Re: Polymorphism

Ulrich Eckhardt <>
Mon, 17 Dec 2007 13:41:35 +0100
Alamelu wrote:

1) C++ doesn't require the user to explicitily say dynamic_cast for
derived to base conversion. Is it users best pratise to say dynamic_cast
during such conversion?

Base *pB1 = new Base;
Derived *pD1 = new Derived;
pB1 = pD1; // Doesn't require explicite dynamic_cast

When is the user supposed to explicitily say dynamic_cast, something like
this Base *pB = dynamic_cast<Derived*>(&D)

Ahem, you're now talking about dynamic_cast, not reinterpret_cast, this is a
totally different beast!

Anyway: most conversions that happen implicitly are safe, which is probably
also the reason that they don't require an explicit conversion. Converting
a pointer-to-derived to a pointer-to-base is safe because of the ISA
relationship (Liskow substitution principle, IIRC), so it doesn't need a

In your example above, you are further converting a pointer-to-derived to a
pointer-to-derived and the compiler then performs an implicit conversion to
a pointer-to-base.

2) Why does c++ allow static_cast for base to derived conversion, when
actually will this be needed?

It isn't needed. The difference between a dynamic_cast and a static_cast is
that a dynamic_cast can fail. If the given object's dynamic type isn't the
required type, it will either return a null pointer or throw a
std::bad_cast (depending on the used syntax). A static_cast is performed
without checking. Further, a static_cast is performed without using RTTI,
which makes it the only thing available for objects that don't have RTTI,
i.e. when using it with a pointer-to-void or other types that don't have
any virtual functions.

"Ulrich Eckhardt" wrote:

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