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Don't mix objects and interfaces

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Mixing objects and interfaces in classic (non-ARC) Delphi is a huge no-no. That part is clear. What that actually means is more of a blur. After all there is a object instance lying behind every interface reference.

So what is wrong with mixing objects and interfaces?
object references are weak, interface references are strongreference counted object instances need at least one strong reference to keep them alive therefore you can't use object reference as primary owning referencereference counted object instances can be safely accessed through temporary, short-lived (weak) object references, as long as there are strong reference(s) keeping the object alive

The real issue behind the mixing of objects and interfaces is not the mixing itself, but the lack of initial strong (interface) reference to object instance, or accessing the reference counted object instance through weak (object) reference after the last strong (interface) reference has gone out of scope and the object has alr…

Optimizing ARC - weakness of [weak]

In Delphi, the [weak] attribute is used to break strong reference cycles between reference counted object instances. It can be used with the ARC compiler on both object and interface references, and with the classic compiler for interface references. Weak interface references were introduced in Delphi 10.1 Berlin.

Now, there is a teeny-weeny problem with the [weak] attribute. It denotes a zeroing weak reference that will be zeroed (niled) when the object it points to is no longer valid. In order to do that, the compiler has to track such objects at runtime and that introduces some overhead. If you are tracking many such references, that can introduce a significant performance loss.

Not all weak references that serve the purpose of breaking reference cycles need to be zeroed out. If you can guarantee that a [weak] reference will never outlive the object it points to, all that tracking and zeroing is useless. To avoid a performance penalty in such cases, you can use the [unsafe] attribu…