Protocol inheritance in Swift - Step-by-Step Execution

Concept Flow - Protocol inheritance

Define Base Protocol

↓

Define Derived Protocol: inherits Base

↓

Create Struct/Class: conforms to Derived

↓

Implement all requirements from Base + Derived

↓

Use instance as Derived or Base Protocol

Protocol inheritance means one protocol can require everything from another, so a type conforming to the child protocol must implement all requirements from both.

Execution Sample

Swift

protocol A {
    func foo()
}

protocol B: A {
    func bar()
}

struct S: B {
    func foo() { print("foo") }
    func bar() { print("bar") }
}

let s = S()
s.foo()
s.bar()

Defines protocol A, then B inherits A. Struct S conforms to B, so it must implement foo() and bar(). Then calls both methods.

Execution Table

Step	Action	Evaluation	Result
1	Define protocol A with foo()	No output	Protocol A ready
2	Define protocol B inheriting A, adds bar()	No output	Protocol B ready (inherits foo())
3	Define struct S conforming to B	No output	S must implement foo() and bar()
4	Implement foo() in S	Prints 'foo' when called	foo() implemented
5	Implement bar() in S	Prints 'bar' when called	bar() implemented
6	Create instance s of S	No output	Instance s created
7	Call s.foo()	Calls foo()	Output: foo
8	Call s.bar()	Calls bar()	Output: bar
9	End of program	No more actions	Execution stops

💡 Program ends after calling both foo() and bar() on instance s

Variable Tracker

Variable	Start	After 6	After 7	After 8	Final
s	undefined	instance of S	instance of S	instance of S	instance of S

Key Moments - 3 Insights

Why does struct S need to implement foo() even though it only says it conforms to protocol B?

Because protocol B inherits protocol A, it requires all methods from A too. So S must implement foo() from A and bar() from B, as shown in steps 3-5 of the execution_table.

What happens if S does not implement foo()?

The code will not compile because S does not fulfill all requirements of protocol B, which includes foo() from protocol A. This is implied in step 3 where S must implement both methods.

Can we use instance s as type A or B?

Yes, since S conforms to B which inherits A, s can be used as either protocol type. This is the purpose of protocol inheritance shown in the flow.

Visual Quiz - 3 Questions

Look at the execution_table, what is printed when s.foo() is called at step 7?

A "foo"

B "bar"

C Nothing

D Error

💡 Hint

Check row 7 in execution_table where s.foo() is called and output is "foo"

At which step does struct S become required to implement both foo() and bar()?

A Step 2

B Step 5

C Step 3

D Step 6

💡 Hint

Look at step 3 in execution_table where S is defined conforming to B and must implement both methods

If protocol B did not inherit protocol A, what would change in the execution_table?

A S would need to implement foo() and bar() anyway

B S would only need to implement bar(), not foo()

C S could implement neither method

D Program would not compile

💡 Hint

Protocol inheritance means B requires A's methods; without it, only B's own methods are required (see concept_flow)

Concept Snapshot

Protocol inheritance syntax:
protocol B: A { ... }
Means B includes all requirements of A.
A type conforming to B must implement all methods from A and B.
Use inheritance to build on protocols.
Allows flexible, reusable interfaces.

Full Transcript

This visual trace shows how protocol inheritance works in Swift. First, protocol A is defined with a method foo(). Then protocol B inherits A and adds method bar(). A struct S conforms to B, so it must implement both foo() and bar(). The execution table walks through defining protocols, struct, implementing methods, creating an instance, and calling methods. The variable tracker shows the instance s is created and used. Key moments clarify why S must implement foo() even though it only says it conforms to B, because B inherits A. The quiz questions check understanding of method calls, implementation requirements, and effects of inheritance. The snapshot summarizes the syntax and behavior of protocol inheritance. This helps beginners see step-by-step how protocol inheritance requires implementing all inherited methods.