How much null checking is enough?

Question

What are some guidelines for when it is not necessary to check for a null?

A lot of the inherited code I\'ve been working on as of late has null-checks

Answer 1

If I receive a pointer that is not guaranteed by language to be not null, and am going to de-reference it in a way that null will break me, or pass out put my function where I said I wouldn't produce NULLs, I check for NULL.

It is not just about NULLs, a function should check pre- and post-conditions if possible.

It doesn't matter at all if a contract of the function that gave me the pointer says it'll never produce nulls. We all make bugs. There's a good rule that a program shall fail early and often, so instead of passing the bug to another module and have it fail, I'll fail in place. Makes things so much easier to debug when testing. Also in critical systems makes it easier to keep the system sane.

Also, if an exception escapes main, stack may not be rolled up, preventing destructors from running at all (see C++ standard on terminate()). Which may be serious. So leaving bad_alloc unchecked can be more dangerous than it seems.

Fail with assert vs. fail with a run time error is quite a different topic.

Checking for NULL after new() if standard new() behavior has not been altered to return NULL instead of throwing seems obsolete.

There's another problem, which is that even if malloc returned a valid pointer, it doesn't yet mean you have allocated memory and can use it. But that is another story.