Re: Weird V-table issue

=?ISO-8859-1?Q?Marcel_M=FCller?= <>
Fri, 25 Jul 2008 00:07:17 +0200
Hi, schrieb:

Can anyone tell me why this is happening? The problem goes away if I
am doing a cast when I am returning the interface pointer. But I am
still not convinced that this is the correct behavior by the compiler.
How does casting affect a V-table of a object pointer?

you are happily to void* and then to another pointer type. This is invalid.

The below program compiles and runs fine on a GNU C++ compiler.

Don't use C-style reinterpret casts.
Don't use void* in C++ programs (except for wrapping C library functions).

And while we are at it do not use char* in C++ programs (You did not do
that in the example, of course). But const char* is fine.

If you follow this advices you won't run into that invalid code without
a compiler warning or error.

  int QueryInterface (void **ppOut) {*ppOut = this; return 0;}
  int QueryInterface2 (void **ppOut) {*ppOut = (INetworkB *) this;

Furthermore these functions are completely superflous, since every class
pointer to an instance of Network is implicitely convertable to a
pointer to all of it's public base classes.

If you want to restrict the interface availibility by some runtime
conditions, derive protected from the interface and write a strongly
typed QueryInterface function that returns NULL or throws an exception
in case of an error.

And even more, when the same interface is implemented at different
levels of the class hierarchy, you should always derive virtual from
the interface. This gives the Java like behavior.

  /* Weirdness happens here */
  /* Get the INetworkB interface using QueryInterface() with no
casting */
  pINetA->QueryInterface ((void **) &pINetB);

You directly assigned a pointer to Network to a pointer to INetworkB.


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