CODEWITHSUNDEEP
objective-carrays
Tortugo
Tortugo

Reputation: 11

Unexpectedly non-nil pointer in C array in Objective-C code

Below is the code

Thing *thingA = [[Thing alloc] init];

thingA.name = @"paper";

Thing   *things[0];
things[0] = thingA;

NSLog(@"%@", things[0].name);
// output paper

[thingA release];
// here dealloc on Thing class is call

NSLog(@"%@", things[0].name);
// output paper yet
- Here I thought that this object would be nill

Why is the element things[0] that points to thingA pointer not nil, thus making the application crash? Why was thingA pointer not nil?

Upvotes: 0

Views: 113

Answers (2)

jlehr
jlehr

Reputation: 15597

You're right in thinking that the memory for thingA has already been deallocated by the time execution reaches the final NSLog statement, but there's no guarantee that the operating system has marked it as invalid at that point. Since the value you placed at that address is still there, and thingA is still a pointer to that value, the NSLog statement still works.

The operating system doesn't automatically set local variables to nil if the memory they refer to gets deallocated. That's one of the reasons pointer variables can be dangerous. If you like, you can programmatically set these variable to nil yourself after sending a release message, particularly if there's any chance they will be used again in the same block of code.

Upvotes: 1

Only You
Only You

Reputation: 2072

If I remember right, Objective-C will not release the memory if there are still other objects/variables referencing that location.

However, it is crashing because your array Still has the memory location address which no longer is available for referencing and thus crashing when you try to acces with thing[0].name

Hope this helps.

Upvotes: 1

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