Class issues: should I use everywhere self as class in objective c? - objective-c

Question

Wow, great issue I have found for myself.
What is it? The candy or the garlic?
something about Objective-C:
Are there any issues not to use 'self' in (+) - class methods as class?
in the deep of a class...
+(NSDate*)dateWithTimeInterval:(NSTimeInterval)interval {
return [self dateWithTimeIntervalSince1970:interval];
}
Ruby here:
For example, in Ruby everything is object and class is object of class Class and there is a good practice to rely on self:
class DateClass
# self is DateClass here, inside of class definition, uh
self.dateWithTimeInterval(interval)
self.dateWithTimeIntervalSince1970(interval)
end
end
Perl here:
Another example was found in perl oop deep: (thanks for this thread)
sub new {
my $proto = shift || die "Must pass a class or object into new()";
my $class = ref($proto) || $proto;
bless {}, $class;
}
So, in Perl and in Ruby guys always rely on $class refs
Maybe example with Perl code not obvious, but it happens all time. Programmers rely on $class reference and take class name with it. also, they can invoke some methods with it:
my $class = 'Class';
$class->new();
or
Class::->new()
After all...
Which pitfalls or caveats could you provide against usage self as class in objective-c?

Answer 1

Usually you use self whenever you can but of course, there are situations when referencing the class by [MyClass class] is desired. Almost all of the scenarios are related to inheritance.
For example, a creator method for a class A.
#implementation A
+ (id)createInstanceWithParam:(NSInteger)param {
return [[self alloc] initWithParam:param];
}
#end
Will work correctly even if we create a subclass B. However, if we decide to implement a class cluster, then we have to reference classes by names:
#implementation SomeDataStructure
+ (id)createInstanceWithType:(NSInteger)type {
if (type == 0) {
return [[DataStructureImpl1 alloc] init];
}
else if (type == 1) {
return [[DataStructureImpl2 alloc] init];
}
}
#end
Another example is the common example of +initialize
+ (void)initialize {
if (self == [MyClass class]) {
...perform initialization...
}
}
And of course, if you are overriding a method, then using self or using [MySelf class] can be a distinction between your overriden implementation and the original implementation. Although super could be used there, too.
TLDR:
self is preferred but be careful with subclasses/superclasses.

Answer 2

For understanding pros and cons of using self vs. class name let's consider one situation:
Class A is subclass of NSDate and implements method +(NSDate*)dateWithTimeInterval:(NSTimeInterval)interval.
Class B is subclass of A and overrides implementation of +dateWithTimeIntervalSince1970:(NSTimeInterval)interval method that declared in NSDate.
Now let's consider two possible implementations of +(NSDate*)dateWithTimeInterval:(NSTimeInterval)interval method in A:
1. Using self
+(NSDate*)dateWithTimeInterval:(NSTimeInterval)interval {
return [self dateWithTimeIntervalSince1970:interval];
}
if run [B dateWithTimeInterval:interval]; then self in above code is kind of B class and as expected custom implementation (in class B) for +(NSDate*)dateWithTimeIntervalSince1970:(NSTimeInterval)interval method would be called.
2. Using directly NSDate
+(NSDate*)dateWithTimeInterval:(NSTimeInterval)interval {
return [NSDate dateWithTimeIntervalSince1970:interval];
}
if run [B dateWithTimeInterval:interval]; then overridden implementation (in class B) would be ignored and instead of it: original implementation (in class NSDate) for +(NSDate*)dateWithTimeIntervalSince1970:(NSTimeInterval)interval method would be called. It's so because we directly send message to NSDate: [NSDate dateWithTimeIntervalSince1970:interval];.
This behavior is unexpected for developer.
For the same reason declare methods in such way:
+(instancetype)dateWithTimeInterval:(NSTimeInterval)interval {
return [self dateWithTimeIntervalSince1970:interval];
}
By using instancetype compiler will know what kind of object is returned by method-initializer. When you call [B dateWithTimeInterval:interval] it returns object of kind B but not NSDate.