Managed types vs. GetMem

I’ve just seen the following question on StackOverflow:

I am getting an unexpected AV in the following code:

program Project65;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  SysUtils;

type
  ITest = interface
  end;

  TTest = class(TInterfacedObject, ITest)
  end;

var
  p: ^ITest;

begin
  GetMem(p, SizeOf(ITest)); 
  p^ := TTest.Create; // AV here
  try
  finally
    p^ := nil;
    FreeMem(p);
  end;
end.

Now interestingly if I change the first line to

GetMem(p, 21);

than the AV is gone. (20 bytes or less fails). What is the explanation to this?

Simple: GetMem, as stated by the documentation, does not initialise the memory it allocates to zero. The chances are, therefore, that p^ is not nil when it comes to be assigned to the new instance of TTest; if that is the case however, then the low-level RTL will assume it points to a valid interface reference, and call _Release accordingly. As it in fact points to random bytes, an AV results. Importantly, the fact it does appear to be zero-initialised for the questioner if 21 not 4 bytes are requested is irrelevant: not ensuring memory is zero-initialised does not imply ensuring memory isn’t zero-initialised.

Moral of the story? If you mix and match relatively high level features (managed types) with relatively low level ones (dynamically allocating raw memory blocks), don’t be surprised if you need to perform bookkeeping for the former that is normally done for you.

Now in this particular case, using AllocMem instead of GetMem would be a quick fix. Nonetheless, I would personally advise not mixing and matching higher and lower level language features like this, unless there’s a good framework-y reason to do so. If you want to use pointers, just use pointers!

[Update: in between me writing this and it coming up on DelphiFeeds.com, two perfectly serviceable answers appeared on StackOverflow – check them out.]