static member variable when declared private

Question

When a static member variable is declared private in a class, how can it be defined?

Suppose i have the following class declaration

class static_demo
{
   private:
      static int a;

   public:
      static int b;

      void set(int x, int y)
      {
         a = x;
         b = y;
      }

      void show()
      {
         cout << "a = " << a << "\n";
         cout << "b = " << b << "\n";
      }
};

Then the following statement to define a will result in compilation error.

int static_demo::a;

So is it possible to have a static data member in the private section of class?

Adding full code as per Greg ,

#include <iostream>

using namespace std;

class static_demo
{
   private:
      static int a;

   public:
      static int b;

      void set(int x, int y)
      {
         a = x;
         b = y;
      }
};

int static_demo::a;
int static_demo::b;

int main()
{
   static_demo::b = 10;
   static_demo::a = 20;

   return 0;
}

The compilation error is:

static_member_variable.cpp: In function `int main()':
static_member_variable.cpp:20: error: `int static_demo::a' is private
static_member_variable.cpp:26: error: within this context

Answer 1

When a static member variable is declared private in a class, how can it be defined?

In the very same way as you define a public static variable in your source(cpp) file.

int static_demo::a = 1;

Access specifiers will not give you error while defining the member. Access specifiers control the access of the member variables, defining a static variable is an excpetion which is allowed.

Compiles cleanly on Ideone here .

EDIT: To answer your Q after you posted code.
Your problem is not the definition of the static member. The error is because you are trying to access the private static member inside main . You cannot do that.

Private members of a class can only be accessed inside the class member functions, the same rule applies even to static members. To be able to modify/access your static members you will have to add a member function to your class and then modify/access the static member inside it.

Answer 2

in your .cpp:

int static_demo::a(0);

when this causes an error, it is most common that you either encountered a linker error for a multiply defined symbol (eg the definition was in the header), or that you may have tried to define it in the wrong scope. that is, if static_demo is in the namespace MON , it would be defined in the .cpp:

#include "static_demo.hpp"
int MON::static_demo::a(0);

for other types, it may simply have been an incorrect constructor.

Answer 3

The problem is not the definition, but the fact that in main() (that's not in the name scope of static_demo, and cannot see a being private), you do an assignment .

The definition of a is what you did at global scope, with int static_demo::a; . You simply need an initializer, if you want a not to start with an undefined value.

int static_demo::a = 1;

will solve the problem.

From than on, a can be manipulated only by functions in static_demo (of friend of it).