Listing 7.3 - add early exit to `.pop()`

[ITHelpDec]

It seems odd to perform work on a node that could potentially be a nullptr from the moment the function is called.

I hope I'm correct in saying that .compare_exchange_weak(...) will return true if head still refers to our original old_head (i.e. no other thread has had a chance to modify it), but if that old_head was nullptr to begin with, then would old_head->next not throw after a successful CAS check?

https://github.com/anthonywilliams/ccia_code_samples/blob/6134deed3865de373f438e2a34fea53bba2c93ba/listings/listing_7.3.cpp#L24-L30

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The same thing applies to void pop(T &val) a few pages prior - an early exit allows the code the run without throwing for bad access to old_head->next.

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Arguably, maybe changing old_head to original_head would be more conducive to helping readers grasp the concept of .compare_exchange_weak(...).