Exercise 1712 in Java

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Exercise 1712
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Within the NameQuery object, the most straightforward internal representation of the location vector is a pointer initialized with the pointer stored within the text location map This is also the most efficient because we'd be copying a single address rather than each location pair within the vector The AndQuery, OrQuery, and NotQuery classes must construct their location vectors based on the evaluation of their operands When the lifetime of one of these class objects ends, the associated location vector must be deleted When the lifetime of a NameQuery object ends, the location vector must not be deleted How might we store the location vector as a poin er within the Query base class, delete the instances associated with the AndQuery, OrQuery, and NotQuery class objects but not that of the NameQuery class objects (Note that we are not permitted to add a flag to the Query class indicating whether or not to delete the pointer to the location vector!)
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Exercise 1713
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What is wrong with the following class definition
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class AbstractObject { public: ~AbstractObject(); virtual void doit() = 0; // };
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Exercise 1714
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Why is it that, given
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NameQuery nq( "Sneezy" ); Query q( nq ); Query *pq = &nq;
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the invocation
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pq->eval();
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invokes the NameQuery instance of eval(), whereas
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qeval();
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invokes the Query instance
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Exercise 1715
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Which virtual function redeclarations of the Derived class are in error
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(a) Base* Base::copy( Base* ); Base* Derived::copy( Derived* ); (b) Base* Base::copy( Base* ); Derived* Derived::copy( Base* ); (c) ostream& Base::print( int, ostream&=cout ); ostream& Derived::print( int, ostream& ); (d) void Base::eval() const; void Derived::eval();
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Exercise 1716
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In practice, our programs are unlikely to run correctly the first time we exercise them and the first time we exercise them against real data It is often useful to incorporate a debugging strategy into the design of our classes Implement a debug() virtual function for our Query class hierarchy that displays the data members of the respective classes Support a level of detail control as (a) an argument to the debug() function and (b) as a class data member (The latter allows individual class objects to turn on or turn off the display of debugging information)
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Exercise 1717
What are likely errors in the following inheritance hierarchy
class Object { public: virtual void doit() = 0; // protected: virtual ~Object(); }; class MyObject : public Object { public: MyObject( string isA );
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string isA() const; protected: string _isA; };
Memberwise Initialization and Assignment
One of our responsibilities in designing a class is to make sure the class behaves correctly and efficiently under memberwise initialization (introduced in Section 146) and memberwise assignment (introduced in Section 147) In this section we consider these operations under inheritance As of yet, we have not provided any explicit handling of memberwise initialization Let's walk through what happens by default with our Query class hierarchy The abstract Query base class defines three nonstatic data members:
class Query { public: // protected: int _paren; set<short> *_solution; vector<location> _loc; // };
_solution, if set, addresses a set allocated on the free store by the _vec2set() member function The Query destructor applies the delete expression to _solution The Query class needs to provide both an explicit copy constructor and explicit copy assignment operator (If this is not apparent to you, please review Section 146) Before we provide these, however, let's first step through default memberwise copy in their absence The NameQuery derived class contains a string member class object and a Query base class subobject Given the following NameQuery object, folk,
NameQuery folk( "folk" );
the initialization of music with folk
NameQuery music = folk;
causes the following steps to occur
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1 The compiler checks to see if the NameQuery class defines an explicit instance of a copy constructor It does not Therefore, the compiler prepares to apply default memberwise initialization 2 The compiler next checks to see if the NameQuery class contains any base class subobjects Yes, it contains a Query base class subobject 3 The compiler checks to see if the Query base class defines an explicit instance of a copy constructor It does not Therefore, the compiler prepares to apply default memberwise initialization 4 The compiler checks to see if the Query class contains any base class subobjects No, it does not 5 The compiler examines each nonstatic member of the Query class in order of declaration If the member is a nonclass object, such as _paren and _solution, it initializes the music object member with the value of folk's member If the member is a class object, such as _loc, it applies step 1 recursively Yes, the vector class defines an explicit instance of a copy constructor The copy constructor is invoked to initialize music_loc with folk_loc 6 The compiler next examines each nonstatic member of the NameQuery class in order of declaration The string member class object is recognized as providing an explicit copy constructor It is invoked to initialize music_name with folk_name The default initialization of music with folk is now complete It is well behaved except for the default copy of _solution that, if permitted, is likely to cause our program to fail We override the default handling by providing an explicit Query class copy constructor One solution is to copy the entire solution set, such as the following:
Query::Query( const Query &rhs ) : _loc( rhs_loc ), _paren(rhs_paren) { if ( rhs_solution ) { _solution = new set<short>; set<short>::iterator it = rhs_solution->begin(), end_it = rhs_solution->end(); for ( ; it != end_it; ++it ) _solution->insert( *it ); } else _solution = 0; }
However, because our implementation of the solution set is to have it calculated on demand, there is really no imperative to copy it now The purpose of our copy constructor is to prevent its default copy It's sufficient to initialize _solution to 0:
Query::Query( const Query &rhs ) : _loc( rhs_loc ), _paren(rhs_paren), _solution( 0 ) {}
The initialization of music with folk follows the same steps 1 and 2 Step 3 now finds that Query defines an explicit copy constructor The copy constructor is invoked Steps 4 and 5 are no longer carried out Step 6 is carried out as before
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