Is there a Necessity for private static Methods?

问题

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?

回答1:

Member functions have access to all private members of the class. If a function needs access to these members, it should be a member. This applies whether or not it's static.

A static function is one that doesn't operate on a specific object. However, it can still receive objects as parameters. For example:

class A
{
    int m;

    static int process_3_objects_in_some_way(A x, A y, A z)
    {
        return x.m + y.m + z.m;
    }
}

Another reason to make a function static is the order of parameters. For example, print:

class A
{
    int m;

    static void print(std::ostream& stream, A x, int options)
    {
        stream << "Value is: " << (x.m * 1000000 + options);
    }
}

回答2:

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.

回答3:

friend functions or classes which are implemented in another implementation file are another example where a private static member function is required.

来源：https://stackoverflow.com/questions/41964745/is-there-a-necessity-for-private-static-methods

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c++

function

methods

static

private