Delphi Programming Tips & Tricks

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 It denotes a zeroing weak reference that will be zeroed (niled) when the object it points to is no longer valid.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] ...

ARC recognizes two types of references: strong and weak . Strong references are ones that participate in the reference counting mechanism, and weak references are references that do not participate in reference counting. While there is more than one way of achieving weak references, the ones that conceptually fit into ARC memory management are zeroing weak references. Zeroing weak references are niled (set to zero) when the object instance they point to is destroyed. A combination of strong and zeroing weak references leaves no room for invalid pointers. At all times, you will be dealing with references that are either nil or point to a valid object instance. Sounds perfect, doesn't it? Well, at least in theory. In practice, just like reference counting itself adds some small amount of overhead, zeroing weak references add some more. In order to zero out weak references after the object instance they point to is destroyed, the application has to track them at runtime and tha...