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POWhy and Where do we use down casting?
 

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  1. POWhy and Where do we use down casting?
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    <p>Are there any cases where we do down casting of objects?</p> <p>If we do, why?</p> <p>I have observed a way of hiding implementation using the below code. Is this the correct way to do? Is there any better way to achieve the same.</p> <pre><code>class A{ public: A(); virtual ~A(); //exposed virtual functions }; class AImpl : public A{ public: AImpl(A *obj); virtual ~AImpl(); //exposed virtual functions++ }; class Helper{ //utility class so i am making constructor and assignment operator as private public: static bool doWork(A *obj){ AImpl *objImpl = dynamic_cast&lt;AImpl *&gt; (obj); return doWork(objImpl); // some internal function } private: Helper(); Helper(const Helper&amp; obj); const Helper&amp; operator=(const Helper&amp; obj); }; </code></pre> <p>The question still does not makes sense. I agree. I still have not figured out a proper way of hiding the implementation details from the client.</p> <p><strong>UncleBens</strong> I termed this thing wrongly as Object slicing. Basically, I was referring to this (Object Slicing) as the information related to derived part is missing.</p> <p><strong>S.Soni</strong> Thanks for giving a wonderful explaination. Now, I can really put forward question.</p> <p>Consider a clients perspective. The only class which are visible to him is class A and the Helper class (because I have hidden implementation behind AImpl</p> <p>Client has to write the following code as he is unaware of AImpl class</p> <pre><code>int main(){ A *a = new A(); Helper.doWork(a); // ... } </code></pre> <p>As you said AImpl * in this case will actually be pointing to the base class object, which is wrong (you have explained it with a great example), so this approach of hiding implementation is not correct. </p> <p>Any attempt to access a derived class member function will result in a crash (and correctly so).</p> <p>How should I go about hiding the implementation in this case? This is a design problem now?</p>
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    1. COThis question still doesn't make sense. dynamic_cast only works on polymorphic types (you must have some virtual methods). But if A were polymorphic, why would you need to downcast it?
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    2. COI don't understand why down_casting would lead to object_slicing...
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    3. COremoved object slicing as downcasting does not slice an object.
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