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    <p>Your example code contains at least 4 syntax errors, so it's hard to judge what you're trying to do.</p> <p>And it logically won't work either. You have a class <code>AImpl</code> that inherits <code>A</code>, then a member function that takes an <code>A</code> and appears to try to dynamic-cast it to an <code>AImpl</code>. But it isn't an <code>AImpl</code>, because it's just an <code>A</code>, as that is how the parameter is declared. If you were to pass an instance of <code>AImpl</code> to that function, it would be sliced down to just an <code>A</code>.</p> <p>You could make it a reference or pointer to an <code>A</code>, and then it could be an <code>AImpl</code>. Dynamic casts are only ever of use on references or pointers.</p> <p>Down-casting is used when we have a variable or parameter that has the static type of a base class but we logically know that it is (or might be) of a derived class. It is avoided wherever possible because it means that the compilation process is not able to completely check the type-correctness of the program, i.e. the answer to the question "are you trying to put a square peg into a round hole" cannot be fully answered until runtime.</p> <p><strong>Update after question was edited</strong></p> <p>It sounds like you want clients of your library to have access to a limited interface to an object, A, but when they pass it to a function in your library you will have access to the full interface. You could just use <code>friend</code> for this.</p> <pre><code>class A { friend class LibraryThing; void visibleToLibraryThing(); public: // ctor, etc. void visibleToAll(); }; class LibraryThing { public: void foo(A &amp;a) { a.visibleToLibraryThing(); } }; </code></pre> <p>The <code>LibraryThing</code> class can access the private members of <code>A</code>, because it is declared as a friend of <code>A</code>.</p> <p>The downside is that <code>LibraryThing</code> can access <em>everything</em> in <code>A</code>, so it means that as the author of the library, you won't be able to benefit from encapsulation. Only users of your library will.</p>
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    1. POWhy and Where do we use down casting?
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    1. USDaniel Earwicker
    1. USDaniel Earwicker
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    1. COYou are so quick. I have made the corrections. Basically I am trying to hide class AImpl using dynamic_cast See, if the code now makes sense to you.
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      1. USpankajt
 

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