C++ multiple inheritance problem

This is a discussion on C++ multiple inheritance problem within the C++ Programming forums, part of the General Programming Boards category; I want to use a class composed of all pure virtual functions as like 'interface' in java. but instantiating a ...

  1. #1
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    Talking C++ multiple inheritance problem

    I want to use a class composed of all pure virtual functions as like 'interface' in java. but instantiating a new derived class as interface class is not allowed.

    like following code:

    class ISampleProvider{
    virtual CObject getSample() = 0;
    }

    class BaseClass{
    method A...
    method B...
    }

    class A : public BaseClass, public ISampleProvider{
    method A...
    mehtod B...

    // implementation for ISampleProvider
    CObject getSample(){
    return m_thisSample;
    }
    }

    void main(){

    //A *a = new A ; // this expression does not make problems.
    BaseClass *a = new A;
    ISampleProvider *sample =
    dynamic_cast<ISampleProvider*>(a);

    if(sample)
    sample->getSample(); // error occurs
    }
    }

    error occurs just like:

    " The value of ESP was not properly saved accross a function call. his is usually a result of calling a function declared with one calling convention with a function pointer declared with a different calling convention."


    Like this, main() can get the sample object if the class implemented ISampleViewable interface, regardless of the type of the class.
    It's a powerful expression of Java, but I think this is not supported in C++ 'cause the typing policy or 'virtual' mechanism of C++
    cannot support it.

    I wanna know whether this expression is not allowed originally in C++ or some another way exists...

    thanx
    Last edited by pongsor; 02-08-2002 at 12:53 AM.

  2. #2
    the hat of redundancy hat nvoigt's Avatar
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    sample.getSample();

    Maybe a -> would help.

    What errormessage pops up ?
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  3. #3
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    I updated the thread I've posted.
    please read again my posting where source and error message.

    thanx for your attention.

  4. #4
    the hat of redundancy hat nvoigt's Avatar
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    PHP Code:
    #include <afx.h>

    class ISampleProvider

    public:
        
    virtual CObjectgetSample() = 0
    }; 

    class 
    BaseClass

    public:
        
    void Test();
    }; 

    class 
    : public BaseClass, public ISampleProvider

    private:
        
    CObjectm_thisSample;

    public:
        
    CObjectgetSample()
        { 
            return 
    m_thisSample
        } 
    }; 

    int main()


        
    BaseClass *= new A

        
    // you are casting the baseclass to ISampleProvider here
        // though this happens to be ok, make sure it's no problem
        // with the design, because BaseClass has no ISampleProvider
        // interface. Only A has, so you might want to cast it to A 
        // fist, then to ISampleProvider.

        
    ISampleProvider *sample = (ISampleProvider*)a

        
    sample->getSample();

        return 
    0;


    // I changed the CObject to CObject*, because CObject has no public
    // copy constructor or assignment operator and you don't want to 
    // copy the element anyway, that would be horribly inefficient.
    // What you have as object in Java is a pointer to an object in C++. 
    This code compiles fine on VC6SP5.
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  5. #5
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    Well, I missed it.
    but,
    I'm sorry that It is not my question point.

    My Question is how any classes cast into specific interface class
    in order to extending facilities to a class by implementing it.

    I know your code above will be compiled well, but It does not work well. It will make a null-pointer error. and you can correct the problem by replacing the line 'BaseClass *a = new A' into 'A *a = new A' simply.

    I guess the real why is the OOP mechanism of C++ wouldn't support it. The declaration type (handle) may effect on casting capabilities.

    thanx.

  6. #6
    the hat of redundancy hat nvoigt's Avatar
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    well, as I mentioned, getting the interface from the baseclass is not possible, only A has this interface. The following works fine:

    PHP Code:
    #include <afx.h>

    class ISampleProvider

    public:
        
    virtual CObjectgetSample() = 0
    }; 

    class 
    BaseClass

    public:
        
    void Test();
    }; 

    class 
    : public BaseClass, public ISampleProvider


    private:
        
    CObjectm_thisSample;

    public:
        
    A()
        {
            
    m_thisSample NULL;
        }

        
    virtual CObjectgetSample()
        { 
            return 
    m_thisSample
        } 
    }; 

    int main()


        
    BaseClass *= new A(); 

        
    // you are casting the baseclass to ISampleProvider here
        // though this happens to be ok, make sure it's no problem
        // with the design, because BaseClass has no ISampleProvider
        // interface. Only A has, so you might want to cast it to A 
        // first, then to ISampleProvider.

        // EDIT: Right, cast it to A first, because Baseclass ( pointer a ) 
        // has NO ISampleProvider Interface. So casting from Baseclass
        // to ISampleProvider will result in errors.

        
    ISampleProvidersample = (ISampleProvider*)(A*)a

        
    printf("%p\n"sample->getSample() );

        return 
    0;


    // this prints out 0x00000000 as expected 
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  7. #7
    Unregistered
    Guest

    Smile

    well, Casting into A first is a good way to accomplish it. but, I assume that there's no information 'bout the class type of interface casting at all.

    following UML notation shows this case:

    BaseClass

    A---o ISampleViewable

    you can do it in Java like following:

    class A extends BaseClass implements ISampleViewable{
    ....
    }

    void engine(BaseClass b){
    __...
    __b.methodA();
    __....

    __//if b is a 'Sample providable'--implements ISampleProvider--, then following codes are available

    __ISampleViewable sample = (ISampleViewable)b; //If not allowed, a casting exception will occurs
    __sample.getSample();
    }

    void static main(...){
    __...
    __engine(new A()); // sample viewable (provider)
    __engine(new BaseClass()); // fundamental functions works only
    }


    Probably, C++'s VTable mechanism cannot support the case like Java.

    The Only thing I doubt is 'reinterpret_cast' do this?
    or another way?

  8. #8
    Snior Member
    Join Date
    Jan 2002
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    I'm not entirely sure if this is what you're after, but you could use dynamic cast (as long as you enable RTTI). Extending nvoigts code -

    Code:
    #include <afx.h>
    
    class ISampleProvider
    { 
    public:
        virtual CObject* getSample() = 0; 
    }; 
    
    class BaseClass
    { 
    public:
        virtual void Test(){};
    }; 
    
    class A : public BaseClass, public ISampleProvider
    { 
    
    private:
        CObject* m_thisSample;
    
    public:
        A()
        {
            m_thisSample = NULL;
        }
    
        virtual CObject* getSample()
        { 
            return m_thisSample; 
        } 
    }; 
    
    int main()
    { 
    
        BaseClass *a = new A(); 
        BaseClass *b = new BaseClass();
    
        ISampleProvider* sample = dynamic_cast<ISampleProvider*>(a); 
    
        if(sample)
            printf("%p\n", sample->getSample() );
    
        ISampleProvider* fail = dynamic_cast<ISampleProvider*>(b);
    
        if(fail==0)
            printf("Throw casting exception\n");
    
        return 0;
    }

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