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These changes to getIterator give the implementation that replaces lines 18-19 of Figure 517
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auto-ptr<Iterator<Object> > getIterator( ) const { return a u t o q t r < I t e r a t o r < O b j e c t > > ( new VectorIterator<Object>( this ) ) ;
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The construction of an auto- ptr<I terator<Object> >, with a Vec tor1terator<Objec tz pointer, implies that we need the STL version
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Composite (Pair)
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A common design pattern is to return two objects as a pair
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In most languages, a function can return only a single object What do we do if we need to return two or more objects There are two possibilities: One is to use reference variables; the other is to combine the objects into a single struct (or class) The most common situation in which multiple objects need to be returned is the case of two objects So a common design pattern, the Composite pattern, stores two objects in one entity We return them as a pair In addition to the situation just described, pairs are useful for implementing maps and dictionaries In both these abstractions, we maintain key-value pairs: Pairs are added to the map or dictionary; we then search for a key, returning its value One common way to implement a map is to use a set, in which we have a collection of items and search for a match If the items are pairs-and the match criterion is based exclusively on the key component of the pair-we can easily write an adapter class that constructs a map on the basis of the set We explore this idea in more detail in 19 The STL defines a pair class An implementation is shown in Figure 520 Note that it is a class only in the technical sense; the data members are public
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Pairs are useful for implementing keyvalue pairs in maps and dictionaries
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The STL defines a
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pair class
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Observer
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The Observer pattern involves a subject and a set of observersThe observers are informed whenever something interesting happens to the subject
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Our last pattern is the Observer pattern, the use of which involves a subject and a set of observers; the observers are informed whenever something interesting happens to the subject The number of observers can vary as the application runs and can be large A simplistic example of an Observer pattern is a windowing operating system, such as Windows or Mac 01s When a window is created or maximized, it covers other windows The windows that are covered are now observers in the sense that they want to be informed when the newly created
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/ / Class (more like a struct) that stores / / a pair of objects
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template <class Typel, class Type2> class pair public: Type1 first; Type2 second; pair( const Type1 & f = Typel( const Type2 & s = Type2 ( : first( f ) , second( s ) { }
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The pair class (basically the same as the STL version)
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The Observer abstract class specifies a protocol: when something interesting happens, Observer is told about it by having its update method called The Subject class defines meth0ds add an observer, remove an observer, and notify all observers that something has happened
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window is minimized or destroyed-or simply moved-as this action might require that the previously hidden windows b e redrawn Another example might be a class that wraps pointers Several wrapper objects may be sharing a pointee that is pointing at some dynamically allocated object If one instance of the wrapper calls delete on the dynamically allocated object, the other wrappers have stale pointers, which gives undefined behavior that can be hard to detect A solution is that when the delete is performed, we inform all the wrappers that are looking at that deleted object, perhaps setting their pointees to NULL to ensure defined behavior Figure 521 contains an abstract base class Observer and a base class (which could be abstract, but is not) for the observee Subject The Observer abstract class specifies a protocol: when something interesting happens, Observer is told about it by having its update method called A subclass can override update to handle the interesting occurrence For instance, in our windowing example, the observer windows will have their u ~ d a t e method called when the window that was coverinrr it is no longer in the way The update method could redraw an appropriate portion of the screen The Subject class is not abstract (but see Exercise 57) Instead it defines methods to add an observer, remove an observer, and notify all observers that something has happened It does so by keeping an internal list of all registered observers (possibly in a vector) The implementations of addobserver and removeobserver are in the online code An illustration of the Observer pattern in action is shown in Figure 522 There we have a subject, which is a Timer object, and we have ~ c h o objects, which are the interested observers The Timer object has a tick method; whenever the tick method is called, any ~ c h objects that are o