Unfinished requirements

• Count the number of objects of a class during program running
• Ensure the security of the program (global variables cannot be used)
• Get the number of current objects at any time (Failure)

Change experiment: an attempt at a solution

#include <stdio.h>

class Test
{
public:
    static int cCount;
public: 
    Test()
    {
        cCount ++;
    }
    ~Test()
    {
        -- cCount;
    }
    int getCount()
    {
        return cCount;
    }
};

int Test::cCount = 0; 

int main()
{
    printf("count = %d\n", Test::cCount);
    
    Test::cCount = 1000;
    
    printf("count = %d\n", Test::cCount);

    return 0;
}

Output:
count = 0
count = 1000

Question:
The security of static variables is not guaranteed

problem analysis

• What do we need?

  • Object independent access to static member variables
  • The security of static member variables must be guaranteed
  • Get the value of static member variable conveniently and quickly

Static member function

• Static member functions can be defined in C + +

  • Static member function is a special member function in a class
  • Static member functions belong to the entire class
  • You can directly access the public static member function through the class name
  • Public static member functions can be accessed through object names
  • this pointer is not implied
  • Cannot access ordinary member variable (function), can access static member variable (function)

• Definition of static member function

  • Modifying member functions directly through the static keyword

class Test
{
public:
    static void Func1() {};
    static int Func2();
};

int Test::Func2()
{
    return 0;
}

Programming experiment: static member function example

#include <stdio.h>

class Demo
{
private:
    int i;
public:
    int getI();
    static void StaticFunc(const char* s);
    static void StaticSetI(Demo& d, int v);
};

int Demo::getI()
{
    return i;
}

void Demo::StaticFunc(const char* s)
{
    printf("StaticFunc: %s\n", s);
}

void Demo::StaticSetI(Demo& d, int v)
{
    d.i = v;                            // Accessing common member variables through objects 
}

int main()
{
    Demo::StaticFunc("main Begin...");  // Accessing static member functions by class name
    
    Demo d;
    
    d.StaticSetI(d, 10);                // Accessing static member functions by object name
    
    printf("d.i = %d\n", d.getI());
    
    Demo::StaticFunc("main End...");

    return 0;
}

Static member function VS ordinary member function

Programming experiment: the final solution

#include <stdio.h>

class Test
{
private:
    static int cCount;
public: 
    Test()
    {
        cCount ++;
    }
    ~Test()
    {
        -- cCount;
    }
    static int GetCount()
    {
        return cCount;
    }
};

int Test::cCount = 0; 

int main()
{
    printf("count = %d\n", Test::GetCount());
    
    Test t1;
    Test t2;
    
    printf("count = %d\n", t1.GetCount());
    printf("count = %d\n", t2.GetCount());
    
    Test* pt = new Test();
    
    printf("count = %d\n", pt->GetCount());
    
    delete pt;
    
    printf("count = %d\n", Test::GetCount());

    return 0;
}

Output:
count = 0
count = 2
count = 2
count = 3
count = 2

Summary

• Static member function is a special member function in a class
• Static member function has no hidden this parameter
• Static member functions can be accessed directly through the class name
• Static member functions can only access static member variables (functions)

The above contents refer to the series courses of Ditai Software Institute, please protect the original!