How can I redifine int main()

Answer 1

What is the need to redefine main() function? it is the entry point for console programs, it is meant to be uniquely identified.

Answer 2

#define hello() main()

#include <iostream>
using namespace std;

int hello() {
cout << "Hello!";
return 0;
}

This works, but completely pointless and confusing for anybody who would read your code.