gistfile1.swift

				/// The current element attributes state
	internal struct TagAttributes
	{
		var tagName: String!
		var URL: NSURL?
		var imageURL: NSURL?
		
		var isUnderlined: Bool = false
		var isStriken: Bool = false
		
		var color: UIColor?
		
		var fontAttributes: TagFontAttributes!
		var textAlignment: NSTextAlignment = .Natural
	}
	
	/// A stack of the element attributes
	internal var tagAttributesStack = [TagAttributes]()
			
			
			
			if var tagAttributes = self.tagAttributesStack.last, let tagName = tagAttributes.tagName where tagName == "img"
			{
				tagAttributes.imageURL = NSURL(string: string)
				
				// replace the value type with the modified version
				self.tagAttributesStack[self.tagAttributesStack.count-1] = tagAttributes
			}

You can have the changed variable inside a box class and it will work:

struct MyStruct {
    var val:String = "initial"
    var boxURL:Box = Box()
}

class Box {
    var url:NSURL = NSURL(string: "http://initial.com")!
}

var a = MyStruct()
var b = a

a.val                 // "initial"
a.boxURL.url   // "http://initial.com"

b.boxURL.url = NSURL(string: "http://changed.com")!

a.val                    // "initial"
a.boxURL.url     // "http://changed.com"

b.val                   // "initial"
b.boxURL.url    // "http://changed.com"

Wouldn't something like this work?

if let tagAttributes = self.tagAttributesStack.last, let tagName = tagAttributes.tagName where tagName == "img"
{
    self.tagAttributesStack.last?.imageURL = NSURL(string: string)
}