What is the proper declaration of main?
What is the proper signature of the main function in C++? What is the correct return type, and what does it mean to return a value from main? What are the allowed parameter types, and what are their meanings?
Is this system-specific? Have those rules changed over time? What happens if I violate them?
Answer
The main function must be declared as a non-member function in the global namespace. This means that it cannot be a static or non-static member function of a class, nor can it be placed in a namespace (even the unnamed namespace).
The name main is not reserved in C++ except as a function in the global namespace. You are free to declare other entities named main, including among other things, classes, variables, enumerations, member functions, and non-member functions not in the global namespace.
You can declare a function named main as a member function or in a namespace, but such a function would not be the main function that designates where the program starts.
The main function cannot be declared as static or inline. It also cannot be overloaded; there can be only one function named main in the global namespace.
The main function cannot be used in your program: you are not allowed to call the main function from anywhere in your code, nor are you allowed to take its address.
The return type of main must be int. No other return type is allowed (this rule is in bold because it is very common to see incorrect programs that declare main with a return type of void; this is probably the most frequently violated rule concerning the main function).
There are two declarations of main that must be allowed:
int main() // (1)
int main(int, char*[]) // (2)
In (1), there are no parameters.
In (2), there are two parameters and they are conventionally named argc and argv, respectively. argv is a pointer to an array of C strings representing the arguments to the program. argc is the number of arguments in the argv array.
Usually, argv[0] contains the name of the program, but this is not always the case. argv[argc] is guaranteed to be a null pointer.
Note that since an array type argument (like char*[]) is really just a pointer type argument in disguise, the following two are both valid ways to write (2) and they both mean exactly the same thing:
int main(int argc, char* argv[])
int main(int argc, char** argv)
Some implementations may allow other types and numbers of parameters; you'd have to check the documentation of your implementation to see what it supports.
main() is expected to return zero to indicate success and non-zero to indicate failure. You are not required to explicitly write a return statement in main(): if you let main() return without an explicit return statement, it's the same as if you had written return 0;. The following two main() functions have the same behavior:
int main() { }
int main() { return 0; }
There are two macros, EXIT_SUCCESS and EXIT_FAILURE, defined in <cstdlib> that can also be returned from main() to indicate success and failure, respectively.
The value returned by main() is passed to the exit() function, which terminates the program.
Note that all of this applies only when compiling for a hosted environment (informally, an environment where you have a full standard library and there's an OS running your program). It is also possible to compile a C++ program for a freestanding environment (for example, some types of embedded systems), in which case startup and termination are wholly implementation-defined and a main() function may not even be required. If you're writing C++ for a modern desktop OS, though, you're compiling for a hosted environment.