Declaring main as friend considered harmful?

Question

Discussion

I know that main can be a friend of a class:

#include <iostream>

class foo {
  friend int main();
  int i = 4;
};

int main() {
  foo obj;
  std::cout << obj.i << std::endl;
}

LIVE DEMO

However, I feel that although this is perfectably allowable it conceals many dangers.

Questions

1. Are there any valuable uses in making main a friend of a class?
2. Are there any reasons that declaring main as friend of a class should be considered harmful?

Answer 1

The choice whether to use or avoid a legal feature becomes moot if the feature is not, in fact, legal. I believe there's serious doubt surrounding this, because the Standard says

The function main shall not be used within a program.

There's already a question regarding whether befriending ::main() is in fact allowed, and you'll find more details in my answer there.

Answer 2

The general frienship considerations should be identical as for any other functions.

---

However I see one possible danger:

C++ Standard :

• Section § 11.3 (Friends)

A function first declared in a friend declaration has external linkage

• Section § 3.6.1 (Main Function)

The linkage of main is implementation-defined

So if your implementation expects main() not to have external linkage and you first declare main() as a friend (as in your example), you contradict the standard.