Are ints always initialized to 0?
Objective CInitializationIntInstance VariablesObjective C Problem Overview
Is it safe to count on int
s always being initialized to 0 in Objective-C?
More specifically, when an object with int
ivars has been newly instantiated, is it safe to assume that its ivars have value 0?
Objective C Solutions
Solution 1 - Objective C
Yes, class instance variables are always initialized to 0 (or nil
, NULL
, or false
, depending on the exact data type). See the Objective-C 2.0 Programming Language:
> The alloc
method dynamically allocates memory for the new object’s instance variables and initializes them all to 0—all, that is, except the isa
variable that connects the new instance to its class.
EDIT 2013-05-08
Apple seems to have removed the above document (now linked to The Wayback Machine). The (currently) active document Programming With Objective-C contains a similar citation:
> The alloc
method has one other important task, which is to clear out the memory allocated for the object’s properties by setting them to zero. This avoids the usual problem of memory containing garbage from whatever was stored before, but is not enough to initialize an object completely.
However, this is only true for instance variables of a class; it is also true for POD types declared at global scope:
// At global scope
int a_global_var; // guaranteed to be 0
NSString *a_global_string; // guaranteed to be nil
With one exception, it is not true for local variables, or for data allocated with malloc()
or realloc()
; it is true for calloc()
, since calloc()
explicitly zeros out the memory it allocates.
The one exception is that when Automatic Reference Counting (ARC) is enabled, stack pointers to Objective-C objects are implicitly initialized to nil
; however, it's still good practice to explicitly initialize them to nil
. From the Transitioning to to ARC Release Notes:
> Stack Variables Are Initialized with nil
>
> Using ARC, strong, weak, and autoreleasing stack variables are now implicitly initialized with nil
In C++ (and C++ objects being used in Objective-C++), class instance variables are also not zero-initialized. You must explicitly initialize them in your constructor(s).
Solution 2 - Objective C
I don't think you should assume any values for initialization. If you are building logic around a "0" value, you should set it to be sure.
Solution 3 - Objective C
Yes, in C global vars are initialized to zero. In Objective-C even local vars are initialized to zero. You can count on it.