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differences in behavior, depending on whether we pass an object itself, or a reference or pointer to the object as we shall now see
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1321 Obtaining a value without knowing the object's type
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Our compare function does the right thing when we call it with a Grad object as an argument because the name function is shared by both Grad and Core objects What if we wanted to compare students, not on the basis of their names, but on the basis of their final grades For example, instead of producing a listing of final grades sorted by name, we might need to produce a listing sorted by final grade As a first cut at solving this problem, we'd write a function that is similar to compare:
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bool compare_grades(const Core& c1, const Core& c2) { return c1grade() < c2grade(); }
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The only difference is that here we're invoking the grade function rather than the name function This difference turns out to be significant! The difference is that Grad redefines the meaning of the grade function, and we have done nothing to distinguish between these two versions of grade When we execute the compare_grades function, it will execute the Core::grade member, just as compare executes Core::name In this case, if we are operating on a Grad object, then the version from Core gives the wrong answer, because the grade functions in Core and Grad behave differently from each other For Grad objects, we must run Grad::grade in order to account for the thesis What we need is a way for compare_grades to invoke the right grade function, depending on the actual type of object that we pass: If c1 or c2 refers to a Grad object, then we want the Grad version of grade; if the object is of type Core, then we want the one from Core We want to make that decision at run time That is, we want the system to run the right function based on the actual type of the objects passed to the function, which is known only at run time To support this kind of run-time selection, C++ provides virtual functions:
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class Core { public: virtual double grade() const; // };
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// virtual added
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We now say that grade is a virtual function When we call compare_grades, the implementation will determine the version of grade to execute by looking at the actual types of the objects to which the references c1 and c2 are bound That is, it will determine which function to run by inspecting each object that we passed as an argument to compare_grades If the argument is a Grad object, it will run the Grad::grade function; if the argument is a Core object, it will run the Core::grade function The virtual keyword may be used only inside the class definition If the functions are defined separately from their declarations, we do not repeat virtual in the definitions Thus, the definition of Core::grade() need not change Similarly, the fact that a function is virtual is inherited, so we need not repeat the virtual designation on the declaration of grade within the Grad class We do have to recompile our code with the new Core class definition Once we have done so, then because the base-class version is virtual, we get the behavior that we need
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1322 Dynamic binding
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This run-time selection of the virtual function to execute is relevant only when the function is called through a reference or a pointer If we call a virtual function on behalf of an object (as opposed to through a reference or pointer), then we know the exact type of the object at compile time The type of an object is fixed: It is what it is, and does not vary at run time In contrast, a reference or pointer to a base-class object may refer or point to a base-class object, or to an object of a type derived from the base class, meaning that the type of the reference or pointer and the type of the object to which a reference or pointer is bound may differ at run time It is in this case that the virtual mechanism makes a difference For example, assume we rewrote compare_grades as follows:
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// incorrect implementation! bool compare_grades(Core c1, Core c2) { return c1grade() < c2grade(); }
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In this version, we say that our parameters are objects, not references to objects In this case, we always know the type of objects represented by c1 and c2: They are Core objects We can still call this function on behalf of a Grad object, but the fact that the argument had type Grad is immaterial In this case, what happens is that what we pass is the base part of the object The Grad object will be cut down to its Core part, and a copy of that portion of the Grad object will be passed to the compare_grades function Because we said that the parameters are Core objects, the calls to grade are statically bound they are bound at compile time to Core::grade This distinction between dynamic binding and static binding is essential to understanding how C++ supports OOP The phrase dynamic binding captures the notion that functions may be bound at run time, as opposed to static bindings that happen at compile time If we call a virtual function on behalf of an object, the call is statically bound that is, it is bound at compile time because there is no possibility that the object will have a different type during execution than it does during compilation In contrast, if we call a virtual function through a pointer or a reference, then the function is dynamically bound that is, bound at run time At run time, the version of the virtual function to use will depend on the type of the object to which the reference or pointer is bound:
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Core c; Grad g; Core* p; Core& r = g; cgrade(); ggrade(); p->grade(); points rgrade(); refers // statically bound to Core::grade() // statically bound to Grad::grade() // dynamically bound, depending on the type of the object to which p // dynamically bound, depending on the type of the object to which r
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The first two calls can be statically bound: We know that c is a Core object, and that at run time, c will still be a Core object Therefore, the compiler can statically resolve this call, even though grade is a virtual function In the third and fourth calls, however, we can't know the type of the object to which p or r refers until run time: They might be Core or Grad objects Hence, the decision as to which function to run in these cases must be delayed until run time The implementation makes that decision based on the type of the object to which p points or to which r refers The fact that we can use a derived type where a pointer or reference to the base is expected is an example of a key concept in OOP called polymorphism This word, from the Greek polymorphos, meaning "of many forms," was already in use in English in the mid-nineteenth century In a programming context, it refers to the ability of one type to stand in for many types C++ supports polymorphism through the dynamic-binding properties of virtual functions When we call a virtual through a pointer or reference, we make a polymorphic call The type of the reference (or pointer) is fixed, but the type of the object to which it refers (or points) can be the type of the reference (or pointer) or
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