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to read, the call to grade is a virtual call, so the right version of grade is automatically invoked to calculate the grade, including a thesis if the object is a Grad and not otherwise The final change is to remember to return to the implementation the space that the object consumed, which we do by calling delete on the pointer that students[i] contains
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1332 Virtual destructors
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Our program almost works The only problem occurs when we delete the objects, as we do inside the output loop When we allocated these objects, we allocated both Grad and Core objects, but we stored pointers to these objects as Core*, and not as Grad* pointers Thus, when we delete them, we are deleting a pointer to Core, and never a pointer to Grad, even if the pointer actually points to a Grad object Fortunately, this problem is easily fixed When we call delete on a pointer, two things happen: The destructor is run on the object, and the space that held the object is freed When the program deletes the pointer in students[i], it could be pointing at either a Core object or a Grad object Neither Core nor Grad explicitly defined a destructor, which means that when the delete runs, it will invoke the synthesized destructor and then return the space that the object consumed The synthesized destructor will run the destructor for each data element in the class But when the delete is executed, which destructor should the system run Should the destructor destroy the members of a Core or a Grad And when the space is freed, how much space should be returned enough to hold a Core or a Grad These questions sound like the kind that the virtual mechanism can resolve and indeed it can In order to have a virtual destructor, the class must have a destructor, which we can then make virtual:
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class Core { public: virtual ~Core() { } // as before };
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Now when we execute delete students[i], the destructor that will be run will depend on the type of object to which students[i] actually points Similarly, the type of the memory that we return to the system will be determined by the type to which students[i] actually points Note that the body of the destructor is empty The only work needed to destroy a Core is to destroy its members, and the system does this work automatically Empty, virtual destructors are not uncommon A virtual destructor is needed any time it is possible that an object of derived type is destroyed through a pointer to base If there is no other reason for the destructor to be defined, then that destructor has no work to do and should be empty There is no need to update the Grad class to add a destructor As with all virtual functions, the fact that the destructor is virtual is inherited Because neither class has any explicit work to do in order to destroy objects, there is no need to redefine the destructor in the derived class Because the derived class inherits the virtual property of its base-class destructor, all we have to do is recompile the program
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134 A simple handle class
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Although the approach that we have just seen is straightforward, it has problems: The program has acquired a lot of extra complexity related to managing the pointers, and has added several pitfalls that could lead to bugs Our users have to remember to allocate space for the records as they read them, and to remember to free that space when they no longer need the data The code is constantly dereferencing the pointers to get at the underlying objects Nevertheless, we have solved the problem of writing a program that can read a file that contains both kinds of records intermixed What we'd like to do is find a way to preserve the good properties of our simpler programs that dealt with either Core objects or Grad objects, and eliminate the problems inherent in our new solution, which can process both kinds of records It turns out that there is a common programming technique, known as a handle class, that will let us do so Our code became cluttered when we realized that we needed to be able to deal with objects whose type we could not know until run time We knew that each object would be either a Core, or something derived from Core Our solution used pointers, because we could allocate a pointer to Core and then make that pointer point to either a Core or a Grad object The trouble with our solution is that it imposed error-prone bookkeeping on our users We can't eliminate that bookkeeping, but we can hide it from our users by writing a new class that will encapsulate the pointer to Core:
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class Student_info { public: // constructors and copy control Student_info(): cp(0) { } Student_info(std::istream& is): cp(0) { read(is); } Student_info(const Student_info&); Student_info& operator=(const Student_info&); ~Student_info() { delete cp; } // operations std::istream& read(std::istream&); std::string name() const { if (cp) return cp->name(); else throw std::runtime_error("uninitialized Student"); } double grade() const { if (cp) return cp->grade(); else throw std::runtime_error("uninitialized Student"); } static bool compare(const Student_info& s1, const Student_info& s2) { return s1name() < s2name(); } private: Core* cp; };
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The idea here is that a Student_info object can represent either a Core or a Grad In that sense, it will act like a pointer However, users of Student_info do not have to worry about allocating the underlying object to which the Student_info is bound The class will take care of these tedious and error-prone aspects of our programs Each Student_info object will hold a pointer, called cp, that points to an object that has either type Core or a type derived from Core As we'll see in 1341/245, in the read function we'll allocate the object to which cp points Therefore, both constructors initialize cp to 0, indicating that the Student_info object is as yet unbound In the
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