On 09/10/2009, at 8:01 PM, Matthias Arndt wrote:
While restructuring some old classes I'm uncertain about the
preferred way to access instance variables within their own instance:
I tend to defines all these variables as properties and use their
implicit getters / setters, but ...
1. ) ... in the designated initializer I have to access the instance
variables directly, because at this moment "self" is not yet
defined, is it?
2.) ... are there significant performance issues or other reasons to
prefer the direct reference to (or direct assignment of) instance
variables? At least the code of the setters / getters might impact
the performance, but I'd sacrifice it for the sake of a consistent
code.
Any comments / suggestions?
Properties and ivars are distinct things, though very often a property
is implemented in terms of an ivar.
In init and dealloc, it is usual & recommended to access the ivar
directly - not because self isn't defined (it is), but because at
those times you are usually only concerned with setting the ivar's
initial value (possibly to an object), or releasing it, without side
effects.
At all other times, including when accessing from within the instance
itself, you should usually use the getters/setters unless you have an
explicit and clear reason not to. Using the getters and setters allows
KVO to work as it should, and for deliberate side effects implemented
both by the class and any subclasses to work normally. It is only if
you definitely don't want these effects to operate that you should
access the ivar directly (and in those cases, it makes for more
readable code to have additional accessors that explain what they do,
e.g. -setSomePropertyWithoutNotifying:)
There is a minor performance penalty in accessing your ivars through
the accessors, but in general this is not something to be concerned
about unless profiling shows it's a problem. The code will be more
readable, and also allows a subclass to override an accessor to
supplement or replace functionality correctly - if the base class
bypasses the accessors very often subclasses will not work as might be
expected. (Though on that score it would be nice if Objective-C
supported the notion of private methods, though you can achieve
something of the same effect using a (Private) category, which I'd
also recommend).
--Graham
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