Is there a Necessity for private static Methods?

Question

The Principle Engineer at my last company had a rule that private static methods should be implemented as functions in the implementation file, not as class methods.

I don't remember if there were any exceptions to his rule. I have stumbled into the motivation for it at my current job: If the arguments or return type of the function in question are objects that would require the inclusion of a definition file in the header, this can cause unnecessary difficulties.

That's enough to steer me away from ever using a private static method again, but before I wrote them off I wanted to know if anyone is aware of a niche they fill that an implementation file function would not?

EDIT:

An example may be helpful here. Say this is the start of the declaration of class Foo, which has other methods which will call void foo() in the implementation file:

class Foo {
    static void foo();

So foo is only accessible by other methods of Foo. Why wouldn't I just define foo in the implementation file, and keep it out of the header all together?

Answer 1

Unlike free-standing static functions in the implementation file, private static member functions can be used in the header of the class. This is important in situations when you want to inline a non-private member function, which calls your private static function:

class Demo {
private:
    static std::string sanitize(const std::string& name);
    std::string name;
public:
    Demo(const std::string& n) : name(sanitize(n)) {
    }
};

Doing the same with free-standing static functions would require implementing Demo's constructor in the cpp file.