Static Members: Class-Level State

A static member belongs to the class, not to any one object: one copy shared by every instance. Static data members are declared in the class and defined once outside it (C++17 allows in-class initialization with inline).

#include <string>
#include <vector>

class IdGenerator {
public:
    static int next() { return ++counter_; }   // static member function: no this

private:
    inline static int counter_{0};             // one shared int
};

class Registry {
public:
    void add(std::string name) { names_.push_back(std::move(name)); }
    static std::size_t count(const Registry& r) { return r.names_.size(); }

private:
    std::vector<std::string> names_;
};

The two kinds, cleanly separated

• static data member — shared state across all instances (counters, caches, config).
• static member function — callable without an object; has no this, so it may not touch non-static members (it may read others' via a parameter).

Accessing

ClassName::member from anywhere with access; member from inside class scope. Use cases that actually justify static state: instance counting, shared immutable configuration, factory helpers. Everything else should be a plain object — global mutable state is a design smell you will meet in debugging challenges.