A Constant Reference Wrapper in Java

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532 A Constant Reference Wrapper
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A reference variable must reference an object Sometimes this requirement is unfortunate
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Reference variables in C++ are different from pointer variables in several ways One important difference is that, whereas pointer variables can point at either an object or NULL, a reference variable must reference an object Sometimes this requirement is unfortunate For instance, when we search for an object in an arbitrary container, we may want to return a reference to it But what if the object is not found
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Adapters and Wrappers
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/ / Class that wraps a constant reference variable / / Useful for return value from a container find method
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template <class Object> class Cref
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public: Cref ( ) : obj ( NULL ) { } explicit Cref( const Object const Object
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obj ( &x
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get(
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const
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if ( isNull( ) ) throw NullPointerException( else return *obj;
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boo1 isNull( ) const ( return obj == NULL; private: const Object *obj;
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Constant reference wrapper
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The solution, as usual, is to wrap the behavior of a reference variable inside a class Our class, Cref,shown in Figure 510, mimics a constant reference variable (Alternatively, we could implement a simple reference class and even use inheritance to make the reference and constant reference classes type compatible; we leave this for you to do as Exercise 518) The implementation is short We store a pointer to the referenced obiect as a private data member The pointer is initialized in the constructor but it is NULL if Cref is constructed with no parameters We provide a get method that returns the constant reference and an i sNull method that returns true if the null reference is being represented
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C r e f wraps the Of a reference variable inside a class
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C r e f provides an that isNull returns true if the reference is being represented
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533 Adapters: Changing an Interface
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The adapter pattern is used to change the interface of an existing class to conform to another Sometimes it is used to provide a simpler interface, either with fewer methods or easier-to-use methods At other times it is used
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Theadapter pattern is change the interface of an class to conform to another
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m x s i g n Patterns
1 / / A class for simulating a memory cell
3 template <class Object>
4 class Storagecell : private MemoryCell<Object> 5 { 6 public: 7 explicit Storagecell( const Object & initialvalue 8 = Object( 9 : MemoryCell<Object>( initialvalue ) { 1 10 const Object & get( ) const 11 12 ( return read( ) ; 1 13 void put( const Object & x ) 14 ( write( x ) ; 1 15 1 ;
An adapter class that changes the MemoryCell interface to use get and put
simply to change some method names In either case, the implementation technique is similar For example, in our MemoryCell class in Section 34 we use read and writeBut what if we wanted the interface to use get and put instead There are two reasonable alternatives One is to use composition Doing so, however means, for instance, that a call to get will then call read,thus adding an extra layer of overhead The other alternative is to use private inheritance We use private inheritance to implement the new class, storagecell, in Figure 51 1 Its methods are implemented by calls to the base class methods As discussed in Section 445, in private inheritance, public methods in the base class are private in the derived class Thus, as Figure 512 illustrates, the only visible methods are the storagecell constructor, get,and
put
lterators
An iterator is the Object that iteration of a collection
Consider the problem of printing the elements in a collection Typically, the collection is an array, so assuming that the object v is an expanded vector template, its contents are easily printed with code like
for( int i = 0; i < vsize( cout << v [ i I < < endl;
Iterators
1 int main( ) 2 i 3 StorageCell<int> ml; 4 StorageCell<string> m2 ( "hello" ) ; 5 6 mlput( 3 7 ) ; 7 m2put( m2get( ) + " world" ) ; 8 cout << mlget( ) << end1 << m2get(
< c endl;
12 13 1
/ / The next line does not compile if uncornrnented / / cout < < mlread( ) << endl;
return 0;