self.instance_var performance hit?

Question

Let mFoo be an instance variable that's a property that has been synthesized, hence it has default setter and getter. I am wondering if one needs to be concerned with the performance hit of using self.mFoo vs.mFooif mFoo is accessed repeatedly in logical statements.

It seems to me that if one is absolutely certain that a method does not have a local variable mFoo declared, and one is doing several logical comparison with mFoo within a method, and the method is called a lot, that it makes sense not to access mFoo through its accessor method but directly.

For example:

NSMutableArray *mFoo;
@property(nonatomic, retain) NSMutableArray *mFoo;
@synthesize mFoo;

-(void)someMethod {
    Bar* b;
    for (int i=0; i<10000; i++) {
        b = [self.mFoo objectAtIndex:i];   // <<<<<<<<<<<<<<<<
        if (b.something == 123) { // do something };
    }
 }

Answer 1

For every access to self.mFoo, about one C function would have to be called if fast Objective C message dispatch is enabled (unless the compiler optimizes that call away). So the performance hit won't be noticeable most of the time. In the example you mentioned, you should use fast enumeration like

for (Bar *b in self.mFoo) {
    // do whatever you wish to here
}

, which will lead to self.mFoo being accessed just once and is faster in general (e.g. if your array is really a list).

Answer 2

If you're accessing the property from another class, you should always use the accessor method or dot notation. That's just basic OO hygiene. In the unlikely case that you discover that this causes a bottleneck in a loop or something, you can think about some tailor-made solution at that point.

If you're accessing the variable from within the class, I think it's ok to access the ivar directly rather than through the property if you are getting the value. However, if you are setting the value you should always use the propery, both in order to avoid memory leaks and because the setter might have logic that should be executed.

So:

CGSize sz = [self size]; //ok
CGSize sz = self.size; //ok
CGSize sz = _size; //ok, but be careful

[self setSize:sz]; //ok
self.size = sz, //ok
_size = sz; // DON'T EVER DO THIS!

However, if you have an ivar that doesn't even have a property associated with it, you should needless to say access it directly (you don't have much choice...). Personally, I usually declare properties for all ivars except some flags or counters with non-object types, but many people don't.