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creating a list pointer of parent type from base type...

This is a discussion on creating a list pointer of parent type from base type... within the C++ Programming forums, part of the General Programming Boards category; I wasn't sure how to phrase the question in the title. But here is my question. First lets say I ...

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    Question creating a list pointer of parent type from base type...

    I wasn't sure how to phrase the question in the title. But here is my question. First lets say I have something like this:

    Code:
    class A
    {
    ...
    };
    
    class B 
    {
    ...
    };
    
    class C : public A, public B
    {
    ...
    };
    Where A and B are "interfaces" meaning they contain mostly if not all pure virtual functions.

    What I would like to do is have a main list, in the main part of my code:
    Code:
    std::list<C*> mainList;
    But in the subsystems of the code that only need to know about A and not about B or C, I would like to have a list something like this:

    Code:
    std::list<A*>* alist = &mainlist;
    So in otherwords is there a way to make pointer to a list of type "pointer to A" from a list of type "pointer to C" without having to create a new list and copy the elements one at a time?

    I hope that makes sense.

    Adam.

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    Quote Originally Posted by Adamkromm View Post
    So in otherwords is there a way to make pointer to a list of type "pointer to A" from a list of type "pointer to C" without having to create a new list and copy the elements one at a time?
    No it is not.
    Right 98% of the time, and don't care about the other 3%.

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    Registered User gardhr's Avatar
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    Quote Originally Posted by Adamkromm View Post
    So in otherwords is there a way to make pointer to a list of type "pointer to A" from a list of type "pointer to C" without having to create a new list and copy the elements one at a time?
    In practice it should work fine. That said, I'm not so sure that it's guaranteed by the standard, and besides that it's probably considered "bad form". A more conventional approach would be to simply make it a list<A*> to begin with. Manipulating the elements as C*'s would be a simple matter of a cast, after all...
    Last edited by gardhr; 04-14-2012 at 02:47 AM.

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    I guess it's not going to be as easy as I'd hoped.

    gardhr: The only problem with that suggestion is that I have more than one subsystem that needs to know about different parents. Eg, one subsystem needs to know about A, another needs to know about B ...

    Thanks for the quick responses.

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    Registered User manasij7479's Avatar
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    Quote Originally Posted by Adamkromm View Post
    Eg, one subsystem needs to know about A, another needs to know about B ...
    That would be quite easy.
    Have your subsystems use pointers (or references) to A s and B s respectively... but pass around a C*.
    That is all you need to do, according to the Liskov Substitution Principle.. (Unless, I'm myself confused about something here..)
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
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    Quote Originally Posted by manasij7479 View Post
    That would be quite easy.
    Have your subsystems use pointers (or references) to A s and B s respectively... but pass around a C*.
    That is all you need to do, according to the Liskov Substitution Principle.. (Unless, I'm myself confused about something here..)
    I know that would work -- if I pass around the C*'s individually -- but what I want is to be able to pass around a list of C*'s.

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    Quote Originally Posted by Adamkromm View Post
    I know that would work -- if I pass around the C*'s individually -- but what I want is to be able to pass around a list of C*'s.
    Now.. that doesn't make sense!
    If a function is declared as :
    Code:
    void foo(std::list<A*> alist);//or B*
    You can definitely call it with a list<C*> object. That IS the point of having interfaces.
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
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    Master Apprentice phantomotap's Avatar
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    You can definitely call it with a list<C*> object.
    You very definitely can't.

    That IS the point of having interfaces.
    The point of interfaces is sharing implementation of logic that uses those interfaces via some form of polymorphisms.

    There is no form of polymorphisms at play in your example.

    Soma
    manasij7479 likes this.

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    Registered User manasij7479's Avatar
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    Quote Originally Posted by phantomotap View Post
    You very definitely can't.
    You're right. I put the example before testing it.
    Apart from being a little embarrassed, I can't understand why it isn't acceptable by the compiler.
    [@Mods:You could split the thread from the point I posted that wrong statement...I'd like some light on it]
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
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    Registered User whiteflags's Avatar
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    The compiler essentially writes different forms of the list class depending on the template parameter(s), and they aren't related to one another in a hierarchy. It just so happens that the type contained by the list, which is otherwise separate from the list, is in a hierarchy.

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    Quote Originally Posted by whiteflags View Post
    The compiler essentially writes different forms of the list class depending on the template parameter(s), and they aren't related to one another in a hierarchy. It just so happens that the type contained by the list, which is otherwise separate from the list, is in a hierarchy.
    That sure seems to be a deficiency in the design, to me.
    A list of circles, for example, should be able to be count as a list of shapes.
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    Registered User whiteflags's Avatar
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    Your design would mean that any of the lists are related to every other kind of list.

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    Registered User manasij7479's Avatar
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    Quote Originally Posted by whiteflags View Post
    Your design would mean that any of the lists are related to every other kind of list.
    No relationship, if the template arguments don't show the relation themselves.
    Manasij Mukherjee | gcc-4.8.2 @Arch Linux
    Slow and Steady wins the race... if and only if :
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    2.The fast and unsteady suddenly falls asleep while running !



  14. #14
    Master Apprentice phantomotap's Avatar
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    Well, whiteflags explained the issue well enough, but I'll tackle the next bits.

    It isn't a deficient design; it is only a design that works within the constraints the committee required.

    The containers were designed to need no virtual functions.

    The containers were designed to have minimal and as uniform as possible template parameters.

    The containers would have no way of knowing which classes in the chain to inherit as it is literally impossible to frame this within C++ as any attempt you might make breaks the moment one adds other classes to the hierarchy.

    It is possible to have a container of one type implicitly converted to the same container of a related type by duplicating the converted pointers with all the relevant overloads in place. This doesn't require virtual inheritance, implicit knowledge of the class hierarchy, and allows uniform template parameters. It wasn't done primarily because the techniques weren't well understood and the necessary functionality wasn't widely available at the time the library was being formalized.

    Soma

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    Master Apprentice phantomotap's Avatar
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    No relationship, if the template arguments don't show the relation themselves.
    This, again, is wrong.

    If `void A::doSomething()' is to call `void C::doSomething()' correctly because of polymorphisms related to inherited types `A' and `C' must be related. This holds true regardless of types `A' and `B'. To prove this to yourself replace `A' with `std::list<A *>', `C' with `std::list<C *>', and `doSomething' with `push_back'.

    [Edit]
    The point here is that for each class in the `A' or `C' hierarchy the container (`std::list<???>') would have to inherit an interface (`std::list<A *>::interface' and `std::list<C *>::interface' for example) for the relationship between the containers to work.
    [/Edit]

    [Edit]
    If your `foo' from post 7 was itself a template the concept works but because of parametric polymorphisms which are very different from those inheritance based.

    Code:
    template<typename T> void foo(std::list<T*> alist);
    Of course, a template of that form has problems of its own.

    Using a typed interface of iterators is certainly a better approach as would be a solution built around the mechanisms I referenced in post 14.
    [/Edit]

    Soma
    Last edited by phantomotap; 04-14-2012 at 02:17 PM.

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