Main entry point and @main attribute in Swift - Step-by-Step Execution

Concept Flow - Main entry point and @main attribute

Program Start

↓

@main Struct or Class

↓

Call static main()

↓

Execute main() code

↓

Program Ends

Swift looks for a type marked with @main and calls its static main() method to start the program.

Execution Sample

Swift

import Foundation

@main
struct MyApp {
    static func main() {
        print("Hello, Swift!")
    }
}

This program uses @main on MyApp struct; Swift calls MyApp.main() to print a greeting.

Execution Table

Step	Action	Evaluation	Result
1	Program starts	Find @main type	MyApp struct found
2	Call MyApp.main()	Execute main() method	Prints "Hello, Swift!"
3	main() completes	No more code	Program ends

💡 Program ends after main() finishes execution

Variable Tracker

Variable	Start	After Step 2	Final
Output	""	"Hello, Swift!\n"	"Hello, Swift!\n"

Key Moments - 3 Insights

Why does Swift look for a static main() method inside the @main type?

Because the @main attribute tells Swift this is the program's entry point, so it calls the static main() method to start execution (see execution_table step 2).

Can we have multiple @main types in one Swift program?

No, only one @main type is allowed because Swift needs a single entry point to start the program (execution_table step 1).

What happens if main() does not exist inside the @main type?

The program will not compile because Swift requires a static main() method in the @main type to know where to start (refer to concept_flow).

Visual Quiz - 3 Questions

Look at the execution table, what is printed during step 2?

A "Program starts"

B "Hello, Swift!"

C "MyApp struct found"

D "Program ends"

💡 Hint

Check the 'Result' column in execution_table row for step 2.

At which step does the program find the @main type?

A Step 3

B Step 2

C Step 1

D Program never finds it

💡 Hint

Look at the 'Action' and 'Evaluation' columns in execution_table step 1.

If the static main() method was missing, what would happen?

A Compilation error occurs

B Program prints nothing but runs

C Program runs normally

D Program ends immediately

💡 Hint

Refer to key_moments about missing main() method.

Concept Snapshot

@main attribute marks the program's entry point.
Swift calls the static main() method inside the @main type.
Only one @main type allowed per program.
main() contains code that runs first.
Program ends when main() finishes.

Full Transcript

In Swift, the program starts by looking for a type marked with @main. This attribute tells Swift where the program's entry point is. Swift then calls the static main() method inside that type. The main() method contains the code that runs first. For example, if main() prints "Hello, Swift!", that message appears when the program runs. The program ends when main() finishes. Only one @main type is allowed, and it must have a static main() method. If main() is missing, the program will not compile.