void pointer syntax

[Stephen Jones]

On Wednesday, 16 May 2012 at 02:55:41 UTC, H. S. Teoh wrote:
> On Wed, May 16, 2012 at 04:01:23AM +0200, Stephen Jones wrote:
> [...]
>> Using Object gives exactly the same problem as the Object super
>> class does not have vcount variable. Casting is not a solution
>> because the reason for throwing different sorts of widgets 
>> into a
>> single array was so I did not have to track what each type of 
>> object
>> was; not tracking what each object in the array is I have no 
>> means
>> of knowing what to cast each Widget in the array to.
>
> If you did not know what type each object was, how do you know 
> they have
> a vcount variable?
>
> If you _do_ know that they have a vcount variable, that means 
> you know
> what type to downcast to. If there is more than one subclass 
> with a
> vcount variable, that's an indication that you want to factor 
> it into a
> common base class that gives a unified access to it, in which 
> case you'd
> downcast to the common base class.
>
>
>> If some one knows void pointer syntax that would be helpful. 
>> As I
>> understand it there is a type called size_t that takes the 
>> address
>> of as a a value. Can I make an array of these and simply 
>> initialize
>> each with &button1, &cursor, etc? Also, how is size_t freed?
>
> Void pointer does not help you at all. You can't access the 
> vcount
> variable through a void pointer unless you know it's a 
> particular type
> that has such a variable -- you'd have to cast the void pointer 
> to that
> type first. And in that case, you might as well be downcasting 
> instead
> because it's type-safe. Blindly casting a void pointer into a 
> particular
> type because you _think_ that's what it is, is a recipe for 
> disaster
> when later on your list changes and the void pointer no longer 
> points to
> what you think it points to.
>
>
> T



Ali you are right about the void pointer requiring a cast as I 
figured out. It does seem kind of odd that you cannot keep an 
array of integers that hold the start address of blocks of memory 
which blocks of memory contain objects such as Buttons etc. For I 
would have thought that the start address would hold something 
akin to a header telling whoever pointed at it that it was of a 
type, whichever type it was. And that if somebody asked for a 
piece of data that was in that block of memory the header would 
provide the index to it. But then I have to speculate about how 
it is things work because compiler design is not a field I am 
familiar with.

Ali your post above, and T your post in the other forum (Simpsons 
bit) is sort of what I was after. I tried both interface and 
abstract class but not outright super class. The problem I have 
with the solution is the same problem I have with header files 
and cpp files; in both instances you need to hunt in other files 
for the variable's definition. If name is a field that will 
contain Bart's name then it should be in the Bart class, 
otherwise I end up wasting time confused about some variable that 
has been initialized or used to initialize some other variable 
and not finding it in the relevant module.

Cain: My understanding is that D is based on "no proper ways of 
doing things" just get the job done. The use of properties when 
you can use public access baffles me. So long as you are careful 
with the namespaces why would you want to  incur function 
overheads that could only possibly "save" the situation if you 
were not careful with namespaces? Another anomaly I have never 
understood is why would you use multiple property calls when you 
could use a single function to set or get multiple items of data 
for the cost of a single function? It is either fashion, 
politics, or I am missing some piece of the puzzle.