On 29.06.2018 17:57, Stefan Glienke wrote:
Change the code to following:
type
TFoo = class
A, B: Vector;
end;
procedure Main;
var
foo: TFoo;
begin
foo := TFoo.Create;
foo.A := DoSomething(3);
foo.A[0] := 42;
foo.B := DoSomething(4);
Writeln(foo.B[0]);
end;
Now we are back to using temp variables (both Delphi and FPC do) but FPC again
reuses its temp variable for A and B while Delphi uses different ones. Now for
some integer this might not be a big issue but imagine you have something else
in these arrays (for example any managed type like an interface).
Not having properly cleared B because it still uses the temporary content from
A might cause some issues.
Change the code to the following:
program Project1;
{$APPTYPE CONSOLE}
type
Vector = array of integer;
TFoo = class
A, B: Vector;
end;
function DoSomething (len: Integer): Vector;
begin
SetLength(Result, len);
end;
var
foo: TFoo;
I: Integer;
begin
foo := TFoo.Create;
for I := 0 to 1 do
begin
foo.A := foo.A + DoSomething(3);
foo.A[0] := 42;
end;
WriteLn(foo.A[3]); // << writes 42 even in Delphi !!!
Readln;
end.
Delphi 10 outputs "42"! Yes, and this is not a bug!
Result is not guaranteed to be cleared. Not in Delphi, nor in FPC.
Yes, you showed us one code example where Delphi clears the result and where
FPC does not but that's all. Delphi behavior can change any time in the future
as it has already been in other cases of undocumented behavior. So your example
can stop working in future Delphi versions, if e.g. some optimizations will be
added.
Ondrej
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