How to detect “dangling pointers” if “Assigned()” can't do it?

Question

In another question, I found out that the Assigned() function is identical to Pointer <> nil. It has always been my understanding that

Answer 1

The bottom line is that you should not be attempting to detect dangling pointers in code. If you are going to refer to pointers after they have been freed, set the pointer to nil when you free it. But the best approach is not to refer to pointers after they have been freed.

So, how do you avoid referring to pointers after they have been freed? There are a couple of common idioms that get you a long way.

• Create objects in a constructor and destroy them in the destructor. Then you simply cannot refer to the pointer before creation or after destruction.
• Use a local variable pointer that is created at the beginning of the function and destroyed as the last act of the function.

One thing I would strongly recommend is to avoid writing if Assigned() tests into your code unless it is expected behaviour that the pointer may not be created. Your code will become hard to read and you will also lose track of whether the pointer being nil is to be expected or is a bug.

Of course we all do make mistakes and leave dangling pointers. Using FreeAndNil is one cheap way to ensure that dangling pointer access is detected. A more effective method is to use FastMM in full debug mode. I cannot recommend this highly enough. If you are not using this wonderful tool, you should start doing so ASAP.

If you find yourself struggling with dangling pointers and you find it hard to work out why then you probably need to refactor the code to fit into one of the two idioms above.

You can draw a parallel with array indexing errors. My advice is not to check in code for validity of index. Instead use range checking and let the tools do the work and keep the code clean. The exception to this is where the input comes from outside your program, e.g. user input.

My parting shot: only ever write if Assigned if it is normal behaviour for the pointer to be nil.