C++ cross-casting practical use case

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Hi,

Just to understand the use of cross casting by practice,
Could any person give me a practical example of using a cross casting ?

Thx.

Posted 7-Nov-11 6:18am

elgaabeb

Updated 10-Oct-17 0:28am

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Richard MacCutchan 7-Nov-11 12:48pm

I know what casting is, but what do you mean by cross casting?

elgaabeb 8-Nov-11 4:11am

Definition by Danny Kalev : A cross cast converts a multiply-inherited object to one of its secondary base classes.

Maximilien 7-Nov-11 13:18pm

this : http://www.objectmentor.com/resources/articles/crosscst.pdf (PDF file) ?

Sergey Alexandrovich Kryukov 7-Nov-11 15:48pm

Could be a complete answer is posted as such.
As to the practical use... not sure, to me it's doubtful...
--SA

elgaabeb 8-Nov-11 4:16am

I've seen this article, it seems to be in a Reseach context and it's not so clear.
Thx for your comment :)

Maximilien 7-Nov-11 15:55pm

I did not wanted to post it as an answer because I have not completely read it yet, and found it by googling; heck, I'm not even certain if it really relates to the original question.

Philippe Mori 7-Nov-11 19:00pm

Multiple inheritances and in particular virtual derivation are generally not recommanded... so cross-casting is rarely useful in real-life. It is mainly academical.

Most of the time when multiple inheritance seems to be a solution, implementing multiple interface is probably adequate.

elgaabeb 8-Nov-11 4:22am

I'm 100% OK with you. After hours of search I've found nothing about practical use of cross-casting.
I've posted a question here to incite developpers expierience hoping to find a response :).

3 solutions

Solution 2

It is typical when you know an object through an interface and you want to access another interface of that same object:

C++

class ia
{
public:
    virtual void a1() =0;
    virtual void a2() =0;
    virtual ~ia() {}
};

class ib
{
public:
    virtual void b1() =0;
    virtual void b2() =0;
    virtual ~ib() {}
};

class agg1:
    public ia,
    public ib
{
public:
    virtual void a1() { std::cout << "a1 in agg1" << std::endl; }
    virtual void a2() { std::cout << "a2 in agg1" << std::endl; }
    virtual void b1() { std::cout << "b1 in agg1" << std::endl; }
    virtual void b2() { std::cout << "b2 in agg1" << std::endl; }
};

class agg2:
    public ia,
    public ib
{
public:
    virtual void a1() { std::cout << "a1 in agg2" << std::endl; }
    virtual void a2() { std::cout << "a2 in agg2" << std::endl; }
    virtual void b1() { std::cout << "b1 in agg2" << std::endl; }
    virtual void b2() { std::cout << "b2 in agg2" << std::endl; }
};

int main()
{
    ia* pa1(new agg1);
    ia* pa2(new agg2);

    pa1->a1();
    pa1->a2();
    pa2->a1();
    pa2->a2();

    ib* pb1(dynamic_cast<ib*>(pa1)); // CROSS CAST from ia to ib thanks to agg1
    ib* pb2(dynamic_cast<ib*>(pa2)); // CROSS CAST from ia to ib thanks to agg2

    if(pb1)
    {
        pb1->b1();
        pb1->b2();
    }
    if(pb2)
    {
        pb2->b1();
        pb2->b2();
    }

    delete pa1;
    delete pa2;
}

Posted 12-Nov-11 3:50am

Emilio Garavaglia

Updated 12-Nov-11 3:51am

v2

Comments

Dalek Dave 14-Nov-11 4:47am

Good Answer.

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Paulo Zemek · Accepted Answer · 2011-11-11T08:13:00

I think I can give an example of cross-casting.
I will use interfaces to give the example, but in C++ it will be another class. So...

In .Net it is very common that many classes implement IDisposable.
Now, you have a list of a common type (let's say: Control) but that common type is not a disposable type.

Using a normal cast to an IDisposable will only make the program crash when calling Dispose.
But then, when clearing the list you want to also Dispose all those objects that may be IDisposable.

In .Net we will use the "as" operator. If the result is different than null then we can dispose it. It will be the same using C++.

Want other samples?
Ok... imagine that you have a base class for Serializable objects (objects that can save themselves to a file).
Serialize only receives the file (stream) to which it will save.
But for some objects, you may want to call a special kind of save. So, you check if the object supports that special save (ie, it inherits from another, unrelated class to the base one). If it supports, then you use it.

elgaabeb · Accepted Answer · 2011-11-13T22:43:00

Hi,

Thank you all for your answears.
Sorry for the late answear :)
I've been looking for practical example but it will depends on the design of a project, so i'll give a generic example.
Imagine we have four class : ANIMAL, FLYER, DOG, CROWN.

class Animal {
public:
	virtual ~Animal() {};
	void talk (string sound) {
	cout << " Grrrr " << (char *)&sound <<  endl;
}
};


class Flyer {
public:
	void fly (string destination) {
	cout << " Flying to " << (char *)&destination <<  endl;
}
}; 

class Dog:public Animal{
};


class Crow:public Animal, public Flyer{
};

To save the object of type Dog and Crow we need to do an upcast to the base class animal.
when we need to call a fly() method of each Crow we need to know if the animal inherits from Flyer class or not, this is done by a cross-cast :

XML

Flyer* myAnimal = dynamic_cast<Flyer*>(pAnimal);

 if(!myAnimal)
       cout<<" my animal can't fly :( ..."<<endl;
else
       cout<<" my animal can flye :) ..."<<endl;

I hope that it would be helpfull :).

C++ cross-casting practical use case

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