[fpc-pascal] Freeing memory with exceptions

Michael Van Canneyt michael at freepascal.org
Mon May 22 22:21:08 CEST 2023

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).


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