{"id":48,"date":"2018-07-09T11:45:32","date_gmt":"2018-07-09T11:45:32","guid":{"rendered":"http:\/\/itp1.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=48"},"modified":"2019-05-14T11:10:00","modified_gmt":"2019-05-14T11:10:00","slug":"classes-objects-ii","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/itp1\/chapter\/classes-objects-ii\/","title":{"rendered":"Classes & Objects – II"},"content":{"raw":"
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In the previous module we have seen that the Classes are basic building blocks of object oriented applications. A well-crafted class can create powerful user defined data type. Whenever we need to store information of that data type, we create variable of that data type or object of that data type. Data members can be private or public. Private members can be accessed only by member functions. Public members can be accessed outside the class through object of the class using dot operator. Whenever we create an object of any class, each object is allocated memory for each data member. For example have a look at the following program.<\/p>\r\n \r\n\r\nNeed of Static Data Members<\/strong>\r\n\r\n \r\n\r\n\"\"\r\n

When we read data from the user in these objects, objects will be holding data (assumed to be given by the user) as follows<\/p>\r\n\"\"\r\n

You will notice here that rollno, name and marks_obtained will have different values in each object as each student has different rollno and name and each will obtain different marks. But total_marks remain same for each student. You would notice that even when total_marks are same for all, they are stored in each student object. This results in memory wastage. Also this unnecessary repetition of total_marks has other problems. If we want to change the value of total_marks, even though it is common value for all, we will have to modify them separately for all objects as follows :<\/p>\r\n\"\"\r\n\r\n \r\n

Also if we forget to change value of total_marks in one of the objects, it will result into data inconsistency as shown below. If we attempt to compute percentage of marks obtained by the students.The students whose total_marks did not get modified and remained 50, will be at disadvantage as correct percentage will not be calculated.<\/p>\r\n\"\"\r\n\r\nStatic Data Members<\/strong>\r\n\r\n \r\n\r\nWe can define data member of a class as static using static keyword static.\r\n\r\n \r\n

A static data member is shared by all objects of the class. There is only one copy of the static member. All static data is initialized to zero when the first object is created, if no other initialization is present. We cannot initialize in the class definition but it can be initialized outside the class.<\/p>\r\n \r\n\r\nclass\u00a0 student\r\n\r\n \r\n\r\n{\r\n\r\nint rollno;\r\n\r\nchar\u00a0 name[20];\r\n\r\n \r\n\r\nint marks_obtained;\r\n\r\nstatic int total_marks;\r\n\r\n \r\n\r\npublic :\r\n\r\n \r\n\r\nvoid readdata();\r\n\r\nvoid displaydata();\r\n\r\nint\u00a0\u00a0 computePercentage();\r\n\r\nint update_totalmarks(int\u00a0 mks) { total_marks = mks;}\r\n\r\n};\r\n\r\n \r\n\r\n\/\/ Initialize static data member int student :: total_marks = 0;\r\n\r\n\"\"\r\n\r\n \r\n\r\nThe advantage of having one copy is that whenever the value of static data member needs to be modified, we need to modify it at one place only as there is only one copy of the same.\r\n\r\n \r\n\r\n\"\"\r\n

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As we can see in the above program that the modification is done only once as there is only one common copy of the same. We need to invoke update_totalmarks() function on any of the objects. In the above program we have invoked this function on object stud1. We do not wish to update total marks for object 1 only, but we have invoked it on stud1 object as being a member function, it needs any object for its invocation. We could have invoked it on stud2 or stud3 as well. Invokation of update_totalmarks() on stud1 doesn\u2019t look logical, and gives impression that we wish to modify total marks of stud1. What can we do to make function invocation look more logical and convincing?<\/p>\r\n \r\n\r\nWell as we can define data members as static, we can also define member functions static. Defining update_totalmarks() as static can remove the need of calling update_totalmarks() on any object.\r\n\r\n \r\n\r\nStatic Members Function<\/strong>\r\n\r\n \r\n\r\nA member function in a class can be declared as static. A static member function has certain special characteristics. These are:\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 A static function can have access to only other static members.\r\n\r\n\u00b7\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 A static member function can be accessed not only by the object name but also with the class name.\r\n\r\n \r\n\r\nFollowing program illustrates the use of static data member:\r\n\r\n \r\n\r\nclass\u00a0 student\r\n\r\n \r\n\r\n{\r\n\r\nint rollno;\r\n\r\nchar\u00a0 name[20];\r\n\r\nint marks_obtained;\r\n\r\nstatic int total_marks;<\/span>\r\n\r\npublic :<\/span>\r\n\r\n<\/div>\r\n \r\n\r\nvoid readdata();\r\n\r\nvoid displaydata();\r\n\r\nint\u00a0\u00a0 computePercentage();\r\n\r\nstatic int update_totalmarks(int\u00a0 mks) { total_marks = mks;}\r\n\r\n \r\n\r\n};\r\n\r\n \r\n