swift“ variable：Type”和“ variable as Type”有什么区别

Question

What is the difference between:

let variable:Double = 23

and "as Type"?

let variable = 23 as Double

Answer 1

let variable:Double = 23

Declaring variable in this way is called Type Annotation in which we are telling complier explicitly that variable is type of Double instead of compiler referring to type of assigned value.

let variable = 23 as Double

This is called type casting .As per definition in Swift,

Type casting in Swift is implemented with the is and as operators. These two operators provide a simple and expressive way to check the type of a value or cast a value to a different type.

So these are not different in terms of functionality. Using first way, we are adding redundant token for declaring a constant as type inference would detect type automatically by value.

By Second way, we are forcing a constant to have a kind of value which would be useful in case of superClass/Subclass type casting.