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copied!<p>I assume you're using <code>boost::shared_ptr</code>... I think you want <a href="http://www.boost.org/doc/libs/1_40_0/libs/smart_ptr/shared_ptr.htm#dynamic_pointer_cast" rel="noreferrer"><code>dynamic_pointer_cast</code></a> or <code>shared_polymorphic_downcast</code>.</p> <p>These require polymorphic types, however.</p> <blockquote> <p>what kind of type <code>Base* const</code> is? <code>const Base*</code> I understand, but <code>Base* const</code>? What does <code>const</code> refer to in this case?</p> </blockquote> <ul> <li><code>const Base *</code> is a mutable pointer to a constant <code>Base</code>.</li> <li><code>Base const *</code> is a mutable pointer to a constant <code>Base</code>.</li> <li><code>Base * const</code> is a constant pointer to a mutable <code>Base</code>.</li> <li><code>Base const * const</code> is a constant pointer to a constant <code>Base</code>.</li> </ul> <p>Here's a minimal example:</p> <pre><code>struct Base { virtual ~Base() { } }; // dynamic casts require polymorphic types struct Derived : public Base { }; boost::shared_ptr<Base> base(new Base()); boost::shared_ptr<Derived> derived; derived = boost::static_pointer_cast<Derived>(base); derived = boost::dynamic_pointer_cast<Derived>(base); derived = boost::shared_polymorphic_downcast<Derived>(base); </code></pre> <p>I'm not sure if it was intentional that your example creates an instance of the base type and casts it, but it serves to illustrate the difference nicely.</p> <p>The <code>static_pointer_cast</code> will "just do it". This will result in undefined behavior (a <code>Derived*</code> pointing at memory allocated for and initialized by <code>Base</code>) and will likely cause a crash, or worse. The reference count on <code>base</code> will be incremented.</p> <p>The <code>dynamic_pointer_cast</code> will result in a null pointer. The reference count on <code>base</code> will be unchanged.</p> <p>The <code>shared_polymorphic_downcast</code> will have the same result as a static cast, but will trigger an assertion, rather seeming to succeed and leading to undefined behavior. The reference count on <code>base</code> will be incremented.</p> <p>See <a href="http://www.tweakbits.com/articles/sharedptr/index.html" rel="noreferrer">(dead link)</a>:</p> <blockquote> <p>Sometimes it is a little hard to decide whether to use <code>static_cast</code> or <code>dynamic_cast</code>, and you wish you could have a little bit of both worlds. It is well known that dynamic_cast has a runtime overhead, but it is safer, whereas static_cast has no overhead at all, but it may fail silently. How nice it would be if you could use <code>shared_dynamic_cast</code> in debug builds, and <code>shared_static_cast</code> in release builds. Well, such a thing is already available and is called <code>shared_polymorphic_downcast</code>.</p> </blockquote>

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