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POwhat is "capability query" in dynamic_cast context and why is this useful?
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  1. POwhat is "capability query" in dynamic_cast context and why is this useful?
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    copied!<p>I am reading some C++ material on dynamic_cast and there the following practice is considered bad:</p> <pre><code>class base{}; class derived1 d1 :public base{}; class derived2 d2 :public base { public: void foo(){} }; void baz(base *b) { if (derived2 *d2= dynamic_cast&lt;derived2 *&gt; (b) ) { d2-&gt; foo(); } } </code></pre> <p>The remedy to this is to use the "capability query" using an empty pure virtual base class like following:</p> <pre><code>class capability_query { public: virtual void foo()= 0; }; class base{}; class derived1 d1 :public base{}; class derived2 d2 :public base, public capability_query { public: virtual void foo(){} }; void baz(base *b) { if (capability_query *cq= dynamic_cast&lt;capability_query *&gt; (b) ) { cq-&gt; foo(); } } </code></pre> <p>My 1st question is why is the first code block considered bad? The way I see it <code>foo</code> is only executed if d2 can be successfully downcasted from b in the <code>baz</code> function. So what is the issue here?!</p> <p>My 2nd question is why is the second code block considered good? and how does this fix the issue, which I don't understand in the first place. </p> <p>FYI, my google search for <code>capability query</code> returned <a href="http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Capability_Query" rel="nofollow">http://en.wikibooks.org/wiki/More_C%2B%2B_Idioms/Capability_Query</a> which seems to be basically code block1 and not code block2. I still don't get why an additional empty base class is considered a better practice?</p> <p><strong>EDIT:</strong> here is the best possible answer I can think of.Since inside <code>baz</code> I am downcasting to a pointer type and not reference, in case the downcast is not successful , I will get a Null pointer and not std::bad_cast. So, assuming the cast goes wrong and I do get NULL pointer , but what if I am not supposed to execute <code>Null-&gt;foo</code> and if I may forget to test for NULL, so code block 1 could be a problem. The way code block 2 fixes this, is by adding an empty class. Even if </p> <pre><code>dynamic_cast&lt;capability_query *&gt; (b) </code></pre> <p>fails and I get a null pointer , you cannot execute <code>null-&gt;foo</code> since inside <code>capability_query</code> class this <code>foo</code> method is pure virtual. This is just a conjecture , but may be I am on the right path??!!</p>
 

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