Assigning Closure to variable in Swift causes 'variable used before being initialized'

Question

I have a problem with a closure that is meant to be created and then being executed within another function over the range of the 2D pixel raster of an image where it shall basically called like this: filter(i,j) and return a value based on its arguments. I thought this code should work but it complains that the closure variable I have created is not initialized. I guess that means that I did not gave it arguments, but I wont within this function as the data is known to the closure at the time when it interacts with the image. How can I setup a closure which does not care about initialization?

Thank you in advance :)

func processFilter(type:FilterType){
 var x = 0
 var y = 0
 //create cloure
 var closure:(i:Int, j:Int)->Int
 if(type == FilterType.MyFilter) {
 x = 1024
 y = 2048
 func filter(i:Int, j:Int)->Int {
 return i*j*x*y*4096
 }
 //compiler does not complain here...
 closure = filter
 }
 //other if statements with different closure definitions follow...
 //This call throws error: variable used before being initialized 
 let image = filterImage(closure)
}

Answer 1

You use the variable closure before the compiler is certain that it is initialized. You can solve this in 2 ways, depending on what you need:

• Add an else-clause to your if and set closure to a default closure.
• Make closure optional by defining it as var closure: ((i: Int, j: Int) -> Int)? and then you can check if it is optional before using it by using closure?(i, j) or if let filter = closure { filter(i, j)}.

Also, try to use better variable names such as filterClosure. closure on its own doesn't really say much.

• Kryptographique: Thank your for your answer! I wanted to set the closure to nil as default which was also not accepted by the compiler and I wanted to prevent having a default closure. Using optionals however seems to be an elegant way! The variable name was just for the example above but could be more meaningful indeed.

Answer 2

The problem is that you define your closure as:

var closure:(i:Int, j:Int)->Int

Then you initialize it only if you enter the if

If not, that var is not initialized, hence the compiler warning

Possible solution:

func processFilter(type:FilterType){
 var x = 0
 var y = 0
 //create cloure
 var filterClosure:((i:Int, j:Int)->Int)?
 if(type == FilterType.MyFilter) {
 x = 1024
 y = 2048
 func filter(i:Int, j:Int)->Int {
 return i*j*x*y*4096
 }
 //compiler does not complain here...
 filterClosure = filter
 }
 //other if statements with different closure definitions follow...
 if let closure = filterClosure { 
 let image = filterImage(closure)
 }
}

• vrwim: Your closure now returns an optional type, you should use parentheses.
• Diego Freniche: Yes! Thanks, a typo! Also: I've seconded your suggestion of naming the closure filterClosure

Answer 3

Your closure is only initialized if the code enters your if block (i.e. if type == FilterType.MyFilter). In the other case it is left uninitialized.