Static and instance members

An instance field exists once per object. A static field exists once for the whole class and is shared by every object.

Time O(1)Space one shared copyCode C++, Python and C#

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The idea

Static members belong to the class itself. You reach them through the class name, Visitor.Count, and they exist even before any object is created.

Code

C++PythonC#

class Visitor {
public:
    static inline int count = 0;   // one copy, shared by the class
    string name;                   // one copy per object
    Visitor(string name) : name(name) { count++; }
};

int main() {
    Visitor a("Asha");
    Visitor b("Ravi");
    Visitor c("Meera");
    cout << Visitor::count << "\n";   // 3
}

class Visitor:
    count = 0  # one copy, shared by the class

    def __init__(self, name: str):
        self.name = name  # one copy per object
        Visitor.count += 1

a = Visitor("Asha")
b = Visitor("Ravi")
c = Visitor("Meera")
print(Visitor.count)  # 3

class Visitor {
    public static int Count;      // one copy, shared by the class
    public string Name;           // one copy per object
    public Visitor(string name) { Name = name; Count++; }
}

var a = new Visitor("Asha");
var b = new Visitor("Ravi");
var c = new Visitor("Meera");
Console.WriteLine(Visitor.Count);   // 3

Tips

A static method has no "this", so it cannot touch instance fields without being given an object.
Shared mutable static state is a common source of bugs in multi-threaded code.

In the .NET library

A static class such as Math or Console cannot be instantiated at all. In a web app, static fields are shared across every request, so prefer dependency injection with a singleton lifetime.