How to prevent object construction in Swift [duplicate]

Question

What ways do you know to prevent an object construction using Swift programming language?

In C++ I can simply make the constructor private like this:

struct A {
 private:
  A() {};
};

int main()
{
  // Doesn't compile because the constructor is private.
  A obj;
  return 0;
}

When I do a similar thing in Swift (I tried it in playground) the code compiles just fine:

class A {
  private init() {}
}

let obj = A()

UPDATE:

Ok, this question is marked as a duplicate. But I think this is a misunderstanding. What I'm asking about is what are the best practices you know to prevent object construction in Swift. All I want to achieve is to make it clear to the users of my class that it should not be constructible.

UPDATE 2:

As this question is still here, I think, it needs some more clarifications for those who still can't comprehend what I really want.

Given a class that is used as a wrapper for some useful constants such as the following:

class Constants {
 static let someConstant1 = "CONSTANT_VALUE1"
 static let someConstant2 = "CONSTANT_VALUE2"
 //....etc...
}

what option can be considered as a best practice:

• Leave it as is and don't worry about the possibility of objects creation outside this class;
• Add private init() {} to prevent creation of the objects outside the current file;
• Use init? and return nil to indicate that the objects must not be created as was suggested in the comments.

Hope the question is more clear now.

Answer 1

From Apple's guide to Swift:

Private access restricts the use of an entity to its own defining source file. Use private access to hide the implementation details of a specific piece of functionality.

Your playground file is all one file, so privacy is not enforced.

For instance, if you create a new project and add a file called Dog.swift to the project that looks like this:

import Foundation

class Dog {
    private init() {
        print("hello")
    }
}

class Cat {
    var d = Dog()
}

in ViewController.swift, you can write:

override func viewDidLoad() {

    let c = Cat()  //=>hello

}

But, if you try:

override func viewDidLoad() {

    let d = Dog() 

}

Xcode will flag that as an error before you even compile the program:

'Dog' cannot be constructed because it has no accessible initializers

Response to comment:

class A {
    init?() {
        return nil
    }

    func greet() {
        print("hello")
    }
}

let x = A()

if let x = x {
    x.greet()
}
else {
    print("nice try")  //=> nice try
}