Inverse Heisenbug - Unit test fails only when debugger is attached

Question

I recently fixed a defect in our product, the symptom of which was an access violation caused by accessing a dangling pointer.

For good practice I added a unit test

Answer 1

Generally, the VC++ debugger will fill heap-allocated memory with some known value when you delete the pointer to that memory. It's been quite a while since I've used Visual Studio, but it seems reasonable to me that 0xcdcdcdcd could be such a value. It seems most likely to me that the application is crashing properly when running in the debugger. When running in Release mode the runtime doesn't waste time overwriting deallocated memory, so some of the time you get "lucky" and the data stored in that memory is still valid.

You can modify your build setting to turn on the option for filling deallocated memory with a known value in Release mode (don't forget to turn it off again when you're done). I'd guess if you did this your application would crash in Release mode.

I appreciate that the value isn't always 0xcdcdcdcd, which may mean I'm wrong or may mean you have more than one path to a dangling pointer.