This is a very basic question. In int main() why is it a int and not a void and why do you have to return 0.

#include <iostream> // The # represens a preprocessor directive and <iostream is a header file>

int main() 
{  
    //std stands for standard
    std::cout << "You are a secret agent breaking into a secure server room...";
    std::cout << std::endl; //endl means end line
    std::cout << "You need to enter the correct codes to continue...";

    const int a = 4;
    const int b = 3;
    const int c = 2;


    const int sum = a + b + c;
    const int product = a * b * c;


    std::cout << std::endl;
    std::cout << sum << std::endl;
    std::cout << product << std::endl;

return 0;

    //use std::cout << std::endl for new line
}

Don't mind the comments. I am taking a udemy class and it says you have to return 0. Why did they say that. Thanks for the help.

The main return value is the exit code of the program:

%  cat x.cc
int main() {
    return 21;
}

# Compile and run it
%  g++ -o x -Wall x.cc
%  ./x

# Ask the shell what exit code the program gave
%  echo $?
21

The ‘diff’ program for example uses that to communicate what the compare result was, from its manual “diff(1)”:

Exit status is 0 if inputs are the same, 1 if different, 2 if trouble

The standard says this runs through std::exit:

https://en.cppreference.com/w/cpp/language/main_function​​ (property number 5)

https://en.cppreference.com/w/cpp/utility/program/exit