Static Member Data in C++

Until now, you have probably thought of the data in each object as unique to that object and not shared among objects in a class. For example, if you have five Cat objects, each has its own age, weight, and other data. The age of one does not affect the age of another.

There are times, however, when you'll want to keep track of a pool of data. For example, you might want to know how many objects for a specific class have been created in your program, and how many are still in existence. Static member variables are shared among all instances of a class. They are a compromise between global data, which is available to all parts of your program, and member data, which is usually available only to each object.

You can think of a static member as belonging to the class rather than to the object. Normal member data is one per object, but static members are one per class. Listing 14.1 declares a Cat object with a static data member, HowManyCats. This variable keeps track of how many Cat objects have been created. This is done by incrementing the static variable, HowManyCats, with each construction and decrementing it with each destruction.

Listing 14.1. Static member data.

1: //Listing 14.1 static data members
3: #include <iostream.h>
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: static int HowManyCats;
14: private:
15: int itsAge;
17: };
19: int Cat::HowManyCats = 0;
21: int main()
22: {
23: const int MaxCats = 5; int i;
24: Cat *CatHouse[MaxCats];
25: for (i = 0; i<MaxCats; i++)
26: CatHouse[i] = new Cat(i);
28: for (i = 0; i<MaxCats; i++)
29: {
30: cout << "There are ";
31: cout << Cat::HowManyCats;
32: cout << " cats left!\n";
33: cout << "Deleting the one which is ";
34: cout << CatHouse[i]->GetAge();
35: cout << " years old\n";
36: delete CatHouse[i];
37: CatHouse[i] = 0;
38: }
39: return 0;
40: }

Output: There are 5 cats left!
Deleting the one which is 0 years old
There are 4 cats left!
Deleting the one which is 1 years old
There are 3 cats left!
Deleting the one which is 2 years old
There are 2 cats left!
Deleting the one which is 3 years old
There are 1 cats left!
Deleting the one which is 4 years old

Analysis: On lines 5 to 17 the simplified class Cat is declared. On line 12, HowManyCats is declared to be a static member variable of type int.

The declaration of HowManyCats does not define an integer; no storage space is set aside. Unlike the non-static member variables, no storage space is set aside by instantiating a Cat object, because the HowManyCats member variable is not in the object. Thus, on line 19 the variable is defined and initialized.

It is a common mistake to forget to define the static member variables of classes. Don't let this happen to you! Of course, if it does, the linker will catch it with a pithy error message such as the following:
undefined symbol Cat::HowManyCats
You don't need to do this for itsAge, because it is a non-static member variable and is defined each time you make a Cat object, which you do here on line 26.

The constructor for Cat increments the static member variable on line 8. The destructor decrements it on line 9. Thus, at any moment, HowManyCats has an accurate measure of how many Cat objects were created but not yet destroyed.

The driver program on lines 21-40 instantiates five Cats and puts them in an array. This calls five Cat constructors, and thus HowManyCats is incremented five times from its initial value of 0.

The program then loops through each of the five positions in the array and prints out the value of HowManyCats before deleting the current Cat pointer. The printout reflects that the starting value is 5 (after all, 5 are constructed), and that each time the loop is run, one fewer Cat remains.

Read also

How to access static members without an object
How to access static members using non-static member functions
Static Member Data in C++ Reviewed by 1000sourcecodes on 02:13 Rating: 5
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