Creating a macro to perform default init - objective-c

Question

I have a lot of methods that repeat this simple boilerplate:
- (id)myObject {
if(!_myObject) {
self.myObject = [_myObject.class new];
}
return _myObject;
}
So I want to replace this with a simple macro:
#define default_init(instance) \
if(!instance) instance = [instance.class new]; \
return instance;
So that I would only have to call:
- (id)myObject {
default_init(_myObject);
}
The above code currently compiles, but the issue is that the macro directly sets the instance variable's value. Instead, I'd like to call self.instance = value;
So instead of
if(!instance) instance = [instance.class new];
I'd like something like;
if(!instance) self.instance = [instance.class new];
But obviously the current code does not allow for this. How might I accomplish something like this?

Answer 1

With this macro:
#define default_init(class, instance) \
if ( ! _##instance ) { \
self.instance = [class new] ; \
} \
return _##instance
I was able to create this instance method:
- (NSMutableArray*) myObject {
default_init(NSMutableArray, myObject) ;
}
I had to add a parameter defining the class, because _myObject is still nil, therefore _myObject.class is nil.
This StackOverflow question and this Cprogramming page recommend wrapping your multi-line macro in do {...} while(0):
#define default_init(class, instance) \
do { \
if ( ! _##instance ) { \
self.instance = [class new] ; \
} \
return _##instance ; \
} while(0)
If you really wanted to, you could make a macro that defines the entire method:
#define default_getter(class, instance) \
- (class*) instance { \
if ( ! _##instance ) { \
self.instance = [class new] ; \
} \
return _##instance ; \
}
And use it thusly:
default_getter(NSMutableArray, myObject)

Answer 2

Instead of a macro to build a getter method, I usually declare the property:
Instance.h
#interface Instance : NSObject
#property NSMutableArray* myObject ;
#end
and override - init to initialize the property:
Instance.m
- (id) init {
self = [super init] ;
if ( self ) {
self.myObject = [NSMutableArray new] ;
}
return self ;
}

Answer 3

Ok, this is my take on it.
Note: this was done just to see if it could be done. I don't think it would be a good idea to use this in shipping code.
First import #import "EXTRuntimeExtensions.h" from libextobjc. Then:
#define underize(name) _##name
#define property_type(property) \
property_attr(property)->objectClass
#define property_attr(propertyName) \
ext_copyPropertyAttributes(class_getProperty(self.class, # propertyName))
#define default_init(propertyName) \
- (id)propertyName { \
if(!underize(propertyName)) self.propertyName = [property_type(propertyName) new]; \
return underize(propertyName); \
} \
Then say you have a property:
#property (nonatomic) NSArray *myArray;
You can do:
default_init(myArray);
And that creates a default getter.