Programming Languages - Main

main is the entry point for many languages:

• either a main() function or in other forms, like the __main__ block in Python.
• main() function:
  • commonly has no returns, but returns an int in C/C++.
  • some with special requirements, e.g. must be in a class in Java, must be in package main in go.

By Language

C++

main() returns an int to indicate exit status; 0 if there's no error.

int main(int argc, char** argv) {
  // ...
  return 0;
}

• argc: argument count
• argv: argument vector

Java

Java code must live in a class.

There's no return value from main(), use System.exit(int status) in case of errors.

class Foo {
  public static void main(String args[]) {
    // ...
  }
}

Kotlin

Similar to Java, however it does not have to be in a class, the compiler will wrap it in a class com.example.HelloWorldKt for a file HelloWorld.kt.

package com.example

fun main() {
    println("Hello, World!")
}

Python

if __name__ == "__main__":
  # running directly
else:
  # imported

Rust

env::args() provides access to raw command-line arguments.

use std::env

fn main(){
  for argument in env::args() {
    println!("{}", argument);
  }
}

Go

In Go, main() function must be in the main package.

There's no parameters passed to main(); os.Args provides access to raw command-line arguments.

package main

import (
    "os"
)

func main() {
  args := os.Args
  // ...
}

JavaScript

JavaScript does not have a main function. The entry point is the beginning of the code.