Re: return *(cast(T*)vPtr) and OutOfMemoryException

[Frits van Bommel]

TSalm wrote:
Hello,
In the code below, why the first Stdout throws a Exception when the 
second doesn't ?

/*  -------- CODE --------   */
import tango.io.Stdout;


struct VoidPtr(T)
{
  void* vPtr;

  void value(T val)
  {
    vPtr = &val;

Here you're storing a pointer to a non-ref parameter. This is the bug; 
the parameter itself is implicitly deallocated on returning from this 
function.
Change to 'ref T val' to fix it.

Essentially, an array is a struct { size_t length; T ptr; }. This means 
'val' refers to a copy in the stack frame of this function, not to 'arr' 
in main().

  }

  T value()
  {
    return *(cast(T*)vPtr);
  }
}


void main()
{
  VoidPtr!(char[][]) vPtr ;

  char[][]  arr = [ "hello" , "you" ];

  vPtr.value =  arr ;

  Stdout( vPtr.value ).newline; // <-- 
[tango.core.Exception.OutOfMemoryException: Memory allocation failed

Here you're calling a new function, overwriting *vPtr with something 
else (probably vptr itself), resulting in a huge array when you try to 
read it later.


  Stdout( *(cast(char[][]*) vPtr.vPtr ) ); // <-- This works good

This reads the (implicitly deallocated) 'val' parameter before it gets 
overwritten, hiding the bug. It's still there: just because the code 
doesn't crash doesn't mean it's correct.

}

/*  ------ END CODE ------  */


Thanks in advance for your help,
TSalm