3.18 Static States of Classes


Each instance of a class has its own states. However, we also have static states allowing us to share a variable between every instance of a class. You can think of a static state as being a part of the class rather than the objects.

Here is a good example of the use of static states in the Account class.

 1 
 2   // Static Member Example
 3  
 4   #include<iostream>
 5   #include<string>
 6  
 7   using namespace std;
 8  
 9   class Account{
10 
11     protected:
12 
13       static int nextAccountNumber; 1
14       int accountNumber;
15       float balance;
16       string owner;
17 
18     private:
19 
20       void construct(); 2
21 
22     public:
23 
24       Account(string owner, float aBalance);
25       Account(float aBalance);
26 
27       virtual void display();
28       virtual void makeLodgement(float);
29       virtual void makeWithdrawal(float);
30   };
31 
32   int Account::nextAccountNumber = 123456; 3
33 
34   Account::Account(string anOwner, float aBalance):
35 	  balance(aBalance),
36 	  owner (anOwner) { construct(); } 4
37 
38   Account::Account(float aBalance) :
39       balance(aBalance),
40 	  owner ("Not Defined") {construct(); }
41 
42   void Account::construct()
43    {
44     accountNumber = nextAccountNumber++; 5
45   }
46 
47   void Account::display(){
48     cout << "account number: " << accountNumber
49          << " has balance: " << balance << " Euro" << endl;
50     cout << "This account is owned by: " << owner << endl;
51   }
52 
53   void Account::makeLodgement(float amount){
54       balance = balance + amount;
55   }
56 
57   void Account::makeWithdrawal(float amount){
58       balance = balance - amount;
59   }
60  
61   int main()
62   {
63     Account *a = new Account("John", 10.50);
64     Account *b = new Account("Derek", 12.70);
65 
66     a->display();
67     b->display();
68   }
69   
70 

(The full source code for this example is here - StaticMemberTest.cpp)

1

A state can simply be defined as static by placing the static keyword in front of its declaration.

2

One of the problems with C++ and multiple constructors is that there is no way to call one constructor directly from the other (like in Java). To prevent us from having to duplicate code we can create a private method (I called it construct()) that is called directly from the actual constructors of the class. If any alterations need to be made to the construct() method then they will impact on all of the constructors.

3

In C++ we are required to define and associate a value with the static value in the implementation of the class. Note that the static keyword is not used again at this point.

4

Note that in the two constructors the construct() method is called which sets the accountNumber value.

5

The private construct() method sets the accountNumber state to the nextAccountNumber and increments the nextAccountNumber after the assignment. Since the static state is shared with every Account object it is incremented on each and every object. This means that when every object is created it will have its own individual account number, without having to pass the value to the constructor.

The output of this example can be seen in Figure 3.18, “The Account class with static state example output.”.

Figure 3.18. The Account class with static state example output.

The Account class with static state example output.

Static variables can be publicprivate or protected.

Static member methods can be defined in the same way (e.g. static int getNextAccountNumber();). Static member methods cannot access non-static states and they cannot be virtual, as they are related to the class and not the object.




These notes are copyright Dr. Derek Molloy, School of Electronic Engineering, Dublin City University, Ireland 2013-present. Please contact him directly before reproducing any of the content in any way.
ċ
StaticMemberTest.cpp
(1k)
Derek Molloy,
19 Oct 2013, 06:09
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