问如何在*.cpp文件中实现静态类成员函数？

EN

class A {
public:
    static int a(int i);
};

#include <iostream>
#include "test.hpp"


int A::a(int i) {
    return i + 2;
}

using namespace std;
int main() {
    cout << A::a(4) << endl;
}

class CFoo
{
public: 
    static bool IsThisThingOn();
};

#include "header.hxx"
bool CFoo::IsThisThingOn() // note: no static keyword here
{
    return true;
}

class helper
{
 public: 
   static void fn1 () 
   { /* defined in header itself */ }

   /* fn2 defined in src file helper.cxx */
   static void fn2(); 
};

#include "helper.hxx"
void helper::fn2()
{
  /* fn2 defined in helper.cxx */
  /* do something */
}

#include "helper.hxx"
A::foo() {
  helper::fn1(); 
  helper::fn2();
}