Initialize static member field outside the class declaration : static « Language Basics « C++ Tutorial

1. C++ Tutorial
2. Language Basics
3. static

#include <iostream>
using std::cout;
using std::endl;

class Box {
  public:
    Box() {
      cout << "Default constructor called" << endl;
      ++objectCount;
      length = width = height = 1.0;
    }

    Box(double lvalue, double wvalue, double hvalue) :
                              length(lvalue), width(wvalue), height(hvalue) {
      cout << "Box constructor called" << endl;
      ++objectCount;
    }

    double volume() const {
      return length * width * height;
    }


    int getObjectCount() const {return objectCount;}

  private:
    static int objectCount;
    double length;
    double width;
    double height;
};
int Box::objectCount = 0;

int main() {
  cout << endl;

  Box firstBox(17.0, 11.0, 5.0);
  cout << "Object count is " << firstBox.getObjectCount() << endl;
  Box boxes[5];
  cout << "Object count is " << firstBox.getObjectCount() << endl;

  cout << "Volume of first box = "
       << firstBox.volume()
       << endl;

  const int count = sizeof boxes/sizeof boxes[0];

  cout <<"The boxes array has " << count << " elements."
       << endl;

  cout <<"Each element occupies " << sizeof boxes[0] << " bytes."
       << endl;

  for(int i = 0 ; i < count ; i++)
    cout << "Volume of boxes[" << i << "] = "
         << boxes[i].volume()
         << endl;

  return 0;
}

Box constructor called
Object count is 1
Default constructor called
Default constructor called
Default constructor called
Default constructor called
Default constructor called
Object count is 6
Volume of first box = 935
The boxes array has 5 elements.
Each element occupies 24 bytes.
Volume of boxes[0] = 1
Volume of boxes[1] = 1
Volume of boxes[2] = 1
Volume of boxes[3] = 1
Volume of boxes[4] = 1