Accessing static members using non-static member functions in C++

1: //Listing 14.3 private static data members 2: 3: #include <iostream.h> 4: 5: class Cat 6: { 7: public: ...

1: //Listing 14.3 private static data members
2:
3: #include <iostream.h>
4:
5: class Cat
6: {
7: public:
8: Cat(int age):itsAge(age){HowManyCats++; }
9: virtual ~Cat() { HowManyCats--; }
10: virtual int GetAge() { return itsAge; }
11: virtual void SetAge(int age) { itsAge = age; }
12: virtual int GetHowMany() { return HowManyCats; }
13:
14:
15: private:
16: int itsAge;
17: static int HowManyCats;
18: };
19:
20: int Cat::HowManyCats = 0;
21:
22: int main()
23: {
24: const int MaxCats = 5; int i;
25: Cat *CatHouse[MaxCats];
26: for (i = 0; i<MaxCats; i++)
27: CatHouse[i] = new Cat(i);
28:
29: for (i = 0; i<MaxCats; i++)
30: {
31: cout << "There are ";
32: cout << CatHouse[i]->GetHowMany();
33: cout << " cats left!\n";
34: cout << "Deleting the one which is ";
35: cout << CatHouse[i]->GetAge()+2;
36: cout << " years old\n";
37: delete CatHouse[i];
38: CatHouse[i] = 0;
39: }
40: return 0;
41: }

Output: There are 5 cats left!
Deleting the one which is 2 years old
There are 4 cats left!
Deleting the one which is 3 years old
There are 3 cats left!
Deleting the one which is 4 years old
There are 2 cats left!
Deleting the one which is 5 years old
There are 1 cats left!
Deleting the one which is 6 years old

Analysis: On line 17, the static member variable HowManyCats is declared to have private access. Now you cannot access this variable from non-member functions, such as TelepathicFunction from the previous listing.

Even though HowManyCats is static, it is still within the scope of the class. Any class function, such as GetHowMany(), can access it, just as member functions can access any member data. However, for a function to call GetHowMany(), it must have an object on which to call the function.

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DO use static member variables to share data among all instances of a class. DO make static member variables protected or private if you wish to restrict access to them. DON'T use static member variables to store data for one object. Static member data is shared among all objects of its class.

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