6.5.3.

//: C05:CountedClass.cpp
// Object counting via static members.
#include <iostream>
using namespace std;

class CountedClass {
  static int count;
public:
  CountedClass() { ++count; }
  CountedClass(const CountedClass&) { ++count; }
  ~CountedClass() { --count; }
  static int getCount() { return count; }
};

int CountedClass::count = 0;

int main() {
  CountedClass a;
  cout << CountedClass::getCount() << endl;   // 1
  CountedClass b;
  cout << CountedClass::getCount() << endl;   // 2
  { // An arbitrary scope:
    CountedClass c(b);
    cout << CountedClass::getCount() << endl; // 3
    a = c;
    cout << CountedClass::getCount() << endl; // 3
  }
  cout << CountedClass::getCount() << endl;   // 2
} ///:~

Listado 6.43. C05/CountedClass.cpp


//: C05:CountedClass2.cpp
// Erroneous attempt to count objects.
#include <iostream>
using namespace std;

class Counted {
  static int count;
public:
  Counted() { ++count; }
  Counted(const Counted&) { ++count; }
  ~Counted() { --count; }
  static int getCount() { return count; }
};

int Counted::count = 0;

class CountedClass : public Counted {};
class CountedClass2 : public Counted {};

int main() {
  CountedClass a;
  cout << CountedClass::getCount() << endl;    // 1
  CountedClass b;
  cout << CountedClass::getCount() << endl;    // 2
  CountedClass2 c;
  cout << CountedClass2::getCount() << endl;   // 3 (Error)
} ///:~

Listado 6.44. C05/CountedClass2.cpp


//: C05:CountedClass3.cpp
#include <iostream>
using namespace std;

template<class T> class Counted {
  static int count;
public:
  Counted() { ++count; }
  Counted(const Counted<T>&) { ++count; }
  ~Counted() { --count; }
  static int getCount() { return count; }
};

template<class T> int Counted<T>::count = 0;

// Curious class definitions
class CountedClass : public Counted<CountedClass> {};
class CountedClass2 : public Counted<CountedClass2> {};

int main() {
  CountedClass a;
  cout << CountedClass::getCount() << endl;    // 1
  CountedClass b;
  cout << CountedClass::getCount() << endl;    // 2
  CountedClass2 c;
  cout << CountedClass2::getCount() << endl;   // 1 (!)
} ///:~

Listado 6.45. C05/CountedClass3.cpp