function declaration without definition

Question

The following code will compile and is deterministic according to cppquiz.org (Question #30)

#include <iostream>
struct X {
  X() { std::cout << "X"; }
};

int main() { X x(); }

The output of the program is nothing, as

X x();

is a function declaration.

But still I wonder why this compiles though this declaration is never defined anywhere?

Answer 1

Since x() is never called, there's nothing to link so no error from linker that it's not defined. It's only declared as a function taking no arguments and returning an X: X x();.

Answer 2

X x(); itself a declaration (prototype), not a function call. If a function call is made prior to seeing its declaration then it would not compile.