On Apr 18, 2008, at 5:56 PM, K. Darcy Otto wrote:
I am working on a program with a complex hierarchy of classes, and I
want to subclass one of the objects a few steps down on that
hierarchy. Do I have to create a parallel hierarchy to do this?
Here is the problem:
AppController instantiates a Deduction object.
The Deduction object instantiates a DeductionLine object.
The DeductionLine object instantiates a Dependency object.
When you say "instantiates," I assume you mean each object has the
next-level object as an instance variable, and the instantiating
happens in the -init method? And the problem is that the
DeductionLine and Dependency classes are hard-coded in the init
methods that instantiate them?
Assuming I've guessed correctly...
Perhaps you could add an init method in each class that takes an
instance of the sub-object as an argument. For example, DeductionLine
could have this as its new designated initializer:
- (id)initWithDependency:(Dependency *)dependency
{
if ((self = [super init]))
{
_dependency = [dependency retain];
}
return self;
}
Then it could override -init to use a plain Dependency as the default:
- (id)init
{
return [self initWithDependency:[[[Dependency alloc] init]
autorelease]];
}
You could do a similar -initWithDeductionLine: method for the
Deduction class.
To create a Deduction that uses an SDependency two levels down, you
could do:
Dependency *sdep =
[[[SDependency alloc] init] autorelease];
DeductionLine *dedLine =
[[[DeductionLine alloc] initWithDependency:sdep] autorelease];
Deduction *ded =
[[[Deduction alloc] initWithDeductionLine:dedLine] autorelease];
You could simplify things by wrapping some of the above in convenience
methods that would enable you to do this...
Deduction *ded = [Deduction deductionWithDependency:[SDependency
dependency]];
...or even this if it's not too implementation-revealing...
Deduction *ded = [Deduction deductionWithSDependency];
Another approach might involve using setter methods like -
setDependency: etc. But it sounds like the structure of your code
requires a DeductionLine to have a valid Dependency instance as soon
as it is instantiated.
--Andy
I want to subclass Dependency and override a few things. Can I do
this without modifying DeductionLine? As it stands, I think I'm
going to have to do this:
AppController instantiates a SDeduction object (where SDeduction is
a subclass of Deduction).
The SDeduction object instantiates a SDeductionLine object (where
SDeductionLine is a subclass of DeductionLine).
The SDeduction object instantiates a SDependency object (where
SDependency is a subclass of Dependency).
The only reason to subclass Deduction (with SDeduction) and then
DeductionLine (with SDeductionLine) is to get an SDependency
object. Is there an easier way? I suppose I could just copy the
Dependency class files and then modify it for this particular
project; but I was hoping for a different way. Thanks.
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