Get number of active class instances

I need to get a number of created and the number of active class instances.

#include <iostream>
#include <stdexcept>
class Set {
  double value;
  int created = 0;
  int active = 0;
public:
  Set();
  Set(double num);
  static int NumberOfCreated();
  static int NumberOfActive();
};
Set::Set() : value(0) {}
Set::Set(double num) : value(num) {
  created++;
  active++;
}
int Set::NumberOfCreated() { return created; }
int Set::NumberOfActive() { return active; }
int main() {
  Set s1, s2(100);
  { Set s3(50); }                      // After "}", "s3" doesn't exist anymore
  std::cout << Set::NumberOfCreated(); // Should print 3
  std::cout << Set::NumberOfActive();  // Should print 2
  return 0;
}

I get two errors:

invalid use of member ‘Set::created’ in static member function

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invalid use of member ‘Set::active’ in static member function

Also, this logic would always give the same output for active and created class instances. Could you give me a better approach to how to find number of active elements?

• Note: main function cannot be changed! Only class should be changed to enable this functionality.

>Solution :

you have multiple problems, here is how it can be done, check comments:

#include <iostream>
#include <stdexcept>

class Set {
  double value;
  static int created; // you need single variable for all instances, this is why static
  static int active;
public:
  Set();
  Set(double num);
  ~Set() {
      --active; // destructor is called when object is destroyed, so number of active instances is decreased
  }
  static int NumberOfCreated();
  static int NumberOfActive();
};

int Set::created = 0;
int Set::active = 0;

Set::Set() : Set(0) {} // this constructor should increase created/active counters as well, this can be achieved via calling another ctor
Set::Set(double num) : value(num) {
  created++;
  active++;
}
int Set::NumberOfCreated() { return created; }
int Set::NumberOfActive() { return active; }

int main() {
  Set s1, s2(100);
  { Set s3(50); }
  std::cout << Set::NumberOfCreated() << std::endl;
  std::cout << Set::NumberOfActive() << std::endl;
  return 0;
}

and of course follow @user17732522 comments about copy/move and thread safety