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NotQuery *pnq; // set pnq // the new expression invokes the // NotQuery copy constructor NotQuery *pnq2 = new NotQuery( *pnq );
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If we are given a pointer to the abstract Query class, however, allocating a duplicate object is considerably less trivial For example:
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const Query *pq = pnq->op(); // how do we duplicate pq
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If we were able to declare a virtual instance of operator new, the problem would be solved The correct instance of the new operator would be invoked automatically Unfortunately, the new operator cannot be made virtual because it is a static member function applied to raw memory prior to the construction of the class object (see Section 158 for a discussion) Although we cannot make the new operator virtual, we can provide a surrogate new operator to allocate and copy our objects onto the free store This surrogate is generally named clone():
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class Query { public: virtual Query *clone() = 0; // };
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Here is one possible implementation of our NameQuery instance:
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class NameQuery : public Query { public: virtual Query *clone() // invokes the NameQuery copy constructor { return new NameQuery( *this ); } // };
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This works exactly right when the target pointer type is that of a Query*, such as the following:
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Query *pq = new NameQuery( "Valery" ); Query *pq2 = pq->clone();
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It works somewhat less well when the target pointer type is an actual NameQuery* In this case we are required to provide a downcast of the returned Query* pointer back to that of a NameQuery*:
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NameQuery *pnq = new NameQuery( "Rilke" ); NameQuery *pnq2 = static_cast<NameQuery*>( pnq->clone() );
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(The reason the downcast is necessary is explained in Section 1911) Earlier we said there is one exception to the requirement that the return type of the derived class must match exactly that of the base class instance The exception is to support this very case If the base instance of a virtual function returns a class type (or a pointer or a reference to a class type), the derived instance may return a class publicly derived from the class returned by the base class instance (or a pointer or a reference to a class type)
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class NameQuery : public Query { public: virtual NameQuery *clone() { return new NameQuery( *this ); } // };
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Now, the initialization of both pq2 and pnq2 can be achieved without the need of an explicit cast:
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// Query *pq = new NameQuery( "Broch" ); Query *pq2 = pq->clone(); // ok // NameQuery *pnq = new NameQuery( "Rilke" ); NameQuery *pnq2 = pnq->clone(); // ok
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Here is the NotQuery clone implementation:
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class NotQuery : public Query { public: virtual NotQuery *clone() { return new NotQuery( *this ); } // };
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The AndQuery and OrQuery instances are implemented in a similar manner In order for these implementations of clone() to succeed, we must provide explicit NotQuery, AndQuery, and OrQuery copy constructor instances We'll do just that in Section 176 Virtual Functions, Constructors, and Destructors As we saw in Section 174, the order of constructor invocations within a derived class object is to first invoke the base class constructor, then that of the derived class For example, when we define a NameQuery class object, such as
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NameQuery poet( "Orlen" );
the order of constructor invocation is first Query, then NameQuery While the Query base class constructor is executing, the NameQuery portion of poet is uninitialized In effect, the poet is not yet a NameQuery; only its Query subobject has been constructed What should happen if within the base class constructor, a virtual function is invoked for which both a base and a derived class instance is defined Which would be invoked If it were possible for the derived class instance of the virtual function to be invoked, and should it access any of the derived class members, the result of the invocation is formally undefined Informally, the program is likely to crash To prevent this, the virtual instance that is invoked within the base class constructor is always the virtual instance that is active within the base class In effect, inside a base class constructor, the derived class object is an object of the base type The same holds true inside the base class destructor for the derived class object: The derived class portion is also undefined, but this time it is not because it has not been constructed, but because it has already been destroyed