В ответ на:

#include <iostream>
using namespace std;
class A
{
protected:
// static double a;
  double a;
public:
 void set(const double i){this->a=i;}
 double* get(){return &(this->a);}
};
class B:public A
{
protected:
  static double a;
public:
};
class C:public A
{
protected:
  static double a;
public:
};
//double A::a= (double) 1;
double B::a= (double) 2;
double C::a= (double) 3;
B b_arr[5];
C c_arr[5];
int main ()
{
  for(int k=0;k<5;++k)
    {
      b_arr[k].set((double) 4);
//      c_arr[k].set((double) 5);
      printf("%d       C::a = %p  %2.0f     B::a = %p %2.0f \n",
         i,
         (c_arr[k].get()), *(c_arr[k].get()),
         (b_arr[k].get()), *(b_arr[k].get())
            );
    }
  return 1;
}
0       C::a = 0x8049ac0   0     B::a = 0x8049a80  4
1       C::a = 0x8049ac8   0     B::a = 0x8049a88  4
2       C::a = 0x8049ad0   0     B::a = 0x8049a90  4
3       C::a = 0x8049ad8   0     B::a = 0x8049a98  4
4       C::a = 0x8049ae0   0     B::a = 0x8049aa0  4

a nado tak:
0 C::a = 0x8049ac0 3 B::a = 0x8049a80 4
1 C::a = 0x8049ac0 3 B::a = 0x8049a80 4
2 C::a = 0x8049ac0 3 B::a = 0x8049a80 4
3 C::a = 0x8049ac0 3 B::a = 0x8049a80 4
4 C::a = 0x8049ac0 3 B::a = 0x8049a80 4