How can I make this work?

[Jack]

On Sunday, 28 February 2021 at 13:15:47 UTC, Adam D. Ruppe wrote:
> On Sunday, 28 February 2021 at 07:05:27 UTC, Jack wrote:
>> I'm using a windows callback function where the user-defined 
>> value is passed thought a LPARAM argument type. I'd like to 
>> pass my D array then access it from that callback function. 
>> How is the casting from LPARAM to my type array done in that 
>> case?
>
> The best way to do this is to put the array inside a struct and 
> pass the address of the struct instead. This way both length 
> and pointer are passed, and you have the option to add more 
> things if you ended up needing it later, and there's fewer 
> weird things to worry about.
>
> int[] arr = [1, 2, 3];
>
> struct MyMessage {
>      int[] arr;
> }
>
> // this far is easy enough
> MyMessage* messagePointer = new MyMessage(arr);
>
> // but we do need to tell the GC we intend to pass this to the 
> outside world
> // failure to do this MIGHT lead to random crashes as the GC 
> can't see it (it can't look inside the Windows message queue), 
> assumes it is unused, and frees it out from under you.
> import core.memory;
> GC.addRoot(messagePointer);
>
> // when the GC has a root, it will consider that pointer live 
> until further notice and not collect it nor its member 
> variables.
>
> // so it is now cool to do this
> PostMessage(hwnd, MSG_WHATEVER, 0, cast(LPARAM) messagePointer);
>
>
> /* then on the other side */
>
> switch(iMsg) {
>    case MSG_WHATEVER:
>        MyMessage* messagePointer = cast(MyMessage*) lParam;
>
>        // need to tell the GC the pointer can be automatically 
> managed normally again. failure to do this will lead to a 
> memory leak
>        import core.memory;
>        GC.removeRoot(messagePointer);
>
>        // now can use it
>        foreach(item; messagePointer.arr) {
>           // yada yada yada
>        }
> }
>
>
>
> And it is the simplest thing, no missing length, no weird 
> property casting. The GC handled with two simple add/remove 
> calls.

This is what I ended up using. using a single pointer such as 
MyMessage makes things much simpler. Thanks for the rememinder of 
GC.removeRoot()

Everyone else in this theread, thank you guys. Always helpful