Vayn Vayn - 26 days ago 8
Swift Question

NULL parameter in Swift

In Objective-C, we can declare a function like this:

- (void)getRect:(CGRect *)aRectRef bRect:(CGRect *)bRectRef
{
if (aRectRef) *aRectRef = CGRectZero
if (bRectRef) *bRectRef = CGRectZero
}


and pass
NULL
to the function:

CGRect rect;
[self getRect:NULL bRect:rect]


There isn't
NULL
in Swift. I can't use
nil
as inout param directly either:

func getRect(aRect aRectRef: inout CGRect?, bRect bRectRef: inout CGRect?) -> Void {
...
}
self.getRect(&nil, bRect: rect) // <- ERROR


I must define a variable with nil value and pass it to the function, even though I don't need the variable totally.

How to pass
nil
to the function?

UPDATE:

null / nil in swift language just explained
nil
in Swift.

Swift optional inout parameters and nil explained how to define a variable with nil value and pass it as inout parameter.

I want to know there is a way to pass nil directly like
&nil
to function or not.

Answer

Your Objective-C method has nullable pointers as parameters, in Swift 3 that would be an optional UnsafeMutablePointer:

func getRect(aRectRef: UnsafeMutablePointer<CGRect>?, bRectRef: UnsafeMutablePointer<CGRect>?) {
    if let aRectPtr = aRectRef {
        aRectPtr.pointee = CGRect(x: 1, y: 2, width: 3, height: 4)
    }
    if let bRectPtr = bRectRef {
        bRectPtr.pointee = CGRect(x: 5, y: 6, width: 7, height: 8)
    }
}

var rect = CGRect.zero
getRect(aRectRef: &rect, bRectRef: nil)
print(rect) // (1.0, 2.0, 3.0, 4.0)

So you can pass nil as an argument. What you can not do (in contrast to Objective-C) is to pass the address of an uninitialized variable, rect must be initialized here.

The same can be written more compactly as

func getRect(aRectRef: UnsafeMutablePointer<CGRect>?, bRectRef: UnsafeMutablePointer<CGRect>?) {

    aRectRef.map { $0.pointee = CGRect(x: 1, y: 2, width: 3, height: 4) }
    bRectRef.map { $0.pointee = CGRect(x: 5, y: 6, width: 7, height: 8) }
}