Error while deleting 3D dynamic array in Pascal

Question

The point is to make it work with pointers. Also, I found similar problem with C++. Its not working with Pascal, well, I prob. missed something.

1) I make types and variable for my pointers:

type

  PReal = ^double;

  PPReal = ^PReal;

var

  data : ^PPReal;

2) This is how I allocate memory (x,y,z are dimensions):

GetMem( data, sizeof(PPReal)*x );

for i:=0 to x-1 do begin

  GetMem( (data+i)^, sizeof(PReal)*y );

  for j:=0 to y-1 do begin

    GetMem( ((data+i)^+j)^, sizeof(real)*z );

  end;

end;

3) And this is how I try to Dispose() it:

for i:=0 to x-1 do begin

  for j:=0 to y-1 do begin

    Dispose( ((data+i)^+j) );

  end;

  Dispose( (data+i) );

end;

Dispose( data );

However, I'm getting 216(memory access) or 204(invalid pointer operation) runtime errors. Like this. Full code (FreePascal IDE)

Any help appreciated.

Answer 1

The last dispose is mirrored to its getmem, but the other two not. Fix that and it works:

  for i:=0 to x-1 do begin
    for j:=0 to y-1 do begin
    writeln(i,' ',j,' *');
    Dispose( ((data+i)^+j)^ );
  end;
  writeln(i,' ',j);
  Dispose( (data+i)^ );
end;
Dispose( data );

p.s. freemem is the logical companion to getmem, as dispose is to new.