When must I set a variable to "Nothing" in VB6?

Question

In one of my VB6 forms, I create several other Form objects and store them in member variables.

Private m_frm1 as MyForm
Private m_frm2 as MyForm

// Later...
Set m_frm1 = New MyForm
Set m_frm2 = New MyForm

I notice that I'm leaking memory whenever this (parent) form is created and destroyed. Is it necessary for me to assign these member variables to Nothing in Form_Unload()?

In general, when is that required?

SOLVED: This particular memory leak was fixed when I did an Unload on the forms in question, not when I set the form to Nothing. I managed to remove a few other memory leaks by explicitly setting some instances of Class Modules to Nothing, as well.

Answer 1

Actually, VB6 implements RAII just like C++ meaning that locally declared references automatically get set to Nothing at the end of a block. Similarly, it should automatically reset member class variables after executing Class_Terminate. However, there have been several reports that this is not done reliably. I don't remember any rigorous test but it has always been best practice to reset member variables manually.

Answer 2

Objects in VB have reference counting. This means that an object keeps a count of how many other object variables hold a reference to it. When there are no references to the object, the object is garbage collected (eventually). This process is part of the COM specification.

Usually, when a locally instantiated object goes out of scope (i.e. exits the sub), its reference count goes down by one, in other words the variable referencing the object is destroyed. So in most instances you won't need to explicitly set an object equal to Nothing on exiting a Sub.

In all other instances you must explicitly set an object variable to Nothing, in order to decrease its reference count (by one). Setting an object variable to Nothing, will not necessarily destroy the object, you must set ALL references to Nothing. This problem can become particularly acute with recursive data structures.

Another gotcha, is when using the New keyword in an object variable declaration. An object is only created on first use, not at the point where the New keyword is used. Using the New keyword in the declaration will re-create the object on first use every time its reference count goes to zero. So setting an object to Nothing may destroy it, but the object will automatically be recreated if referenced again. Ideally you should not declare using the New keyword, but by using the New operator which doesn't have this resurrection behaviour.