Covariance with virtual functions and stl containers

This is a discussion on Covariance with virtual functions and stl containers within the C++ Programming forums, part of the General Programming Boards category; I have two classes, one derived from the other, and a virtual get function is supposed to return a pointer ...

  1. #1
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    Covariance with virtual functions and stl containers

    I have two classes, one derived from the other, and a virtual get function is supposed to return a pointer to a container with different element types depending on who's getting called. Simplified:

    Code:
    class A {
    public:
    virtual stl_container<T1>* get_something();
    };
    
    class B : public A {
    public:
    stl_container<T2>* get_something();
    };
    And no, the classes themselves are not templates.

    Anyway, when I try to compile this, it gives me an error message about "invalid covariant return types." So I thought it might help if the containers were recast into an isa relationship or something:

    Code:
    template <typename C = T1>
    class C1 : public stl_container<C> {};
    
    class C2 : public C1<T2> {};
    
    class A {
    public:
    virtual C1<>* get_something();
    };
    
    class B : public A {
    public:
    C2* get_something();
    };
    But that didn't work either.

    I'm almost tempted to return a void* and cast it as it comes out. Is there another solution?

    Oh, the reason the simplest solution (turning class A and B into a templated class) is not possible because the T1, T2 types are custom classes and different things are supposed to happen to them depending on the class.
    Last edited by cunnus88; 03-31-2009 at 07:12 PM.

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by cunnus88
    I have two classes, one derived from the other, and a virtual get function is supposed to return a pointer to a container with different element types depending on who's getting called.
    Suppose you did manage to get this setup. How would you call this virtual function? What would be the type of the pointer used to store the return value of a call to this virtual function? It cannot be a pointer to stl_container<T1>, since a pointer to stl_container<T2> might be returned, and vice versa. Basically, you are unable to use polymorphism, since you have to know precisely at the site of the call which function is called in order to correctly store (or even use) its return value.

    I think that you need to relook your fundamental design here. What exactly are you trying to do?
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    Yeah, you're right. The more I think about it, it seems that I need to rethink my design.

  4. #4
    Cat without Hat CornedBee's Avatar
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    templ<T1> and templ<T2> are never related, even if T1 and T2 are. There are very good reasons in type theory for this. In short, let's assume that for T2 : T1, templ<T2> : templ<T1>. Then, by the Liskov Substitution Principle, templ<T2> has to be usable everywhere a templ<T1> is. Since it's not allowed to insert a T1 into a templ<T2>, though, that is not given.
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    Hard to tell with the toy example classes, but I am always suspicious of the design of a class (other than a simple data class) that has get methods. The usual pattern followed is...

    Code:
    Stuff s = dumbObject.getStuff();
    // operate on s
    dumbObject.putStuff(s);
    Where it might be better to have something like...

    Code:
    smartObject.doStuff();
    But again, hard to tell without seeing real code.

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