Tuesday, April 3, 2012

## common c++ run time errors

In java, the undisputed #1 common run time error is NPE, so much so that half of all error checks are null-pointer-checks. In c++, divide-by-zero is similarly a must. But there are more...

- divide by zero
- pointer-move beyond bounds -- remember all array sizes are compile-time constants, even with realloc()
- read/write a heap object (heapy thingy) via a reference variable after de-allocation
- return by reference an object allocated on the stack -- bulldozed
- dereference (unwrap) a dangling pointer
- c-style cast failure
- double-free
- dereference (unwrap) a null pointer -- undefined behavior, unlike java NPE. We should always check null before dereferencing.

----less common
- delete a pointer to non-heap
- free a pointer to non-heap
- hold pointer/reference to a field of an object, not knowing it's soon to be reclaimed/bulldozed. Object could be on heap or stack. More likely in multi-threaded programs.
- misusing delete/delete[] on a pointer created with new/new[]. The variable always looks the same -- plain pointers.

-------------
For any of the above, if it happens in a dtor, you are in double trouble, because the dtor could be executing due to another run time error.

Now, JVM "wraps" up all these runtime error Conditions into exceptions. Java creators generally prefer to push these error conditions to the compilation phase, to reduce the variety of Runtime errors. What remain are wrapped up as various RuntimeExceptions. It's rather remarkable that JVM let's you catch  all(?) of these exceptions. No undefined behavior.

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