Static member

In order to realize data sharing among multiple objects of a class ,C++ Put forward Static members The concept of .

Static members include ：

Static data members and static member functions

The role of static members :

1. Static members do not belong to an object , and Belongs to a class （ Belongs to all objects ), No matter how many objects are created , All share static members —— It is a bridge connecting various objects .
2. Static data members are mainly used to share data among objects . Such as : Statistics of total 、 Average, etc .

Definition of static data members ：

static data type Data member name ;

class student
{
private:
	int num;
	char* name;
	static float score;
};

Static data members can be initialized , But only outside the class

Format ： data type   Class name ：： Static data members = initial value ;

explain :
1） Static data members can be described as public 、 Private or protected , If the description is public, it can be accessed directly through the class name .
         Class name ：： Static member name
2） Private or protected static data members can only be accessed through public member functions .
3） Static members are different from global variables —— Encapsulated inside a class , Replace global variables with static data members , Make data more secure ;

Conclusion : When a data member needs to be shared in a class, it should be defined as a static data member .

#include<iostream>
using namespace std;

class box
{
public:
	int width; int length;
	static int height;
	box(int w, int l);
	int volume();
};
int box::height = 10;
box::box(int w, int l)
{
	width = w;
	length = l;
}
int box::volume()
{
	return width * length * height;
}
int main()
{
	box a(10, 20), b(20, 30);
	cout << a.height << endl;
	cout << b.height << endl;
	cout << a.volume() << endl;
	cout << b.volume() << endl;
	return 0;
}

Storage of static data members :
Separate storage space outside all objects , As long as static data members are defined in the class , Even if no object is defined , Space is also allocated for static data members , It can refer to... By class name when the object has not been established .

After using static data members in a class , As long as an object changes its value , The value of the corresponding static data member of the object of the whole class changes .

Effective solutions :

Static member functions —— Access static data members

Definition of static member function :
static  Return type   Function name ( parameter list );

        { … };
Such as : static float averge( )
     {return (sum/3) ;}

Static member function call :
1） Static member functions belong to a class , Call... With the class name .

          Format :    Class name :: Static member function name ( Argument table );

2） Static members can call... With objects .

Format : Object name  .  Static member function name ( Argument table );

explain :
1) Static member functions Sure Define... Inside a class , You can also define... Outside the class .

Such as : Class : static float averge();
Off class definition :  float student::averge( )
                  {return (sum/3); }
2） Static member function: none this The pointer (this A pointer belongs to an object , Static members belong to a class ), Mainly used to access static data members , Non static data members cannot be accessed directly , If you want to access non static data members , Object name is required .

3） Ordinary member functions can call static member functions ;

4） Constructors and destructors cannot be static .
Conclusion : When static data members are defined in a class , In general, you should define static member functions to access static data members .

#include<iostream>
using namespace std;

class student
{
public:
	student(int n,string m,float s):num(n),name(m),score(s){}
	void total();
	static float averge();
private:
	int num;
	string name;
	float score;
	static float sum;
};
float student::sum = 0;
void student::total() { sum += score; }
float student::averge() { return (sum / 3); }
int main()
{
	student stu[3] = {student(1,"zhangsan",89),student(2,"lisi",85),student(3,"wangwu",75)};
	for (int i = 0; i < 3; ++i)
	{
		stu[i].total();
	}
	cout << "averge=" << student::averge() << endl;
	return 0;
}