278611 - Implement dynamic casts through QueryInterface

Status: RESOLVED INVALID

(Core :: XPCOM, enhancement)

Description

[Daniel Kraft]

User-Agent:       Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.8a6) Gecko/20041227
Build Identifier: Mozilla/5.0 (Windows; U; Windows NT 5.0; en-US; rv:1.8a6) Gecko/20041227

Because dynamic_cast<> is not very good portable, a macro like NS_DYNAMIC_CAST
is missing, even if that could be used good instead of many
QueryInterface-calls. As we can't use dynamic casts as the C++ standard
describes them, we could implement them using the QueryInterface-mechanism (e.
g. using a template method nsDynamicCast<castTo>; macro isn't possible because
the target type can't be a pointer (T*) as it would).

Reproducible: Always

Steps to Reproduce:

Comment 1

[Daniel Kraft]

If you think this could be used well and there isn't already such a mechanism,
I'll try to implement such a function.

Comment 2

[Benjamin Smedberg]

We already have such a templating mechanism, it's called nsCOMPtr<> and 
do_QueryInterface.

Comment 3

[Daniel Kraft]

Of course, but if you have to deal with raw pointers (because of calls such as
GetChildNodes, where you get one returned) it would sometimes be superfluous to
declare a nsCOMPtr<> too (and convert it to a raw pointer again for the next
interface call) just because you can't easy QI without.
I know do_QueryInterface and thought of this function to be the base of dynamic
casting.

Comment 4

[Boris Zbarsky [:bzbarsky]]

The problem is that QueryInterface addrefs.  So you need _something_ a la
nsCOMPtr to manage the refcount...

Comment 5

[Christian :Biesinger (don't email me, ping me on IRC)]

well, such a nsDynamicCast template could do it, like:

template<class T> class nsDynamicCast {
 public:
  nsDynamicCast(nsISupports* p) { CallQueryInterface(p, &mPtr); }
  operator T* { return mPtr; }

  ~nsDynamicCast() { NS_IF_RELEASE(mPtr); }

 private: T* mPtr;
};

of course, this way, it would only be usable as an inline thing, not to store
temporaries.