How to check for Nil

Question

I am currently studying objective-c and the basic c programming language.

I have a questions about a particular line of code:

if (!balance)

Answer 1

First you need to understand how if works. Basically, any non-zero value is treated as true and a zero value is treated as false.

Something as simple as if (10) will be treated as true while if (0) is treated as false.

Any expression evaluates to either a value of zero or a non-zero value.

An object pointer is just a number - a memory address. A nil pointer is simply an address of 0. Any non-nil pointer will have a non-zero address.

The ! operator negates that state. A non-zero value will be treated as a zero value and visa-versa.

So now combine all of this.

Foo *bar = nil;
if (bar) // false since bar is nil (or zero)

Foo *bar = [[Foo alloc] init]; // some non-nil value
if (bar) // true since bar is non-zero

Foo *bar = nil;
if (!bar) // true since !bar mean "not bar" which is "not zero" which is "non-zero"

Foo *bar = [[Foo alloc] init]; // some non-nil value
if (!bar) // false since !bar mean "not bar" which is "not non-zero" which is "zero"