Thread: Help.....can't even get started. "Multiple definitions of 'main' " error

Im trying to learn C++ for my next year at Uni, started using codeblocks to write in C and so was happy to keep using it for C++, but Im having an issue that I can't seem to find an answer to.

I bought the book, "Jumping into C++" and followed the instructions, opened up a new project and basically started from scratch again. Hello world works great! However when I build the second program, it comes up with an error telling me that there are multiple definitions of main, and that it's first defined in line 5 of hello.cpp

Ive tried renaming files but I havent got a clue what Im doing and I have no idea how to fix this......

hello.cpp

Code:

#include <iostream>using namespace std;


int main()
{
    cout << "Hello World: \n";
}

readnum.cpp

Code:

#include <iostream>
using namespace std;


int main()


{
    int number;


    cout << "Please enter a number: ";
    cin >> number;
    cout << "You entered: " << number << "\n";
}

Please help...

The following explanation is probably an oversimplification and not 100% correct, but it will do to get the point across. When you compile and link your program, the linker must figure out where control flow begins in your program (i.e. where to start running). It does that by looking for a function named main. Acceptable function signatures (to my knowledge) are:

int main();

and

int main( int argc, char** argv );

Anyway, if your program has multiple functions named main, the linker won't know which one is supposed to be your program entry point, so it just quits and dies.

Originally Posted by Elkvis

as I understand it, main is not required to have any particular signature apart from its name and return type. section 3.6.1.2 of the standard says that the signature of main is implementation-defined. this is why windows and apple are allowed to add extra parameters. it must be called main, and must return an int, but the standard does not specify what arguments it takes, with the following exceptions: it specifies that all implementations must allow int main(void) and int main(int argc, char** argv).

Yeah, you're right:

3.6.1 Main function
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:

Code:

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

and

Code:

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

Well, I did say my explanation likely wasn't going to be 100% correct.