Local variables set to nil? (Objective-C)

问题:

I'm reading a book on Objective-C and the author said that if local variables aren't assigned a value they will be set to nil, but static variables will be set to zero. So, I set up int a and didn't assign it a value. Then NSLog(@"%i", a) to display it and a was displayed as zero. I was a little confused on that and I was wondering if someone could clarify it for me?

回答1:

With ARC enabled, your Objective-C object pointer variables will be set to nil regardless of where you create them.

Without ARC, and for built in C types, your variables will not be initialized.

Instance variables of Objective-C objects are always set to 0 (or nil) when you allocate an object.

Statics are set to 0.

I've gotten in the habit of always giving a default value to variables, though. It's been a good habit to have.

回答2:

No². Just as in "plain" C, local variables are not assigned a default value. (Although you may get lucky the first time part of the stack is used: do not rely on this!.)

Anyway, nil is 0¹ -- that is, nil == 0 is always true -- so NSLog("@%i", nil) says "hey, log the argument as an integer", which is ... 0.

Happy coding.

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¹ See nil in gdb is not defined as 0x0? which covers the technical definition, including the Objective-C++ case, in more detail. Note that the type changes depending upon architecture as well so "@%i" could very well be wrong for a particular system.

² See wbyoung's answer for ARC-specific rules.