What does an exclamation mark mean in the Swift language?

Question

The Swift Programming Language guide has the following example:

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apar         


        

Answer 1

The entire story begins with a feature of swift called optional vars. These are the vars which may have a value or may not have a value. In general swift doesn't allow us to use a variable which isn't initialised, as this may lead to crashes or unexpected reasons and also server a placeholder for backdoors. Thus in order to declare a variable whose value isn't initially determined we use a '?'. When such a variable is declared, to use it as a part of some expression one has to unwrap them before use, unwrapping is an operation through which value of a variable is discovered this applies to objects. Without unwrapping if you try to use them you will have compile time error. To unwrap a variable which is an optional var, exclamation mark "!" is used.

Now there are times when you know that such optional variables will be assigned values by system for example or your own program but sometime later , for example UI outlets, in such situation instead of declaring an optional variable using a question mark "?" we use "!".

Thus system knows that this variable which is declared with "!" is optional right now and has no value but will receive a value in later in its lifetime.

Thus exclamation mark holds two different usages, 1. To declare a variable which will be optional and will receive value definitely later 2. To unwrap an optional variable before using it in an expression.

Above descriptions avoids too much of technical stuff, i hope.