Are inner functions automatically declared in C++?

Consider a file functions.cpp:

// functions.cpp

void f1(){/* do something */}

void f2()
{
    f1();
}

I wish to use the function f2 is a different cpp file, so I create a header file functions.h:

// functions.h

#ifndefine F2
#define F2

void f2();

#endif

Now, the main.cpp file is where I call f2:

// main.cpp

#include "functions.h"

int main()
{
    f2();
    return 0;
}

Compiling this works fine, but I haven't declared f1 in functions.h. So when f2 is called, how does the compiler know about f1?