CODEWITHSUNDEEP
iosobjective-c
Linh
Linh

Reputation: 61039

Why NSNumber *a = 0 do not display error

I know the correct way to initial a NSNumber is NSNumber *a = @1;

and when I declare NSNumber *a = 1;, I will got the error

Implicit conversion of int to nsnumber is disallowed with arc

But I don't know why when I declare NSNumber *a = 0; there is no error

In my case, I have write some function in NSNumber category and then

Upvotes: 3

Views: 584

Answers (5)

vaibhav
vaibhav

Reputation: 4096

A nil value is the safest way to initialize an object pointer if you don’t have another value to use, because it’s perfectly acceptable in Objective-C to send a message to nil. If you do send a message to nil, obviously nothing happens.

Note: If you expect a return value from a message sent to nil, the return value will be nil for object return types, 0 for numeric types, and NO for BOOL types. Returned structures have all members initialized to zero.

In the last Apple Doc Working with nil

Upvotes: 0

Avi
Avi

Reputation: 7552

The value 0 is synonymous with nil or NULL, which are valid values for a pointer.

It's a bit of compatibility with C that leads to this inconsistent behavior.

History

In the C language, there is no special symbol to represent an uninitialized pointer. Instead, the value 0 (zero) was chosen to represent such a pointer. To make code more understandable, a preprocessor macro was introduced to represent this value: NULL. Because it is a macro, the C compiler itself never sees the symbol; it only sees a 0 (zero).

This means that 0 (zero) is a special value when assigned to pointers. Even though it is an integer, the compiler accepts the assignment without complaining of a type conversion, implicit or otherwise.

To keep compatibility with C, Objective-C allows assigning a literal 0 to any pointer. It is treated by the compiler as identical to assigning nil.

Upvotes: 14

meaning-matters
meaning-matters

Reputation: 23016

Objective-C silently ignores method calls on object pointers with value 0 (i.e. nil). That's why nothing happens when you call a method of your NSNumber category pointer which you assigned the value 0.

Upvotes: 0

Ketan Parmar
Ketan Parmar

Reputation: 27448

You can consider 0 as nil or null that can be assign to object but 1 is integer and can't allow to object or non integer.

Upvotes: 1

gnasher729
gnasher729

Reputation: 52632

0 is a null pointer constant. A null pointer constant can be assigned to any pointer variable and sets it to NULL or nil. This was the case in C for the last 45 years at least and is also the case in Objective-C. Same as NSNumber* a = nil.

Upvotes: 3

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