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The rounded box represents the class itself which contains the three member functions and the data member count. The pub1 ic members are above the line and the private member(s) are below it. Each member function maintains a pointer (named this") which points to the object that owns the current function call. This snapshot shows the status during the execution of the last line in the program: three widgets (w, X , and Y) exist, and w is calling the numwidgets ( ) function which returns the value of the private data member count. Note that this data member resides within the class itself; the class objects have no data.

Like any ordinary member function, the numwidge t s ( > function in Example 8.15 requires that it be owned by some instance of the class. But since it returns the value of the s tat ic data member count which is independent of the individual objects themselves, it doesn t matter which object calls it. We had w call it each time, but we could just as well have had x or Y or z call it when they exist.. Moreover, we couldn t call it at all until after some object had been created. This is rather arbitrary. Since the action of the function is independent of the actual function objects, it would be better to make the calls independent of them too. This can be done simply by declaring the function to be s tat ic.

The Widget class maintains a s tat ic data member count which keeps track of the number of Widget objects in existence globally. Each time a widget is created (by the constructor) the counter is incremented, and each time a widget is destroyed (by the destructor) the counter is decremented.

class Widget { , public: Widget0 { ++count; > -Widget() { --count; > static int num() ( return count; } private: static int count;

8.1 8.2 8.3 8.4 8.5 8.6 8.7 Introduction Object Model Data Flow Modeling Behavioral Modeling Data Dictionary System Diagrams IEEE Standard for Software Requirements Speci cation

int Widget::count = 0; main0 1 tout << Now there are Widget w, x; tout -c-K Now there are 1 Widget w, x, y, z; tout << Wow there > tout << "Now there are Widget y; tout << "Now there are

<< I1 widgets.\xP;

<< 'I widgets.\n";

are 1 << Widget::num() I' CC Widget::num() ' << Widget::num()

112 112 113 114 116 117 118

<< ' widgets.\n";

<< 'I widgets.\n"; << 'I widgets.\n";

Declaring the num ( ) function to be static renders it independent of the class instances. So now it is invoked simply as a member of the Widget class using the scope resolution operator : : . This allows the function to be called before any objects have been instantiated.

The Drevious fiEure showing relationships among the class, its members, and should now looks liki this:

9.1 9.2 9.3 9.4 9.5 9.6 Introduction Phases of the Design Process Design Concepts Measuring Cohesion Measuring Coupling Requirements Traceability

The difference is that now the member function num( > has no this" pointer. As a static member function, it is associated with the class itself, not with its instances. Static member functions can access only s tat i c data from their own class.

8.1 8.2 8.3 8.4