[fpc-pascal] Freeing memory with exceptions

Michael Van Canneyt michael at freepascal.org
Sun May 21 09:47:15 CEST 2023

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;

  A : TObject;
     // call some code in other unit which raise an exception

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.


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