Thread: Better way than casting??

  1. #1
    Registered User
    Join Date
    May 2006

    Better way than casting??


    Class B inherits from class A
    class D inherits from class A

    D has a function say, fun_D()

    B has a function say, fun_B(A* Dtype)
    void main()
    	D *pD = new D();
    	B *pB = new B();
    I want to call fun_D() from B. But I don't want to include fun_D in A.

    The best way I could think was,

    void B::fun_B(A *Dtype)
    	D *lDtype = static_cast <D*>(Dtype);
    But I am not happy with this casting. Is there any better way to do this?

    Best Regards,

  2. #2
    Lurking whiteflags's Avatar
    Join Date
    Apr 2006
    United States
    If it doesn't completely bother you, you can take your foo_function out of the class implementation, and have it work on a const reference to a B object.

    And because of scope, there shouldn't be any conflicts if D uses it's own foo_function while B uses the global one.

  3. #3
    Registered User
    Join Date
    Jan 2005
    >> Is there any better way to do this?

    In many (but not all) cases, yes. The way to get rid of the cast is to identify why fun_B has an A* parameter while it only expects D* arguments to be passed to it. Why not just have fun_B take a D* instead?

    If the reason is that sometimes you will be passing a D, and other times you will be passing A or other classes derived from A that are not D's, then you should be using dynamic_cast to verify that the pointer is in fact a D, not an A or some other derived class. That won't fix your design problem though.

    To fix the design problem, you have to separate the use of D specific functions to places where only a D can be used. If fun_D makes sense as part of the interface to A, but B objects will do nothing and D objects will do something, then you could add fun_D as a virtual function in A and you would no longer need the cast. That is closer to good design, although it might not be the best solution especially if the fun_D interface makes no sense anywhere but for D objects.

  4. #4
    carry on JaWiB's Avatar
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    Feb 2003
    Seattle, WA
    On an unrelated note, main returns int
    "Think not but that I know these things; or think
    I know them not: not therefore am I short
    Of knowing what I ought."
    -John Milton, Paradise Regained (1671)

    "Work hard and it might happen."

  5. #5
    Confused Magos's Avatar
    Join Date
    Sep 2001
    You're doing a down-cast (down the inheritance hiearchy) and thus dynamic_cast would be better suited as it's not guaranteed that the A* is a D* (in the general case).

    Give a man a fish and you feed him for a day.
    Teach a man to fish and you feed him for a lifetime.

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