Feb. 22, 2011, 8:21 p.m.
posted by freeheart
In Figure, notice that the statement that creates the Time object looks as though it is invoking a Time( ) method:
Time timeObject = new Time( );
In fact, a member method is invoked whenever you instantiate an object. This method is called a constructor. Each time you define a class, you are free to define your own constructor, but if you don't, the compiler will provide one for you invisibly and automatically.
The job of a constructor is to create an instance of the object specified by a class and to put it into a valid state. Before the constructor runs, the object is just a blob of memory; after the constructor completes, the memory holds a valid instance of the class.
The Time class of Figure does not define a constructor, so the compiler implicitly provides one. The constructor provided by the compiler creates the object but takes no other action.
If you do not explicitly initialize your member variables, they are initialized to innocuous values (integers to 0, strings to the empty string, etc.). Figure lists the default values assigned to various types.
Typically, you'll want to define your own constructor and provide it with arguments, so that the constructor can set the initial state for your object. In Figure, you want to pass in the current year, month, date, and so forth, so that the object is created with meaningful data.
You declare a constructor like any other member method, except:
If there are arguments to be passed, you define an argument list just as you would for any other method. Figure declares a constructor for the Time class that accepts six arguments, one each for the year, month, date, hour, minute, and second for the new Time object you are creating.
Creating a constructor
The output looks like this:
In this example, the constructor takes a series of integer values and initializes all the member variables based on these parameters.
When the constructor finishes, the Time object exists and the values have been initialized. When DisplayCurrentTime( ) is called in Main( ), the values are displayed.
Try commenting out one of the assignments and running the program again. You'll find that each member variable that isn't assigned-to by you is initialized by the compiler to zero. Integer member variables are set to zero if you don't otherwise assign them. Remember that value types (such as integers) must be initialized; if you don't tell the constructor what to do, it sets innocuous values.