How to I get pointer to an Array and cast to a void * and back ?

[Dennis]

On Thursday, 24 June 2021 at 14:06:11 UTC, seany wrote:
>  void f() {
>         a[] * rd;	
>
>     // DO SOME WORK HERE ....
>
>     this.dataSet = & rd_flattened;	
>     rd = cast (a [] *)  dataSet;
>
>     write("length of rd is : "); writeln((*rd).length); // <--- 
> this works..
>     // do some work on rd
>
>     this.dataSet = rd;
>     rd = cast (field.rawData [] *)  dataSet;
>
>     write("length of rd for a second time is : ");
>     writeln((*rd).length); // <--- this ALSO works..
>     }
>
> Now outside `f`, in the same class, i call :
>
>     void f2() {
>
>     f();
>
>     a[] *aa ;
>     aa = cast (a [] *)  this.dataSet; // recall dataset is 
> public global
>     // if i print the address of this.dataSet here, this is the 
> same as inside f()
>     write("after calling f, count is: "); 
> writeln((*aa).length); readln();
>     // here the situation completely blows up . the length is 
> wrong.
>     }
>

> What is causing this issue ?

Your variable `a[] rd_flattened;` is a local variable to function 
`f()` allocated on the stack. Stack memory expires as soon as you 
return from the function. What `f2()` accesses through your 
global variable is a dangling pointer, a pointer to the expired 
stackframe of `f()`, which is why the `.length` is garbage.