Private static member - compilation error?

Question

I am following simple examples to build up my understanding on private static members. However, I am getting compilation errors.

Here is the example, I am following -

https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.1.0/com.ibm.zos.v2r1.cbclx01/cplr038.htm

have changed the c++ version to see if it something to do with c++ version.

class test1
{
  static int i;
  static int j;
  static int k;
  static int f() { return 0;}
  int p;

public:
   test1()
   {
     p = 0;
   }
};

int main(void)
{
  test1 t;
  int test1::i = test1::f();
  int test1::j = test1::i;
  return 0;
}

Compiling this code -

Why does it fail here whereas the example link says it works.

$g++ -std=c++14  t1.cpp -o t1.out
t1.cpp: In function ‘int main()’:
t1.cpp:19:16: error: qualified-id in declaration before ‘=’ token
   int test1::i = test1::f();
                ^
t1.cpp:20:16: error: qualified-id in declaration before ‘=’ token
   int test1::j = test1::i;

It works if I do like this -

class test1
{
  static int i;
  static int j;
  static int k;
  static int f() { return 0;}
  int p;

public:
   test1()
   {
     p = 0;
   }
};

int test1::i = test1::f();
int test1::j = test1::i;

int main(void)
{
  test1 t;
  return 0;
}

Why it has to be defined outside of main() ? It works with private also. However, it cannot be defined inside main function. My understanding is that static int inside class is just a declaration. It needs to be defined somewhere. I am defining it within main() but it doesn't work. It needs to be global and I don't know why it needs to be global? It has nothing to do with private/public in my opinion. Please clarify my doubt here.

Answer 1

Why it has to be defined outside of main() ? It works with private also. However, it cannot be defined inside main function. My understanding is that static int inside class is just a declaration. It needs to be defined somewhere. I am defining it within main() but it doesn't work. It needs to be global and I don't know why it needs to be global? It has nothing to do with private/public in my opinion. Please clarify my doubt here.

Yes, the static int k; inside the class definition is indeed just a declaration of the static data member and not a definition. And yes, you need to provide a definition of the static data member somewhere. That somewhere cannot be inside the main function, or any function for that matter. And yes, it has nothing to do with private vs public. This is just about where a definition of a static data member can occur. Definitions of static data members (that are not defined inline) have to reside in a namespace scope enclosing the class definition. Because the rules of the C++ language say so (specifically, in [class.static.data]/2). The global namespace is one such place where a static data member can be defined…