Can the main (or entry-point) function be implemented as a lambda?

Question

Is this valid under the recently updated standard?

auto main = [](int argc, char* argv[]) -> int
{
    return 0;
};

My best guess is that it depends on whether main() MUST be a function, or if it is allowed to be any globally scoped symbol that is callable (with ()).

Answer 1

No, main is required to be a global function and cannot be a function object or anything else. See ISO/IEC 14882:2011 § 3.6.1 Main Function.

A program shall contain a global function called main, which is the designated start of the program.

And from paragraph 2

All implementations shall allow both of the following definitions of main:

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

and

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

There is no requirement for implementations to allow any other definitions.

Answer 2

No, and here's why:

[n3290: 3.6.1/1]: A program shall contain a global function called main, which is the designated start of the program. It is implementation-defined whether a program in a freestanding environment is required to define a main function. [ Note: In a freestanding environment, start-up and termination is implementation-defined; startup contains the execution of constructors for objects of namespace scope with static storage duration; termination contains the execution of destructors for objects with static storage duration. —end note ]

Lambdas are not functions, but function objects or functors:

[n3290: 5.1.2/3]: The type of the lambda-expression (which is also the type of the closure object) is a unique, unnamed nonunion class type — called the closure type — whose properties are described below. [..]

Answer 3

main() must be a function because of the way it's called from within with the system libraries . It is part of the POSIX.1 standard and governs the way C linkage works

The main linkage has to be an extern global, it cannot be inlined or made static because it's called from within the libc and typically from a function called _start.

As an example, typical implementation of _start in glibc is:

int _start() {
     __libc_init(argc, argv, __environ);
     exit(main(argc, argv, __environ));
}

Various libc implementations will do it in a similar fashion.

In C++ the main function must be declared in the global scope (i.e.) ::main(); again because it is called from an init-like function such as _start for libc on *nix function above after execution...