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CHAP. 10]
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cout << "Now there are " << w.numWidgets() << " widgets.\n"; } cout << "Now there are " << w.numWidgets() << " widgets.\n"; Widget y; cout << "Now there are " << w.numWidgets() << " widgets.\n"; } This works the same way as Example 10.2. But now that the static variable count is private, we need the access function numWidgets() to read count in main().
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The relationships among the class, its members, and its objects can be visualized like this:
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Widget Widget() x ~Widget()
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numWidgets() w count 3 y
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The rounded box represents the class itself which contains the three member functions and the data member count. The public 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. 10.13 static FUNCTION MEMBERS Like any ordinary member function, the numWidgets() function in Example 10.2 requires that it be owned by some instance of the class. But since it returns the value of the static 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 static. EXAMPLE 10.16 A static Function Member
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The Widget class maintains a static 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: Widget() { ++count; }
CLASSES
[CHAP. 10
~Widget() { --count; } static int num() { return count; } private: static int count; }; int Widget::count = 0; int main() { cout << "Now there are " Widget w, x; cout << "Now there are " { Widget w, x, y, z; cout << "Now there are } cout << "Now there are " Widget y; cout << "Now there are "
<< Widget::num() << " widgets.\n"; << Widget::num() << " widgets.\n"; " << Widget::num() << " widgets.\n"; << Widget::num() << " widgets.\n";
<< Widget::num() << " widgets.\n"; } Declaring the numWidgets() 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 previous figure showing relationships among the class and its instances should now looks like this:
Widget Widget() x ~Widget()
Widget
numWidgets() w count 3 y
Widget Widget
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 static data from their own class. Review Questions
10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 Explain the difference between a public member and a private member of a class. Explain the difference between the interface and the implementation of a class. Explain the difference between a class member function and an application function. Explain the difference between a constructor and a destructor. Explain the difference between the default constructor and other constructors. Explain the difference between the copy constructor and the assignment operator. Explain the difference between an access function and a utility function. Explain the difference between a class and a struct in C++.
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