Type Casting in swift

Question

I am bit confused with typecasting in Swift.

Have a small doubt. What is the difference between as?,as! and only as. And can we say "as" is similar to is.

Thanks in advance.

Lebar

Please search first. This has been satisfactorily explained many times already. Even better, learn Swift. – Difference

Yes,I have searched and i got answers on "as?" and "as!" but not only "as" – Lebar

as is used for casting when it is guaranteed to succeed (String to NSString, [AnyObject] to NSArray, etc.) – Bridgetbridgetown

Then whats the difference between "as" and "as!" – Lebar

as! is used if it cannot be guaranteed. – Bridgetbridgetown

Answer 1

The 'as' keyword is used for casting.

'as' example:

let calcVC = destinationViewController as CalculatorViewController

This line casts the destinationViewController to a CalculatorViewController. However, this would crash if destinationViewController was not a CalculatorViewController or a subclass thereof.

To protect against a crash, you can use 'if let' with 'as?'...

'as?' example:

if let calcVC = destinationViewController as? CalculatorViewController {
   // ... write code to execute if destinationViewController is in fact a CalculatorViewController
}

You can even check before you even try to do 'as' with the 'is' keyword...

'is' example:

if destinationViewController is CalculatorViewController {
   //...
}

Diverse

Thanks ,Nice explanation. But still not clear about only "as" . I tried var button = somebutton as UIButton and it was throwing an error. – Lebar

What is the error you are getting? Is somebutton an instance of type UIButton or a subclass of UIButton? If not then an error will be thrown. You can only cast one object to a certain class if it is an instance of that class or an instance of a subclass. – Diverse

Answer 2

is is used to check the type of a value whereas as is used to cast a value to a different type.

The conditional form, as?, returns an optional value of the type you are trying to downcast to. 

Example:

let number: Int = 1 
if number as? Int {
print ("Int")
} 

You are "asking" if number is of type Int.

Using the as! you're forcing, for example, your let number to be of a certain type, in case it is not, a crash you're going to take!

Example:

if number as! String { //CRASH

}

Prader

Answer 3

Answer in simple words ->

as! -> it is used for casting one data type to other data type forcefully(Use this only if you are sure..). We even call it force downcast means we downcast a particular type from superclass to subclass.

as -> This keywprd is used to raise an object to superclass type. So in other words we can say its upcasting.

as? -> So in this case unless you are certain that this cast is going to work then better option to add ? instead of !. Its also downcasting.

Hydra