C++ class & friend visual studio link error

Question

 #include <iostream>

    using namespace std;

    class A
    { 
        static int x; 
        int y; 

     private: 
        friend void f(A &a); 

     public: 
     A() 
     { 
          x=y=5; 
     } 

     A(int xx,int yy) 
     { 
        x=xx; 
        y=yy; 
     } 

     //static void getVals(int &xx, int &yy); 
     void getVals(int *xx, int *yy) 
     { 
        *xx=x; 
        *yy=y; 
     } 

     void f(A &a) 
     { 
        int x,y; 
        a.getVals(&x,&y); 
        cout << a.x << "; " <<a.y << endl; 
      } 
    }; 

    int main() 
    { 
        A a1; 
        A a2(3,8); 

        f(a1); 
        f(a2); 

        return 0;
    } 

I got 2 link errors with visual studio:

Error 1 error LNK2019: unresolved external symbol "void __cdecl f(class A &)" (?f@@YAXAAVA@@@Z) referenced in function _main

Error 2 error LNK2001: unresolved external symbol "private: static int A::x" (?x@A@@0HA)

please help to resolve these errors

Answer 1

Static member variables exists once and are shared between the objects of the class. Because static member variables are not part of the individual objects, you must explicitly define the static member. Usually the explicitly definition is placed in the source file (cpp) of the class:

header file:

class A
{ 
  static int x;
};

source file:

int A::x = 0; // <- explicitly definition and initialization