Re: Casting from void*
doug@dbassoc.com wrote:
I have a routine that gets passed a void* to a structure.
So far, so bad... For use as callbacks, do you know of e.g. Boost.Function
and Boost.Bind?
In the routine I cast it to the correct type, e.g.
SomeFunc(void* pMsg) {
printf("Event = %d\n",
static_cast<struct MyEventMsg*>(pMsg)->Event);
}
For the record, you can omit the "struct" here, since in C++ enumerations,
unions, structures and of course classes are already recognized as types
without this.
If I am accessing many members of the structure the code looks pretty
messy. I can create a new pointer of the correct type and assign it
the cast void*, but it seems like it shouldn't be necessary to create
a temporary variable just for this purpose. Is there some way to cast
the parameter pMsg to the correct type?
You can not change the type of an object, so there is no way to do what you
are asking for directly. What you can and should do is to change the
parameter that the function takes, so that it reflects the type that it
really expects. If you can't do that, typically in the case of some C-ish
callback, you should do this instead:
assert(ptr);
MyEventMsg const& msg = *static_cast<MyEventMsg const*>(ptr);
This serves multiple purposes:
1. It documents that your function doesn't allow null pointers as
parameters.
2. It documents that the pointer really points to a different type in
exactly one place.
3. It documents that you are not modifying what you are given as parameter,
provided that is really the case.
Don't worry about the reference this creates, any halfway modern compiler
will be able to optimize this to an extent that nothing remains in the final
executable. You shouldn't worry about such microoptimizations anyway.
Cheers!
Uli
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