Re: New as function

22 Jul 2005


      Waldek Hebisch wrote:
...
Peter N Lewis wrote:
...
Does New as a function return nil or does it still give a runtime error?
If I want to allocate memory, but handle it gracefully if it fails, 
what are the recommended approaches?
The modern way is to raise an exception. Unfortunatly, GPC still
do not support exceptions (I have started working on this, but my
code ATM is non-working).
Lacking exceptions you can just jump out of memory allocator
(if you provide your own) or error handler (IIRC GPC allows you to
install custom error handler).
You could also trap runtime errors, using `AtExit', or use the
`Trap' unit (which does this internally). This comes somewhat closer
to exception, though it's not quite the same. Indeed, this may be
the best solution available. It's a bit code to write, but it uses
`New' and has no disadvantages in this regard.

program Trap2Demo;

uses Trap;

type
  p = ^t;
  t = record
    Next: p;
    Data: array [1 .. 50000000] of Integer
  end;

function SafeAllocate = Res: p;

  procedure TryAllocate (Trapped: Boolean);
  begin
    if Trapped then
      begin
        TrappedExitCode := 0;
        TrappedErrorAddr := nil;
        Res := nil
      end
    else
      New (Res)
  end;

begin
  TrapExec (TryAllocate)
end;

var
  i: Integer;
  v, w: p;

begin
  w := nil;
  for i := 1 to 5 do
    begin
      if i > 1 then WriteLn ('Starting again');
      repeat
        v := SafeAllocate;
        if v <> nil then
          begin
            WriteLn ('Allocated v at address ', PtrCard (v));
            v^.Next := w;
            w := v
          end
      until v = nil;
      WriteLn ('Error occurred, continuing');
      while w <> nil do
        begin
          v := w;
          w := w^.Next;
          WriteLn ('Disposing v at address ', PtrCard (v));
          Dispose (v)
        end
    end
end.

Frank

-- 
Frank Heckenbach, frank@g-n-u.de, http://fjf.gnu.de/, 7977168E
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