Why does my C++ function, only when it's placed after main(), not work?

Question

I recently picked up C++ and decided to try out making a function. However, I've run into a problem with my function func() where, even if declared beforehand, it only works if it's placed before the main() function.

If I place it after the main() function, the system tells me there is "no matching function for call to func".

Note: the functionfunc2 on the other hand works even if placed before or after the main() function.

So here's the code :

#include <stdio.h>
#include <iostream>

void func2();

int func();

int main()
{
  int y=2;

  std :: cout << "Hello World\n" << func(y) << "\n";
  func2();
  return 0;
}

int func(int x)
{
 x *= 2;
 return x;
}

void func2()
{
 std :: cout << "Hello there";
}

Answer 1

In C language, the declaration int func(); means a function with an unspecified number of arguments of any type, returning a int.

In C++ language, the same declaration int func(); means a function without any arguments, returning a int.

And therefore, in C++, the definition of func with an argument of type int is an overload. For the compiler, it is a different function, which in the original code is not declared before use, so an error is emitted.

But in C, it would be perfectly legal.