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This does not compile in C++:
class A { }; class B : public A { }; ... A *a = new B(); B *b = dynamic_cast<B*>(a); 
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The primary purpose for the dynamic_cast operator is to perform type-safe downcasts. A downcast is the conversion of a pointer or reference to a class A to a pointer or reference to a class B , where class A is a base class of B .
If the cast is successful, dynamic_cast returns a value of type new-type. If the cast fails and new-type is a pointer type, it returns a null pointer of that type. If the cast fails and new-type is a reference type, it throws an exception that matches a handler of type std::bad_cast.
dynamic_cast will no longer throw an exception when type-id is an interior pointer to a value type, with the cast failing at runtime. The cast will now return the 0 pointer value instead of throwing.
For example, dynamic_cast uses RTTI and the following program fails with the error “cannot dynamic_cast `b' (of type `class B*') to type `class D*' (source type is not polymorphic) ” because there is no virtual function in the base class B.
Because dynamic_cast can only downcast polymorphic types, so sayeth the Standard.
You can make your class polymoprphic by adding a virtual destructor to the base class. In fact, you probably should anyway (See Footnote). Else if you try to delete a B object through an A pointer, you'll evoke Undefined Behavior.
class A { public: virtual ~A() {}; }; et voila!
There are exceptions to the "rule" about needing a virtual destructor in polymorphic types.
One such exception is when using boost::shared_ptr as pointed out by Steve Jessop in the comments below. For more information about when you need a virtual destructor, read this Herb Sutter article.
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