Re: What the hell is dynamic_cast for?

James Kanze <>
Thu, 29 May 2008 01:13:58 -0700 (PDT)
On May 29, 3:49 am, ""
<> wrote:

On May 29, 1:46 am, Juha Nieminen <nos...@thanks.invalid> wrote: wrote:

Up-casting is safe while down-casting is not. dynamic_cast
seems to do down-casting, so why is dynamic_cast intenting
to do the dangerous work?

Dynamic casting is intended to do the "dangerous work" more
safely. While a static_cast will simply cast to the derived
type regardless of whether the object is of that type,

dynamic_cast<T>(pointer) is used with an object pointer, while
static_cast<T>(object) is the implicitly compatible conversion
and works with objects also. Am I right?

Both dynamic_cast and static_cast can be used on pointers and
references. (static_cast can also be used in a lot of other
cases.) When used on pointers or references:

 -- static_cast allows moving up or down in the hierarchy (but
    not sideways); it also allows converting to or from void*.
    When moving down (Base* to Derived*), it makes no
    verifications; if the object actually pointed to isn't a
    Derived, anything can happen, so it is extremely dangerous
    for this sort of casting. Note too that there are some
    restrictions on static_cast when virtual base classes are

 -- dynamic_cast only makes sense on pointers or references to
    polymorphic types---otherwise, it only allows casting up
    (Derived* to Base*), where it has exactly the same semantics
    as static_cast. On pointers to polymorphic types, it allows
    arbitrary navigation within the hierarchy, including
    sideways. It verifies the type: if you dynamic_cast to
    Derived*, and the pointer doesn't actually point to a
    Derived*, it returns a null pointer (or raises an exception
    if it is a reference cast). Except in very exceptional
    cases (when the profiler says you must), you should use it
    exclusively for down casting.

dynamic cast actually checks that the object is of that
type, and if it isn't, it returns a null pointer. You can
check the returned pointer to see if the cast was
successful. (With static_cast you probably just get a
segmentation fault.)

``is of that type'' means that


(*pointer) is declared as T or T's derived classes, right?

This cast fails if (*pointer) is declared as T's parent
classes or other un-related classes. Am I right?

I'm not too sure I understand. If pointer is of type Derived*,
dynamic_cast< Base* >( pointer ) always succeeds, of course,
returning a pointer to the Base sub-object of the Derived. If
pointer is of type Base*, dynamic_cast< Derived* >( pointer )
will return a null pointer if the actual object doesn't contain
an unambiguous Derived in its inheritance hierarchy, and a
pointer to the Derived if it does.

There are situation where downcasting may be unavoidable or
the path of least resistance. Thus it's good to have a tool
to make it safer.

I read some past posts, some said dynamic_cast indicated flaw
in design :)

There is no requirement here for someone to be competent before
posting. dynamic_cast is easily abused, but there are cases
where it corresponds to the best design, and there are cases
where it represents the best engineering compromize.

Having a string of dynamic_cast usually is a sign of poor

James Kanze (GABI Software)
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