Question

I have an app submitted to Apple Store, and it passed. But I found there was a bug will make the app crash. Finally, I found the real problem is a snap of code has the opposite result.

code:

CGPoint now;
CGPoint old;
UIView *lastDot;
now = dotView.center;
if(lastDot){
    //release version, will go here. then crashed.
    old = lastDot.center;
}else{
    //debug version, will go here. run ok.
}

---

I declare a UIView pointer, and check it with “if statement”,

UIView *lastDot;

the debug version consider the lastDot as nil, but the release version think is not null.

can anyone give me any idea? thank you.

Answer 1

The issue is due to uninitialized pointer.

Initialize it to nil for avoiding such troubles.

UIView *lastDot = nil;

I know you already got the answer.

I just want to add some points too:

Pointers are just like any other variable: if you don’t explicitly set
them to a value, the value will be undefined means a random value. If
you then accidentally access the variable, very bad things can happen.

Sometimes people recommend that pointers be assigned to NULL, the
universal value meaning “this pointer doesn’t point at anything”,
because a lot of code already checks for NULL pointers. For example,
if you call [nil release], it’s guaranteed to do nothing. If you
passed an uninitialized pointer in who knows what would happen.

It’s really just a safeguard against programmer error. If you
initialize pointers to a reasonable value, it’s less likely that an
uninitialized pointer will cause a system crash sometime in the
future.

When you don’t initialize a pointer and then try to use it, you have 3
problems:

• It might be pointing at memory you don’t have access to, in which case it causes a segmentation fault and crashes your program
• It might be pointing at real data, and if you don’t know what it’s pointing to, you’re causing unpredictable (and very hard to debug)
  changes to your data.
• You have no way of knowing if it’s been initialized or not – because how do you tell the difference between a valid address and the address
  that happened to be there when you declared the pointer?

Initializing every pointer to nil seriously decreases or eliminates
those problems:

• If I try and use it, it will still segfault, but at least I can test if it’s NULL and act accordingly – I can know that it WILL segfault,
  and do something else. If it’s a random value, I don’t know anything
  until it crashes.
• If you initialize it to nil, I can’t make it point to data unless I explicitly tell it to. So I only modify what I meant to.
• As implied above, I can tell when I’ve initialized it and when I haven’t, and make a decision.

Obviously it’s a matter of style, and it is possible to write an
application where variables are only initialized to their intended
value, but I feel it is safer and easier to initialize them to nil
always. Even the best programmers make typos – nil makes it easier
to know when that’s happened.

Reference :

1. eskimo
2. Initializing pointers always necessary