c++ - Declare static functions as friend function? -

i have class myclass declaration in header file interface.h , static functions (foo , bar , few more) in file1.cpp. static functions used inside file1.cpp need modify private/protected members of myclass`.

// in "interface.h" class myclass { // maybe declare friend? // friend static void foo(myclass &ref); private:     double someval; }  // in "file1.cpp" static void foo(myclass &ref) {     ref.someval = 41.0; } static void bar(myclass &ref) {     ref.someval = 0.42; }  // function uses foo/bar void dosomething(myclass &ref) {     foo(ref);            } 

idea 1: somehow declare them friends of myclass?

why not good: static , in different compilation unit. besides expose them user of myclass not need know them.

idea 2: don't have idea 2.

sort of linked: is possible declare friend function static?

sort of linked: possible declare friend function static?

personally find whole friend thing bit of hack breaks encapsulation you've asked valid question , answer can achieve want helper class:

file1.h

class myclass { private:   double someval;    friend class myclasshelper; }; 

file1.cpp

#include "file1.h"   struct myclasshelper {   static void mutatemyclass(myclass& ref) {     ref.someval=42;   } };   // in "file1.cpp" static void foo(myclass &ref) {   myclasshelper::mutatemyclass(ref); } 

are sure want this? sure don't want encapsulate myclass's mutators inside myclass itself?