Static member

Static members

Static data members

You can define static members of a class , It can realize data sharing among multiple objects of the same kind . The advantage of using static members of classes is ：

• The name of the static member is in the scope of the class , Therefore, you can avoid conflicts with members of other classes or global object names
• Static members can implement encapsulation , Can be a private member , And global objects cannot
• Static members are associated with specific classes , The structure is clear

（1） Static data members
example ：

class employee {
     // Employee class definition 
	private：
		int empNo;
		int id;
		char *name;
		static int count; // Static data members 
		...
};

Static data members are special data members of classes , It takes the keyword static Start , The declaration form is ：

class  Class name  {
     // The class body 
	...
	static  Data member type   List of data member names ; // Static data member declaration 
	...
};

for example ：

class Data {
     //Data Class definition 
	public:
		static int count; // Static data members 
		int maxlevel; // Non static public data member 
		Data(int i=0) {
     ...,count++; } // Constructors 
	private:
		int level; // Non static private data members 
};
int Data::count=0; // Static data members are defined and initialized 

count The purpose of the design is to count Data How many instantiated objects does the class have in total

Description of static data members ：
（1） Usually , Non static data members exist in every object of class type , Static data members are independent of any object of the class , Separate storage space outside all objects . The space allocated for the object does not include the space occupied by static data members
（2） If only a class is declared and no object is defined , Then the non static data members of the class do not occupy storage space , Only when defining objects , To allocate space for the data members of the object . As long as the static member is defined in the class , Even if no object is defined , Space is also allocated for static data members , It can be referenced before the object is created
（3） When accessing static members, you also need to abide by public and private access rules
（4） Static data members must be defined outside the class once （ Only once ）, Static members cannot be initialized by class constructors , Instead, it is initialized when it is defined outside the class . The way to define static data members is ：

 Data member type   Class name :: Static data member name  =  Initialization ;

（5） Static data members can be used as default arguments , Non static data members cannot be used as default arguments , Because its value cannot be used independently of the object it belongs to . for example ：

class Data {
     //Data Class definition 
	...
	Data& setbkcolor(int bkcolor);
	static const int bkcolor = 5;
};

（6） With static data members , Data sharing is realized among objects , Therefore, you can not use global variables

class test {
    
	private:
		int x;
		int y;
	public:
		static int num;
		static int Getnum() {
    
			x+=5; // error , Static member functions cannot call non static data members 
			num+=15;
			return num;
		}
};

int test::num=10;

int main() 
{
    
	test a;
	cout<<test::num<<endl; // Output 10
	text::num=20;
	cout<<test::num<<endl; // Output 20
	cout<<test::Getnum()<<endl; // Output 35
	cout<<a.Getnum()<<endl; // Output 50
	return 0;
}

Static member functions

Member functions can also be defined as static , Before declaring a function in a class, add static It becomes a static member function , The general form of the declaration is ：

class  Class name  {
     // The class body 
	...
	static  Return type   Function name （ type 1  Parameter name 1, type 2  Parameter name 2,...）;
	...
};

for example ：

static int getcount() {
     return count; } // Static member functions 

Just like static data members , Static member functions are part of a class , Not part of the object . If you want to call a public static member function outside the class , You can use the class scope operator （::） And calling static member functions through object names , for example ：

cout<<Data::getcount()<<'\t'<<d.getcount();

The fundamental difference between static member functions and non static member functions is ： Non static member functions have this The pointer , And static member functions don't have this The pointer . therefore , Static member functions cannot access non static members in this class . Static member functions are designed to access static data members
Static member functions cannot be declared as const

Examples of static members ：

#include<iostream>
using namespace std;

class CTest {
    
	public:
		CTest() {
     s_total++; id=s_total; cout<<" structure "<<id<<" "; }
		~CTest() {
     s_total--; cout<<" destructor "<<id<<" "; }
		static int gettotal() {
     return s_total; }
	private:
		static int s_total;
		int id;
};
int CTest::s_total=0;
int main() {
    
	CTest a,b,c;
	CTest *p=new CTest;
	cout<<" total ="<<CTest::gettotal()<<" ";
	delete p;
	cout<<" total ="<<CTest::gettotal()<<" ";
	return 0;
}

Examples of static members

class Test {
    
	public:
		void init() {
     }
		static void output() {
     }
};
int main()
{
    
	Test::init();
	Test::output();
	return 0;
}

• Compilation error , Because non static member functions of a class cannot be called by class name init

Examples of static members

class Test {
    
	public:
		void init() {
     }
		static void output() {
     }
};
int main()
{
    
	Test t;
	t.init();
	t.output();
	return 0;
}

• Compile and pass , Class objects can use static member functions and non static member functions

Examples of static members

class Test {
    
	public:
		void init() {
     }
		static void output() {
     cout<<x<<endl; }
	private:
		int x;
};
int main()
{
    
	Test t;
	t.output();
	return 0;
}

• Compilation error , Because non static members cannot be referenced in static member functions
• Static member functions belong to the entire class , Space is allocated before the class instantiates the object , The non static members of a class must have memory space after the class instantiates the object

Examples of static members

class Test {
    
	public:
		void init() {
     output(); }
		static void output() {
     }
};
int main()
{
    
	Test t;
	t.output();
	return 0;
}

• Compile and pass , Because non static member functions of classes can call static member functions , But not vice versa

Examples of static members

class Test {
    
	public:
		Test() {
     m++; }
		~Test() {
     m--; }
		static void output() {
     cout<<m<<endl; }
	private:
		static int m;
};
int main()
{
    
	Test t;
	t.output();
	return 0;
}

• link error , Because static data members of a class must be initialized before use
• If in main Add... Before the function int Point::m=0;, Recompile the link without error , Running the program will output 1