On Mon, 22 May 2023, Steve Litt via fpc-pascal wrote:
Michael Van Canneyt via fpc-pascal said on Sun, 21 May 2023 09:47:15
+0200 (CEST)
On Sun, 21 May 2023, Hairy Pixels via fpc-pascal wrote:
I've never really used exceptions myself in Pascal (with the
exception of breaking out of deeply recursive function calls) so I
don't know all the rules.
In this example lets say you call Test() which allocates some memory
and then calls out to another function in another unit which raises
(the programmer doesn't know this because FPC doesn't mark the
function as throwing exceptions). Now the Test() function will exit
early before freeing the memory.
What are you supposed to do here? The only thing I can think of is
to wrap every function in try..finally which COULD raise an
exception but that's a huge mess because literally any function
could raise.
====================================
procedure DoThis;
begin
raise Exception.Create('dead');
end;
procedure Test;
begin
TObject.Create;
// call some code in other unit which raise an exception
DoThis;
end;
Your example will leak memory in any case, even if there is no
exception, since you're not freeing the object anywhere..
Assuming the result of A is not used outside of Test, the following is
the only solution:
procedure Test;
var
A : TObject;
begin
A:=TObject.Create;
Try
// call some code in other unit which raise an exception
DoThis;
finally
A.Free
end;
end;
You can try to use interfaces, they will be managed by the compiler.
Alternatively, using generics and management operators you can
create a record that will automatically free the object at the
end of the procedure.
Michael.
Am I correctly understanding you that Pascal objects come off the heap
and not the stack by default? Ouch! This is one of the C-isms I'd like
to get away from.
C has no objects at all. C++ has, and they're on the stack by default.
Is there a way to create objects from stack memory
so there's no need to free?
Yes, you can use objects instead of classes. objects are on the stack.
But they cannot be mixed with classes (for obvious reasons).
Michael.
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