Is there any kind of garbage collector?

InterSystems IRIS employs a memory management approach related to object references (OREFs) rather than a traditional garbage collector mechanism. Here's how it works:

1. Automatic Memory Management for Object References:

   • When an object is created, InterSystems IRIS keeps a reference count to track how many variables are pointing to the object. This count is incremented when a reference is made to the object and decremented when the reference is removed (e.g., the variable goes out of scope, is explicitly killed, or assigned a new value).
   • If the reference count reaches zero, the in-memory object is automatically destroyed and its %OnClose() method (if defined) is executed. This is a form of automatic cleanup tied to the object’s lifecycle and reference count [1].

2. Manual Management of Object References:

   • Although %Close() exists in some older object classes, it is deprecated and does not actually remove object references. To remove an object reference, you should assign an empty string ("") to it, or use the KILL command to remove the variable holding the reference. When there are no remaining references to the object, it is removed from memory [2][1].

3. Special Functions for Object Lifecycle Checks:

   • The $ISOBJECT(expr) function can check whether a given variable holds a valid object reference, returning 1 for valid references and -1 for invalid ones. This is useful to determine if an object still exists or needs cleanup [2].

This overall mechanism ensures efficient memory management of objects and avoids memory leaks, as objects are destroyed once no references point to them. While this approach achieves similar results to garbage collection, it is not automatic in the same sense and requires developers to account for object scope and references properly.

Sources:

Although the %Close method doesn't do anything, there is a useful %OnClose callback, where you can do some cleanup if needed, such killing a temporary global.