Protocol as types in Swift - Time & Space Complexity

Time Complexity: Protocol as types

O(n)

Understanding Time Complexity

When we use protocols as types in Swift, we want to know how the program's running time changes as we work with more objects that follow the protocol.

We ask: How does the time to call protocol methods grow when we have many such objects?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

protocol Drawable {
    func draw()
}

func drawAll(shapes: [Drawable]) {
    for shape in shapes {
        shape.draw()
    }
}
    

This code defines a protocol and a function that calls the draw method on each object in a list of protocol types.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling draw() on each object in the array.
• How many times: Once for each object in the shapes array.

How Execution Grows With Input

As the number of shapes grows, the total calls to draw() grow the same way.

Input Size (n)	Approx. Operations
10	10 calls to draw()
100	100 calls to draw()
1000	1000 calls to draw()

Pattern observation: The number of operations grows directly with the number of shapes.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the function grows in a straight line as you add more shapes.

Common Mistake

[X] Wrong: "Using a protocol as a type makes the code slower in a way that changes the time complexity."

[OK] Correct: Calling methods on protocol types does add a small fixed cost, but this cost stays the same per call and does not change how the total time grows with more objects.

Interview Connect

Understanding how using protocols as types affects performance helps you explain your design choices clearly and shows you know how code structure relates to speed.

Self-Check

What if the draw() method itself contained a loop over a collection? How would that change the overall time complexity?

Practice

1. What does it mean to use a protocol as a type in Swift?

easy

A. Any object that follows the protocol's rules can be used as that type.

B. Only classes can conform to the protocol.

C. Protocols can only be used to create new classes.

D. Protocols define stored properties for types.

Solution

1. Step 1: Understand what a protocol defines

   A protocol defines a set of rules or features that a type must implement.

2. Step 2: Using protocol as a type

   When a protocol is used as a type, any object that implements those rules can be assigned to that type.

3. Final Answer:

   Any object that follows the protocol's rules can be used as that type. -> Option A

4. Quick Check:

   Protocol as type means flexible object use = A [OK]

Hint: Protocol as type means any conforming object fits [OK]

Common Mistakes:

• Thinking protocols only apply to classes
• Believing protocols define stored properties
• Confusing protocol usage with inheritance

2. Which of the following is the correct way to declare a variable of a protocol type named Drawable?

easy

A. var shape: Drawable

B. var shape: Drawable()

C. var shape: Drawable = Drawable()

D. var shape: Drawable[]

Solution

1. Step 1: Understand protocol type declaration

   To declare a variable of a protocol type, use the protocol name without parentheses or brackets.

2. Step 2: Analyze options

   var shape: Drawable correctly declares a variable of type Drawable. Options A and B incorrectly try to instantiate a protocol, which is not allowed. var shape: Drawable[] uses invalid syntax.

3. Final Answer:

   var shape: Drawable -> Option A

4. Quick Check:

   Protocol type variable = var name: ProtocolName [OK]

Hint: Declare protocol type without parentheses or initialization [OK]

Common Mistakes:

• Trying to instantiate a protocol directly
• Using parentheses after protocol name
• Using array brackets incorrectly

3. What will be the output of this Swift code?

protocol Greetable {
    func greet() -> String
}

struct Person: Greetable {
    func greet() -> String {
        "Hello"
    }
}

let greeter: Greetable = Person()
print(greeter.greet())

medium

A. greet()

B. Compile error

C. Person

D. Hello

Solution

1. Step 1: Understand protocol conformance

   Person struct conforms to Greetable and implements greet() returning "Hello".

2. Step 2: Using protocol as type and calling method

   Variable greeter is of type Greetable but holds a Person instance, so calling greet() returns "Hello".

3. Final Answer:

   Hello -> Option D

4. Quick Check:

   Protocol method call returns implemented string = Hello [OK]

Hint: Protocol variable calls conforming type's method [OK]

Common Mistakes:

• Expecting protocol name or method name as output
• Thinking protocol cannot be used as variable type
• Confusing compile error with runtime behavior

4. Identify the error in this Swift code using protocol as a type:

protocol Vehicle {
    func start()
}

class Car: Vehicle {
    func start() {
        print("Car started")
    }
}

var myVehicle: Vehicle = Vehicle()
myVehicle.start()

medium

A. Variable myVehicle should be a class, not protocol

B. Missing method implementation in Car

C. Cannot instantiate protocol directly with Vehicle()

D. start() method should return a value

Solution

1. Step 1: Check protocol instantiation

   Protocols cannot be instantiated directly, so Vehicle() causes an error.

2. Step 2: Analyze variable assignment

   Variable myVehicle must hold an instance of a type conforming to Vehicle, like Car(), not Vehicle() itself.

3. Final Answer:

   Cannot instantiate protocol directly with Vehicle() -> Option C

4. Quick Check:

   Protocols can't be created with () = Error [OK]

Hint: Protocols can't be instantiated directly [OK]

Common Mistakes:

• Trying to create an instance of a protocol
• Confusing protocol with class instantiation
• Ignoring method return types

5. Given the protocol and classes below, which code correctly stores different types conforming to Animal in an array and calls sound() on each?

protocol Animal {
    func sound() -> String
}

class Dog: Animal {
    func sound() -> String { "Bark" }
}

class Cat: Animal {
    func sound() -> String { "Meow" }
}

hard

A. let animals: [Animal] = [Dog(), Cat()]
for animal in animals {
print(animal.sound)
}

B. let animals: [Animal] = [Dog(), Cat()]
for animal in animals {
print(animal.sound())
}

C. let animals: [Dog] = [Dog(), Cat()]
for animal in animals {
print(animal.sound())
}

D. let animals = [Dog(), Cat()]
for animal in animals {
print(animal.sound())
}

Solution

1. Step 1: Understand protocol array usage

   To store different types conforming to Animal, declare array as [Animal].

2. Step 2: Call method correctly

   Use animal.sound() with parentheses to call the method. let animals: [Animal] = [Dog(), Cat()] for animal in animals { print(animal.sound()) } does this correctly.

3. Final Answer:

   let animals: [Animal] = [Dog(), Cat()] for animal in animals { print(animal.sound()) } -> Option B

4. Quick Check:

   Protocol array with method calls = let animals: [Animal] = [Dog(), Cat()] for animal in animals { print(animal.sound()) } [OK]

Hint: Use protocol array type and call methods with () [OK]

Common Mistakes:

• Using concrete type array instead of protocol type
• Calling method without parentheses
• Mixing incompatible types in array