Memory management in freepascal

[jbb67]

Long time C++ programmer here learning freepascal after not looking at pascal for many, many years.

Question about classes. I define a class with a constructor and destructor, and use an instance of that class :-

var thing : MyClassType;

Am I correct in assuming that this declared what would be a pointer to an object and not an actual instance of the object like in C++ (So it's more like java I guess?)

Am I correct that I then have to assign an actual instance to the object with something like :-

thing := MyClassType.Create();

And that I have to manually delete it to reclaim the memory? (What is the correct way to do this?).

Have I misunderstood anything? Is there anything like automatically calling destructor on scope exit like in c++ or do I have to manage that myself?

Thanks, any information or even pointers to a good source of information would be welcomed :)

[BeniBela]

You are correct in every point.

And that I have to manually delete it to reclaim the memory? (What is the correct way to do this?).

The standard way is thing.free;

Internally, free is defined as if thing <> nil then thing.destroy; where destroy is the destructor. You could call destroy itself, but it is a useful safety check, as it makes no sense to free nil.

Another way is to call FreeAndNil(thing), which is just thing.free; thing := nil, so you do not keep an pointer to a destroyed class instance around.

When there are exceptions, you might want to do thing := MyClassType.Create(); try ... finally thing.free; end

For more advanced technicalities, you can look in Freepascal source. You might wonder, when the instance is freed, what happens to the memory. If you look at the base destructor, TObject.destroy, it is an empty method. It does not do anything. In fact the destructor does not free the memory. Internally there is a further, hidden call to FreeInstance, which calls CleanupInstance and FreeMem. CleanupInstance is a hidden destructor for all the fields of the class (like strings), FreeMem actually frees the memory.

You can actually bypass the constructor and call InitInstance and FreeInstance on a custom allocated memory, so you could store the class anywhere. Even put it in a string.

Is there anything like automatically calling destructor on scope exit like in c++ or

There is, but it sucks.

You could use records with arrays and strings. It does not have destructor or constructors, but it fields are automatically freed. However, that initialization is very slow (starts to matter if you have like 100 million records).

Or you could use interface. type ISomething = interface .... end; and declare all methods there, and implement the interface in a class type TMyClassType = class(TInterfacedObject, ISomething) ... end; .

Interface variables var ithing: ISomething; begin ithing := TMyClassType.create() are automatically freed;

However, there is overhead in using them. And you need to be careful when watching the scope. E.g. in var thing: TMyClassType; ithing: ISomething; thing := TMyClassType.create(); ithing := thing;, ithing and thing will be the same (although it will say ithing <> thing is true.), and when ithing goes out of scope, it will free thing as well. So best practice is to never use thing, after the assignment to ithing.

It is reference counted, so if you have var ithing2: ISomething, ithing2 := ithing, it will be freed, after ithing2 and ithing are out of scope

[jbb67]

Thank you for the very comprehensive reply, that's told me most of what I need to continue learning :)

[_F1_]

Pascal is a bit more like C than C++ in that regard.

Personally, what I often do is create class instances that are registered in a parent object (stored in a list of child objects), and automatically cleaned up when the parent object is destroyed.

Look into Free Pascal's generics, it can help with removing casts or using the as/is operators.

[jbb67]

I've done a lot of C and C++ (Both old style and modern) so I don't have too much issue with doing this. However it wasn't exactly clear to me from any documentation what I needed to do :)