On 6 Apr '08, at 9:17 PM, Scott Andrew wrote:
You aren't supposed to EVER use direct ivar acess. Every document i have ever read regarding ObjectiveC has always said to define acessors.
Ummm ... for access to another object's instance data, that's true. You should avoid the temptation to declare ivars as public and grope them directly. But it's fine to access ivars of 'self' by name, and most of the Obj-C code I've seen does so.
So what prefix you use on your ivars don't matter. To set m_object. I am going to either define get/set functions or use properties.
You can't do this if your getters or setters are non-trivial. Let's say I have a property whose value is computed lazily, a very common design pattern:
- (NSImage*) icon {
if( ! _icon ) {
// allocate _icon and draw into it
}
return _icon;
}
You can't replace "_icon" with "self.icon" because you'd be calling
the method you're defining, leading to infinite regress and stack
overflow.
I guess you could create a separate private property (with ivar) called "cachedIcon" and have the -icon getter act on that, but that's starting to get silly.
I'll also re-iterate that calling getters and setters is Extremely Expensive compared to directly accessing ivars. As in, something like 4x as much code at the call-site, and at least 10x as many cycles to execute. (Probably more like 20x or 30x if you use the default 'atomic' style synthesized properties, which do autoreleasing and thread synchronization.) It also defeats a number of compiler optimizations.
—Jens
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