Why does int main() {} compile?

Question

(I'm using Visual C++ 2008) I've always heard that main() is required to return an integer, but here I didn't put in return 0; and and it compiled with 0 errors and 0 warnings! In the debug window it says the program has exited with code 0. If this function is named anything other than main(), the compiler complains saying 'blah' must return a value. Sticking a return; also causes the error to appear. But leaving it out completely, it compiles just fine.

#include <iostream>
using namespace std;

int main()
{
    cout << "Hey look I'm supposed to return an int but I'm not gonna!\n";
}

Could this be a bug in VC++?

Answer 1

I'm pretty sure VC++ just inserts a return 0 if you don't include one in main functions. The same thing can happen with functions too, but in those cases at least you'll get a warning.

Answer 2

This is part of the C++ language standard. An implicit return 0 is generated for you if there's no explicit return statement in main.

Answer 3

Microsoft has always had it's own implementations of things. That's the way they roll. If you were to use the GNU C++ library instead, I'm sure that would trigger an error. I haven't done C++ for quite some time now, but I'm sure Dev-C++ used to trigger an error for these sort of things.

Answer 4

3.6.1 Main function

....

2 An implementation shall not predefine the main function. This function shall not be overloaded. It shall have a return type of type int, but otherwise its type is implementation-defined. All implementations shall allow both of the following definitions of main:

int main() { /* ... */ }

and

int main(int argc, char* argv[]) { /* ... */ }

.... and it continues to add ...

5 A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling exit with the return value as the argument. If control reaches the end of main without encountering a return statement, the effect is that of executing return 0;

attempting to find an online copy of the C++ standard so I could quote this passage I found a blog post that quotes all the right bits better than I could.

Answer 5

Section 6.6.3/2 states- "Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.".

An example is the code below which at best gives warning on VS 2010/g++

int f(){
   if(0){
      if(1)
         return true;
   }
}

int main(){
   f();
}

So the whole point is that 'main' is special as the previous responses have pointed out.