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POAdding Functionality To An Object Without Changing The Interface
 

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  1. POAdding Functionality To An Object Without Changing The Interface
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    <p>I have an object presented as a reference/pointer to an interface. I would like to call a method on the concrete object if that method is present, without changing the interface, breaking encapsulation, or writing any horrible hacks. How can it be done?</p> <p>Here's an example.</p> <p>I have an interface:</p> <pre><code>class IChatty { public: virtual ~IChatty() {}; virtual std::string Speak() const = 0; }; </code></pre> <p>And multiple concrete implementation of this interface:</p> <pre><code>class SimpleChatty : public IChatty { public: ~SimpleChatty() {}; virtual std::string Speak() const override { return "hello"; } }; class SuperChatty : public IChatty { public: void AddToDictionary(const std::string&amp; word) { words_.insert(word); } virtual std::string Speak() const override { std::string ret; for(auto w = words_.begin(); w != words_.end(); ++w ) { ret += *w; ret += " "; } return ret; } private: std::set&lt;std::string&gt; words_; }; </code></pre> <p>The <code>SuperChatty::AddToDictionary</code> method is not present in the abstract <code>IChatty</code> interface, although it could be included in another, new interface.</p> <p>In the real world, these objects are constructed through factories, themselves concrete instantiations of an abstract interface. However for our purposes that's orthogonal to the problem at hand:</p> <pre><code>int main() { IChatty* chatty = new SuperChatty; chatty-&gt;AddToDictionary("foo"); std::cout &lt;&lt; chatty-&gt;Speak() &lt;&lt; std::endl; } </code></pre> <p>Since <code>AddToDictionary</code> isn't part of the <code>IChatty</code> interface (and <em>can't</em> be part of it), I can's call it.</p> <p>How can I call <code>AddToDictionary</code> on the <code>chatty</code> pointer without breaking encapsulation, writing some horrible hack, or taking any other design shortcuts?</p> <p>NOTE: In the real world, the dictionary is part of the <code>SuperChatty</code> object itself, and cannot be separate from it.</p> <p>NOTE2: I do not want to downcast to the concrete type.</p>
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    1. COdynamic_cast is your friend.
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    2. CO@snipes83 Er... no it's not.
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    3. CO@snipes83: If you mean something like, `dynamic_cast<SuperChatty*>(chatty)`, then `dynamic_cast` is a hack. Relying on the ability to cast a pointer to an abstract interface to a pointer to a concrete type defeats the purpose of using the abstract interface in the first place.
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