C++static member (variable hoặc function) thuộc về class, không thuộc về instance cụ thể nào — tất cả object chia sẻ cùng 1 bản.
cpp
class Counter {
public:
static int total; // 1 biến cho toàn bộ class
Counter() { total++; }
~Counter() { total--; }
static int getTotal() { return total; } // không có `this`
};
int Counter::total = 0; // phải định nghĩa ngoài class
Counter a, b, c;
std::cout << Counter::total; // 3
std::cout << Counter::getTotal(); // gọi qua class nameĐặc điểm:
- Static member variable phải định nghĩa ngoài class (trừ inline static C++17).
- Static method không có this — không truy cập non-static member.
- Dùng cho singleton, factory method, global counter, shared config.
A static member (variable or function) belongs to the class itself, not to any particular instance — all objects share the same single copy.
cpp
class Counter {
public:
static int total; // one variable for the entire class
Counter() { total++; }
~Counter() { total--; }
static int getTotal() { return total; } // no `this`
};
int Counter::total = 0; // must be defined outside the class
Counter a, b, c;
std::cout << Counter::total; // 3
std::cout << Counter::getTotal(); // call via class nameKey points:
- Static member variable must be defined outside the class (except inline static in C++17).
- Static methods have no this — cannot access non-static members.
- Used for singletons, factory methods, global counters, shared config.