objective-c note

Apple Guide And Sample Guide

Objects normally have a longer life than the simple scope of a method call. In particular, an object often needs to stay alive longer than the original variable that was created to keep track of it, so an object’s memory is allocated and deallocated dynamically.

Note: If you’re used to using terms like the stack and the heap, a local variable is allocated on the stack, while objects are allocated on the heap.

This requires you to use C pointers (which hold memory addresses) to keep track of their location in memory, like this:

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- (void)myMethod {
    NSString *myString = 
    [...]
}

Although the scope of the pointer variable myString (the asterisk indicates it’s a pointer) is limited to the scope of myMethod, the actual string object that it points to in memory may have a longer life outside that scope. It might already exist, or you might need to pass the object around in additional method calls, for example.

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.

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+ (id)alloc

• Working with nil

Working with nil

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.