if I define a function in the implementation (.cpp) file without defining it in the header file what happens?

Question

I'm a C++ noob. I have a foo.cpp file which defines a function,

void sort() {
   // details of sort algorithm
}

I have not defined this in the corresponding header file, but I get no compilation error. I noticed this is taken as a static method. Is that the case? What exactly have I done here?

EDIT:

I've noticed that inside this method I can't use this, (ex: this->bar), hence my assumption that this is a static function. This actually happened when I wanted a 'private' method, but forgot to declare it in the header file. Is there an actual use of such a function?

Answer 1

It is not invisible outside of that compilation unit, it is not bound to a single compilation unit, it is just a definition that needs a declaration in order to be called. Like any other definition.

wtf.cpp:

#include <iostream>
void f(){ std::cout<< "f();"; }

omg.cpp:

void f(); // I can declare wherever I use it.
int main(){ f(); }

$ g++ wtf.cpp omg.cpp && ./a.out

Output: f();

Answer 2

No, it is not taken as a static function. It is just not visible to the other transllation unnits, and so you cannot use it.

The difference in regard to a static method is, that the linker sees the function. So if you define a function with the same name in another translation unit, you will probably either get a linker error (best case) or the wrong method called at times.

PS: I talk about a functions rather than methods here, because in C++ a method typically is part of a class, and you cannot define a method for a class if that method isn't declared in that class' declaration.