CODEWITHSUNDEEP
swiftprotocolsinstantiation
Cortado-J
Cortado-J

Reputation: 2257

In swift, why can't I instantiate a protocol when it has an initialiser?

I understand that generally I cannot instantiate a protocol. But if I include an initialiser in the protocol then surely the compiler knows that when the protocol is used by a struct or class later, it will have an init which it can use? My code is as below and line: 

protocol Solution {
  var answer: String { get }
}

protocol Problem {
  var pose: String { get }
}

protocol SolvableProblem: Problem {
  func solve() -> Solution?
}

protocol ProblemGenerator {
  func next() -> SolvableProblem
}

protocol Puzzle {
  var problem: Problem { get }
  var solution: Solution { get }

  init(problem: Problem, solution: Solution)
}

protocol PuzzleGenerator {
  func next() -> Puzzle
}

protocol FindBySolvePuzzleGenerator: PuzzleGenerator {
  var problemGenerator: ProblemGenerator { get }
}

extension FindBySolvePuzzleGenerator {
  func next() -> Puzzle {
    while true {
      let problem = problemGenerator.next()
      if let solution = problem.solve() {
        return Puzzle(problem: problem, solution: solution)
      }
    }
  }
}

The line:

return Puzzle(problem: problem, solution: solution)

gives error: Protocol type 'Puzzle' cannot be instantiated

Upvotes: 6

Views: 2570

Answers (5)

Ahmad F
Ahmad F

Reputation: 31645

I understand that I can't do it - I just want to understand why the compiler can't do it?

Because protocols in Swift represent abstraction mechanism. When it comes to abstraction, you could think about it as a template, we don't have to care about the details of how it behaves or what's its properties; Thus it makes no sense to be able to create an object from it.

As a real world example, consider that I just said "Table" (as an abstracted level), I would be pretty sure that you would understand what I am talking about! nevertheless we are not mentioning details about it (such as its material or how many legs it has...); At some point if I said "create a table for me" (instantiate an object) you have the ask me about specs! and that's why the complier won't let you create object directly from a protocol. That's the point of making things to be abstracted.

Also, checking: Why can't an object of abstract class be created? might be helpful.

Upvotes: 3

Caleb
Caleb

Reputation: 124997

But if I include an initialiser in the protocol then surely the compiler knows that when the protocol is used by a struct or class later, it will have an init which it can use?

Protocols must be adopted by classes, and there might be a dozen different classes that all adopt your Puzzle protocol. The compiler has no idea which of those classes to instantiate.

Protocols give us the power to compose interfaces without the complexity of multiple inheritance. In a multiple inheritance language like C++, you have to deal with the fact that a single class D might inherit from two other classes, B and C, and those two classes might happen to have methods or instance variables with the same name. If they both have a methodA(), and B::methodA() and C::methodA() are different, which one do you use when someone call's D's inherited methodA()? Worse, what if B and C are both derived from a common base class A? Protocols avoid a lot of that by not being directly instantiable, while still providing the interface polymorphism that makes multiple inheritance attractive.

Upvotes: 3

ielyamani
ielyamani

Reputation: 18581

When you instantiate an object, the Operating System has to know how to allocate and deal with that kind of object in the memory: Is it a reference type (Classes)? Strong, weak or unowned reference? Or is it a value type (Structs, Strings, Int, etc)?

Reference types are stored in the Heap, while value types live in the Stack. Here is a thorough explanation of the difference between the two.

Only Reference and Value types (objects) can be instantiated. So, only the objects that conform to that protocol can then be instantiated, not the protocol itself. A protocol is not an object, it is a general description or schema of a certain behavior of objects.

As to Initialization, here what the Apple docs say:

Initialization is the process of preparing an instance of a class, structure, or enumeration for use. This process involves setting an initial value for each stored property on that instance and performing any other setup or initialization that is required before the new instance is ready for use.

Upvotes: 2

Amadan
Amadan

Reputation: 198314

Imagine protocols are adjectives. Movable says you can move it, Red says it has color = "red"... but they don't say what it is. You need a noun. A Red, Movable Car. You can instantiate a Car, even when low on details. You cannot instantiate a Red.

Upvotes: 8

Taier
Taier

Reputation: 2119

Unfortunately swift does not allow that even with such "hack"

You would need to use a class that confirms to that protocol as an object you refer to.

Upvotes: 1

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