Use a static member variable independent of any object.

#include <iostream>
using namespace std;

class myclass {
public:
  static int i;
  void setInt(int n) { 
     i = n; 
  }
  int getInt() { 
     return i; 
  }
};

int myclass::i;

int main()
{
  myclass object1, object2;

  
  myclass::i = 100;                      // set i directly,  no object is referenced.

  cout << "object1.i: " << object1.getInt() << '\n'; // displays 100
  cout << "object2.i: " << object2.getInt() << '\n'; // also displays 100

  return 0;
}

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Use a static member variable independent of any object. : Static « Language « C++

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