C ++ static static members

01 Basic Concepts

Static members: the previously defined keywords add a static member.

class A
{
public:
    A(int a, int b):m_a(a),m_b(b)
    {
        num += m_a + m_b;
    }
    
    ~A(){ }
    
    void Fun();             // 普通成员函数
    static void PrintNum()  // 静态成员函数
    {
        // 在静态成员函数中，不能访问非静态成员变量，也不能调用非静态成员函数
        std::cout << num << std::endl; 
    }
    
private:
    int m_a;         // 普通成员变量
    int m_b;         // 普通成员变量
    static int num;  // 静态成员变量
};

// 静态成员必须在定义类的文件中对静态成员变量进行初始化，否则会编译出错。
int A::num = 0;

int main()
{
    A a1(1,1);
    A::PrintNum(); // 访问静态函数
    A a2(1,1);
    A::PrintNum(); // 访问静态函数
    
    return 0;
}

Output

2
4

• Ordinary member variables of each object has its own copy, while static member variables of a total of one, shared by all objects.

It should be noted that the sizeofoperator does not calculate the static member variable size, chestnut follows:

class CTest
{
    int n;
    static int s;
};

It is sizeof(CTest)equal to 4

• Ordinary members have specific functions act on an object, and the static member function not specifically act on an object .
• So static members do not need to be able to visit through the object to ask, because he is shared.

02 How to access static members

1) the class name :: Member Name

A::PrintNum();

2) object name. Member Name

A a;
a.PrintNum();

3) pointer -> member name

A *p = new A();
p->PrintNum();

4) references. Member Name

A a;
A & ref = a;
ref.PrintNum();

03 Summary

• It is a static member variable nature of global variables, even if an object does not exist, like static member variables there.
• Function on a global function is essentially a static member.
• Set a static member of such a mechanism is the purpose of global variables and functions and some closely related write in the class inside, looks like a whole, easy to maintain and understand.
• In the static member function, you can not access non-static member variables, we can not call non-static member functions.
• Static members must be initialized static member variables in a class definition file, otherwise it will compile error.