Static member variables for class 25th

1. Review of member variables

(1) Access to public member variables by object name

(2) The member variables of each object are exclusive

(3) member variables cannot be shared between objects

2. static member variables

(1) Static member variables in C + +

① static member variables belong to the entire class all

② the lifetime of a static member function variable is not dependent on any object

③ can access public static member variables directly through the object name or through the class name

④ static member variables for all objects shared classes

(2) Characteristics of static member variables

① is modified directly by the static keyword when defined

② static member variables are located inside the program within the global data area , so you need to allocate space outside of the class . (Because objects in new or stack will only allocate space for non-static members, not static members, you must allocate space for them separately outside of the class)

③ The syntax rules for allocating space for static member variables:

Type classname::varname = value; // to be placed outside the function as if you were defining a global variable

The use of static member variables for "programming experiments"

#include <stdio.h>classtest{Private:    Static intCcount; Public: Test () {ccount++;} ~test () {--Ccount;} intGetCount () {returnccount;}};//You must allocate space to static member variables outside of the class before useintTest::ccount =0;//indicates that you want to be in the global zone and can only allocate space in the global zone and initialize to 0Test gtest;//ccount increased by 1intMain () {//int test::ccount = 0; error, attempting to allocate space for static members hereTest T1;//ccount increased by 1Test T2;//ccount increased by 1printf ("sizeof (Test) =%d, sizeof (t1) =%d\n",                sizeof(Test),sizeof(t1));//equals 1, indicating that ccount is allocating space in the global zone,//the variable does not belong to an object itself. printf ("count =%d\n", Gtest.getcount ());//3, accessing static member variables by object nameprintf"count =%d\n", T1.getcount ()); printf ("count =%d\n", T2.getcount ()); Test* pt =NewTest ();//ccount increased by 1printf ("count =%d\n", Pt->getcount ());//4    DeletePt//ccount minus 1 .printf ("count =%d\n", Pt->getcount ());//3//printf ("Count =%d\n", test::ccount);//error, only public static member variables can be accessed through the class name    return 0;}

3. Summary

(1) A static member variable can be defined in a class by using the static keyword

(2) static member variable belongs to class all

(3) A static member variable can be accessed by each object

(4) Static member variable allocates space in global data area

(5) The lifetime of the static member variable is the program run time

Static member variables for class 25th