Static Member

A static member is a member that is bound to the class itself and not to any object. Thus it is a shared copy for all objects of the same class. Static members are accessed with the scope resolution operator (::) on the class name instead of the dot operator on an object:

class Shape {
public:
  static int count;
};

int Shape::count = 0;

int main() {
  Shape::count = 2;
}

A static member variable must be defined once outside the class and cannot be initialised inline:

class Shape {
public:
  static int count = 0;  // ERROR.
};

Static members defined outside the class must omit the static keyword:

int Shape::count = 0;         // OK.
static int Shape::count = 0;  // ERROR.

Inline initialisation is possible with the inline keyword:

class Shape {
public:
  static inline int count = 0;
};

A static member function can be called without an object. It can access static members but not the fields of any object since it has no object to work with:

class Shape {
public:
  static int getCount() { return count; }  // OK.
  static int count;

  static int getSides() { return sides; }  // ERROR.
  int sides;
};

int Shape::count = 0;

Static members can also be accessed through an object with the dot operator, which still refers to the shared copy:

Shape a;
Shape b;
a.count = 5;
b.count;  // 5

A static constexpr member can be initialised inside the class, which is the usual way to define a class-level constant:

class Shape {
public:
  static constexpr int MAX_SIDES = 12;
};

int limit = Shape::MAX_SIDES;