dynamic cast not working for non polymorphic base class?

Question

Here the second cast gives an error saying

cast.cc:35:35: error: cannot dynamic_cast ‘base’ (of type ‘class CBase*’) to type ‘class CDerived*’ (source type is not polymorphic)

 CBase * base = new CDerived; CBase* pb; CDerived * der = new CDerived; CDerived* pd; pb = dynamic_cast<CBase*>(der); // ok: derived-to-base pd = dynamic_cast<CDerived*>(base); // wrong: base-to-derived

What is meannt by this ??

And why this works if I make the base class polymorphic ?

Can some one please let me know the basic concept behind this.

Oliver Charlesworth · Accepted Answer · 2012-05-09 22:41:51Z

Because the standard says so (see the section [expr.dynamic.cast] in the C++ standard):

... dynamic_cast<T>(v)

... v shall be a pointer to or an lvalue of a polymorphic type

In practice, because the run-time type information (RTTI) required to make dynamic down-casts (i.e. from base to derived) possible are generated along with the vtbl/vptr mechanism, which isn't required if there are no polymorphic member functions.

Up-casts (i.e. derived to base), on the other hand, require no RTTI (there's no run-time decision to be made). Quoting from the same section of the standard:

struct B { }; struct D : B { }; void foo(D* dp) { B* bp = dynamic_cast<B*>(dp); // equivalent to B* bp = dp; }

Does the compiler substitute a static_cast for the dynamic_cast when it knows it's a derived-to-base conversion? That's what you're implying, right?
@MarkRansom: I guess that's what I'm implying, but I'm not going to make it explicit, in case it's wrong!

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