everyone, especially Daved. I used your first suggestion and it works perfectly.
    Why drink and drive when you can smoke and fly?

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