Static vs Non-Static Variables

In C++, the primary difference between static and non-static variables lies in their lifetime and scope.

Non-static variables are instance variables, meaning each instance of a class has its own copy of these variables. For example, if you have a Vampire class with a Health variable, each Vampire object will have its own Health:

1#include <string>
2
3class Vampire {
4public:
5  int Health{100};
6};
7
8int main() {
9  Vampire A;
10  A.Health = 50;
11
12  Vampire B;
13  B.Health = 200;
14}

In this example, Vampire A has a Health of 50, while Vampire B has a Health of 200. Each instance operates independently with its own copy of Health.

Static variables, on the other hand, are shared among all instances of a class. They belong to the class itself, not to any particular instance. When a variable is declared as static, all instances of the class share the same memory location for that variable:

1#include <iostream>
2
3class Vampire {
4 public:
5  static inline std::string Faction{"Undead"};
6};
7
8int main() {
9  Vampire A;
10  Vampire B;
11  if (&A.Faction == &B.Faction) {
12    std::cout << "They're the same";
13  }
14}

1They're the same

Here, Faction is static, meaning it is shared by all Vampire objects. Changing Faction for one Vampire changes it for all:

1#include <iostream>
2
3class Vampire {
4 public:
5  static inline std::string Faction{"Undead"};
6};
7
8int main() {
9  Vampire A;
10  Vampire B;
11  A.Faction = "Demonic";
12  std::cout << B.Faction;  
13}

1Demonic

In summary:

• Non-static variables: unique to each instance, different instances can have different values.
• Static variables: shared across all instances, one value for the entire class.

Understanding the difference helps you decide whether a variable should be unique to each instance or shared among all instances.