[fpc-pascal] Freeing memory with exceptions

[Michael Van Canneyt]

On Thu, 25 May 2023, Hairy Pixels via fpc-pascal wrote:

>
>
>> On May 24, 2023, at 8:46 PM, Michael Van Canneyt via fpc-pascal <fpc-pascal at lists.freepascal.org> wrote:
>>
>> I see no problem, I am used to this.
>>
>> This is Pascal, you need to manage your memory and hence your references, this is true even in the absence of exceptions: In your example you would
>> also need to remember to remove your instance from the list after you free
>> it, even if there are no exceptions.
>
> My example is still not good.  The problem I have is that once exceptions
> are used in a program calling any function now has a side effect of
> completely exiting the calling function so you need to be paranoid and
> wrap everything in try blocks just in case.  A new added burden for manual
> memory management.
>
> Why is this better than handling errors as values like a normal function result?

Because it centralizes exit code in case of an error, and allows to have a
meaningful result.

In C you need to do something like this:

Function MyFunction(out theresult : TResultType) : Integer;

begin
   Allocate A
   ...
   error condition 1:
     Free A, exit (1);
   ...
   Allocate B
   error condition 2:
     Free A, Free B, exit(2)

   Allocate C
   error condition 3:
    Free A, Free B, free C, exit(3);
   ...
   // etc
   // All went well, report 0
   theresult:=X;
   Free A, Free B, free C exit(0);
end.

(you can use a goto, which we all agree on is a bad idea to introduce in a
language)

With exceptions, this becomes

Function MyFunction : TResultType;

begin
   try
     Allocate A
      ..
     allocate B
     ...
     Allocate C
     ...
     Result:=X;
   finally
     free A,B,C
   end;
end.

I'll take the latter any time.

Michael.