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Possible Duplicate:
Why Free crashes when called twice
I just want to know what exactly happened when we delete the pointer that already been deleted, and what cause the crash?
It’s hard to predict exactly what will happen — it depends a little on the compiler, and a lot on the standard library. Officially, it’s just undefined behavior, so nearly anything can happen.
The most common thing that’ll happen is that the heap will get trashed. It may not check that what delete is valid, so when it gets two blocks at the same address it may (for example) still treat them as two separate blocks, so when you allocate memory later you may get that same block of memory twice. In some other cases, it may (for example) just complement a bit to say whether that block of memory is in use, so the second time you delete it, you actually end up marking it as being in use again, so that memory can never be allocated again, and you’ve basically just created a leak.