Why can static member functions only be called at global scope if they have a return value?

Question

I found a peculiar thing: static member functions of a class/struct cannot be called a global scope, unless they have a return value.

This program does not compile:

struct test
{
  static void dostuff()
  {
      std::cout << "dostuff was called." << std::endl;
  }
};

test::dostuff();

int main()
{  
   return 0;
}

Giving us the following under GCC v4.8.3:

main.cpp:12:16: error: expected constructor, destructor, or type conversion before ';' token
test::dostuff();
               ^

However, by adding a return value to dostuff() and assigning it to a global variable, the program compiles and works as intended:

struct test
{
  static int dostuff()
  {
      std::cout << "dostuff was called." << std::endl;
      return 0;
  }
};

int i = test::dostuff();

int main()
{
   return 0;
}

This yields the expected output:

dostuff was called.

Can anyone explain to me why this is the case and if there is a work-around that doesn't involve creating global variables?

Answer 1

You cannot call functions at global scope because, from [basic.link]

A program consists of one or more translation units (Clause 2) linked together. A translation unit consists of a sequence of declarations.

test::dostuff(); is not a declaration - so you cannot have it as a standalone function call (the fact that it's a static member function is irrelevant - could be a free function, a non-static member function called on a global object, etc.). On the other hand, int i = test::dostuff(); is a declaration: of a variable i of type int at global scope. That's why it is allowed. Note that it's not the fact that dostuff() has a return value that is relevant - it's the fact that you're declaring a variable with that return value.