Swift - what's the difference between metatype .Type and .self?

Declaration

typealias AnyClass = AnyObject.Type

.Type

The metatype of a class, structure, or enumeration type is the name of that type followed by .Type. The metatype of a protocol type—not the concrete type that conforms to the protocol at runtime—is the name of that protocol followed by .Protocol. For example, the metatype of theclass type SomeClass is SomeClass.Type and the metatype of the protocol SomeProtocol is SomeProtocol.Protocol.

_{From Apple : metaType Type}

Under the hood AnyClass is

Basically where ever you see AnyClass, Any.Type, AnyObject.Type, its because it's in need of a type. A very very common place we see it is when we want to register a class for our tableView using register func.

_{If you are confused as to what does 'Swift.' do then above, then see the comments from here}

The above could have also been written as:

.self

You can use the postfix self expression to access a type as a value. For example, SomeClass.self returns SomeClass itself, not an instance of SomeClass. And SomeProtocol.self returns SomeProtocol itself, not an instance of a type that conforms to SomeProtocol at runtime. You can use a type(of:) expression with an instance of a type to access that instance’s dynamic, runtime type as a value, as the following example shows:

_{From Apple : metaType Type}

Where is it used?

If you are writing/creating a function that accepts a type e.g. class, not an instance then to you would write T.Type as the type of the parameter. What it expects as a parameter can be: String.self, CustomTableView.self, someOtherClass.self.

In continuation of the tableView code:

---

Playground code:

Easy example

struct Something {

var x = 5

}

let a = Something()

type(of:a) == Something.self // true

Hard example

class SomeBaseClass {

class func printClassName() {

print("SomeBaseClass")

}

}

class SomeSubClass: SomeBaseClass {

override class func printClassName() {

print("SomeSubClass")

}

}

let someInstance: SomeBaseClass = SomeSubClass()

/*                      |                |

compileTime       Runtime

|                |

To extract, use:       .self          type(of)

The compile-time type of someInstance is SomeBaseClass,

and the runtime type of someInstance is SomeSubClass */

type(of: someInstance) == SomeSubClass.self // TRUE

type(of: someInstance) == SomeBaseClass.self // FALSE

I highly recommend to read Apple documentation on Types. Also see here

https://stackoverflow.com/questions/31438368/swift-whats-the-difference-between-metatype-type-and-self