In a Swift method you can access a property using self.property or simply property.
Is there any difference? Is one of these preferred? Is there any style guide published by Apple which requires one or the other?
In practice, you don’t need to write self in your code very often. If
you don’t explicitly write self, Swift assumes that you are referring
to a property or method of the current instance whenever you use a
known property or method name within a method.
The main exception to this rule occurs when a parameter name for an
instance method has the same name as a property of that instance. In
this situation, the parameter name takes precedence, and it becomes
necessary to refer to the property in a more qualified way. You use
the self property to distinguish between the parameter name and the
property name.
Reference: The Swift Programming Guide from Apple.
Related
Swift appears to share with python one characteristic of requiring the class instance reference to access the members - even inside the class itself. The default in both languages is self. In particular
self.someClassMethod()
This is identical between python and swift. I also dislike this requirement finding it to be intrusive: it attracts my attention to self and away from which method is actually being invoked. In python I reduce (though do not remove) the annoyance by using s instead:
def someInstanceMethod(s, param1, param2)
instead of the standard
def someInstanceMethod(self, param1, param2)
Then inside the method I can access other instance methods :
s.someOtherInstanceMethod()
I'm not going to fight any battles on this: PEP folks will jump up and down about it -but it is more readable to me and others in my team. Is there any such way to do a shortcut in swift? I noticed typealias and tried to use it:
fileprivate let tp = U.tprint // Any non-critical logging statements will happen with this
But then it is necessary to do this:
self.tp("Loaded synthesizer settings")
Without the reference to self the following error occurs:
(162, 25) reference to property 'tp' in closure requires explicit 'self.' to make capture semantics explicit
I would prefer just
tp("Loaded synthesizer settings")
but that is not apparently possible. Can we get closer to that - along the lines of s.<method> instead of self.<method> ?
It's a little unclear what the question is, or what you think is the purpose of passing self around, so here's a quick summary of the key facts:
There are instance members and type members (type members are marked static or class).
If a method is an instance method, it does not need to say self to access instance members.
If a method is a type method, it does not need to say self to access type members.
If a method is an instance method, it can say Self to access type members.
If a method is a type method, there is no instance so instance members cannot be accessed.
Apple's book "The Swift Programming Language (Swift 3.1)" states the following:
As with function and method parameters, initialization parameters can have both a parameter name for use within the initializer’s body and an argument label for use when calling the initializer.
However, initializers do not have an identifying function name before their parentheses in the way that functions and methods do. Therefore, the names and types of an initializer’s parameters play a particularly important role in identifying which initializer should be called. Because of this, Swift provides an automatic argument label for every parameter in an initializer if you don’t provide one.
I don't understand the last sentence, as I didn't notice any difference between functions/methods and initializers when it comes to parameters names/labels. How is the argument label automatically provided for an initializer?
The feature being described is this: Given a struct:
struct Point {
let x: Double
let y: Double
}
Swift will automatically generate Point.init(x: Double, y: Double). If you add an init method of your own in the main struct definition, then Swift won't create that automatic init. (If you add an init in an extension, then you will get an automatic init. This is why people often add convenience init in an extension for structs.)
The point the last paragraph is trying to make is that Point(x:y:) is preferable to Point(_:_:). The labels in an initializer are even more valuable than labels in method names, because all initializers have the same "base" name ("init"). They're just explaining why they didn't choose a more implicit default that some people might expect coming from other languages.
In short, sometimes unlabeled parameters make sense in methods depending on what the name of the method is and how unambiguous that makes the first parameter. But in init, unlabeled parameters should be eyed with strong suspicion.
In languages like Java, PHP, Swift, there are keywords like this, $this, and self, respectively, which are reflexive pointers to a particular instance of the containing class. Both Java and Swift allow the programmer to omit this statement entirely if no other local variables share the same identifier. My question is what is the recommended way to write this in production? For example, is it acceptable for a programmer in production to omit self when it is not necessary?
var name: String = ""
init(name: String) {
self.name = name
}
func doSomeMethod() {
print(name)
}
or should a developer in production always use the self clause when accessing instance properties in general like
var name: String = ""
init(name: String) {
self.name = name
}
func doSomeMethod() {
print(self.name)
}
The documentation describes it very well
The self Property
Every instance of a type has an implicit property called self, which
is exactly equivalent to the instance itself. You use the self
property to refer to the current instance within its own instance
methods.
The increment() method in the example above (see the example in the linked guide) could have been written
like this:
func increment() {
self.count += 1
}
In practice, you don’t need to write self in your code very often.
If you don’t explicitly write self, Swift
assumes that you are referring to a property or method of the current
instance whenever you use a known property or method name within a
method. This assumption is demonstrated by the use of count (rather
than self.count) inside the three instance methods for Counter. (Counter is a class mentioned in the section).
The main exception to this rule occurs when a parameter name for an
instance method has the same name as a property of that instance. In
this situation, the parameter name takes precedence, and it becomes
necessary to refer to the property in a more qualified way. You use
the self property to distinguish between the parameter name and the
property name.
Source: The Swift Language Guide: Methods
I am a big fan of always using this in production code.
It has no affect on the emitted machine code, and making things easier for programmers is pointless as opposed to making things easier for the variety
of other tools you might want to use. (i.e. code searching tools, lint-type tools, and etc.)
Also, the time saved in avoiding stupid typo bugs is much greater than the time saved in typing.
There's currently a proposal on the swift-evolution repository to require self when accessing instance properties. It makes a fairly compelling argument for always requiring it.
I'm reading through the Swift documentation about type methods and type properties, and I cannot for the life of me figure out why it says this particular thing (in bold):
Within the body of a type method, the implicit self property refers to
the type itself, rather than an instance of that type. For
structures and enumerations, this means that you can use self to
disambiguate between type properties and type method parameters, just
as you do for instance properties and instance method parameters.
More generally, any unqualified method and property names that you use
within the body of a type method will refer to other type-level
methods and properties. A type method can call another type method
with the other method’s name, without needing to prefix it with the
type name. Similarly, type methods on structures and enumerations
can access type properties by using the type property’s name without a
type name prefix.
So, why is this pointing out structures and enumerations being able to do these things when, as far as I know, you can do these things with any kind of type methods/parameters (i.e. classes as well)? It makes me think I'm missing something.
The page in the documentation I'm looking at is here: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Methods.html
Apparently I wasn't the only one with this question. I'm not sure how I didn't see this post before: https://softwareengineering.stackexchange.com/questions/276962/static-properties-and-implicit-self-property-in-structures-and-enumerations-vs
How does Swift implement ARC in property attributes? For example, how do I make my String variable use copy instead of strong in Swift?
You can use the #NSCopying attribute when you want the copy behaviour from Objective-C.
From the Swift Book:
Apply this attribute to a stored variable property of a class. This
attribute causes the property’s setter to be synthesized with a copy
of the property’s value—returned by the copyWithZone method—instead of
the value of the property itself. The type of the property must
conform to the NSCopying protocol.
The NSCopying attribute behaves in a way similar to the Objective-C
copy property attribute.
However, in the specific case of String properties, it's not necessary to do so.
Strings are a value type in Swift. As such, when an existing String is assigned to a new variable, the variable actually stores a copy of the String, rather than a reference to the existing one.
Swift’s String type is a value type. If you create a new String value,
that String value is copied when it is passed to a function or method,
or when it is assigned to a constant or variable. In each case, a new
copy of the existing String value is created, and the new copy is
passed or assigned, not the original version.
So, the #NSCopying attribute is to be used when you have properties of a reference type that you want to set using the copy behaviour.