void* is not a pointer to object type - Casting error from void**

Question

I've got a function Quit that is receiving a void** and I need to pass it to a new function who's receiving a DataStructure*.

The problem is that the data in ds is replaced with garbage. What can I do ?

void Quit(void** DS){
       DataStructure* ds = (DataStructure*) *DS;
       return ds->Quit(); //'void*' is not a pointer to object type
}

when the definition of the second Quit() is:

void DataStructure::Quit();

and the call in main is done as follow:

DataStructure *data;
Quit((void**) &ds);

We get the same result if we cast as follow:

 DataStructure* ds = reinterpret_cast<DataStructure*>(*DS);

you need to provide full compile error and/or complete example — apple apple, Dec 07 '16 at 12:32
If `data` doesn't point to a valid `DataStructure`, you can't expect anything to work. — molbdnilo, Dec 08 '16 at 11:45

score 2 · Answer 1 · answered Dec 07 '16 at 12:39

2

The cast works, it just doesn't change the type of DS like you expect it to do.

((DataStructure*)* DS) converts the value of *DS and throws the result away.
(Your compiler might have warned you that it has no effect.)

You need to save the result of the conversion:

void Quit(void** DS){
       DataStructure* ds = (DataStructure*) *DS;
       return ds->Quit();
}

answered Dec 07 '16 at 12:39

molbdnilo

1

or better yer, `DataStructure* ds = reinterpret_cast(*DS);` – YSC Dec 07 '16 at 12:43
It pass the compilation but I lose what was in DS (for every answer). I don't want to lose that data. Indeed, DS is of a DataStructure** type but my function get only void**. – R.Gad Dec 07 '16 at 13:48
@R.Gad Post the code that creates this `void**` as well. – molbdnilo Dec 07 '16 at 13:52

score 1 · Answer 2 · answered Dec 07 '16 at 12:36

1

If your DS variable actually is a DataStructure* you can simply cast it:

void Quit(void** DS)
{
       DataStructure* ds = (DataStructure*)DS;
       return ds->Quit();
}

answered Dec 07 '16 at 12:36

nvoigt

2 Answers2