This is a good question, and the answer is part of the fundamental philosophy of C++: Because its not quite as efficient.You can see from the previous assembly-language output that instead of one simple CALL to an absolute address, there are two more sophisticated assembly instructions required to set up the virtual function call. Declaration virtual funct_name (parameter_list) {. The VPTR points to the starting address of the VTABLE. This is also pushed on the stack because it is the starting address of the object of interest. It cannot safely create an object of an abstract class, so you get an error message from the compiler. You can recognize a pure virtual function because it uses thevirtualkeyword and is followed by= 0. In each class with virtual functions, it secretly places a pointer, called thevpointer(abbreviated as VPTR), which points to the VTABLE for that object. This example required at least one data member. Thus, it is also a way to preventobject slicing(which will be described shortly). Note that, in this example, ifdescription()were made into a pure virtual function in the base class (which is not unreasonable, since it doesnt really do anything in the base class), then the compiler would prevent object slicing because that wouldnt allow you to create an object of the base type (which is what happens when you upcast by value). A virtual function call, however, reaches forward or outward into the inheritance hierarchy. Thats fine if you are starting your class from scratch, but what if you already have a class such asstringthat you want to be able to put onto theStack? It can be helpful to see the assembly-language code generated by a virtual functioncall, so you can see that late-binding is indeed taking place. You also saw a problem arise, which is embodied in the following code: The functiontune()accepts (byreference) anInstrument, but also without complaint anything derived fromInstrument. There are some less-common cases in which you may know that the pointer actually points to an object of a specific subclass. The output isInstrument::play. If a function is declared asvirtualin the base class, it isvirtualin all the derived classes. The pure virtual function has no definition in the base class, and all the inheriting derived classes has to redefine it. A pure virtual function is a virtual function that has no definition within the class. If a more-derived constructor is called afterwards, that constructor sets the VPTR toitsVTABLE, and so on, until the last constructor finishes. This brings up an interesting question: What is the point of a pure virtual destructor? AllInstrumentobjects or objects derived fromInstrumenthave their VPTR in the same place (often at the beginning of the object), so the compiler can pick the VPTR out of the object. The response of the intellectual left to Russia's war against Ukraine has largely involved blaming NATO, an aversion to any kind of military involvement, and generic calls to pacifism. So the VPTR it uses must be for the VTABLEofthatclass. There is a distinct difference between passing the addresses of objects and passing objects by value when using polymorphism. Sound familiar? This is an important restriction because the compiler must guarantee that you can polymorphically call the function through the base class, and if the base class is expecting anintto be returned fromf(), then the derived-class version off()must keep that contract or else things will break. Often you will want to call a base-class version of a function, even if it is virtual. Because the class has nothing but pure virtual functions, we call it apure abstract class: Pure virtual functions are helpful because they make explicit the abstractness of a class and tell both the user and the compiler how it was intended to be used. To see it, heres an example to examine the sizes of classes that use virtual functions compared with those that dont: With no virtual functions, the size of the object is exactly what youd expect: the size of a single[55]int. But in this case, all those functions have been declared with thevirtualkeyword, so something different happens. But what happens when you inherit and add new virtual functions in thederivedclass? So youll always see one more than the number of arguments pushed on the stack before a member function call (except forstaticmember functions, which have nothis). The following sections go into this process in more detail. For example, consider a system that deals with matrices, vectors and scalar values, all three of which are derived from classMath: For simplicity, only theoperator*has been overloaded. As you might guess, since theres such a thing asupcasting moving up an inheritance hierarchy there should also bedowncastingto move down a hierarchy. The avoidance of downcasting is covered in Volume 2 of this book.). First, becausedescribe()accepts aPetobject(rather than a pointer or reference), any calls todescribe()will cause an object the size ofPetto be pushed on the stack and cleaned up after the call. Heres how it might work: In this program, a new feature is used that is not fully described until Volume 2 of this book, where a chapter is given to the topic: C++srun-time type information(RTTI) mechanism. You can see that there is no explicit type information stored in any of the classes. A virtual function makes its class apolymorphic base class. So the address is fetched and called all at once in the statement. To cause late binding to occur for a particular function, C++ requires that you use thevirtualkeywordwhen declaring the function in the base class. In theInstrumentexamples, the compiler creates a default constructor that does nothing except initialize the VPTR. Because it has been declared virtual in your BaseClass. First, as simply a better C, because C++ forces you to declare all functions before using them and is much pickier about how variables are used. Its not transparent at first, but if you stare at the example for awhile it should begin to make sense. In a normal member function, construction has already taken place, so all the members of all parts of the object have been built. Encapsulation creates new data types by combining characteristics and behaviors. Theres more to RTTI thantypeid, and you can also imagine that it would be fairly easy to implement your own type information system using a virtual function. The compiler creates a new VTABLE for your new class, and it inserts your new function addresses using the base-class function addresses for any virtual functions you dont override. The compiler places the addresses of the virtual functions for that particular class in the VTABLE. In addition, it can be manipulated through an address of the base type. A dummy member is inserted into objects that would otherwise be zero-sized. public: virtual void f () =0; // =0 means pure virtual. The line. syntax: class B {. Pure virtual functions are used to create "interface . The fact that the following compiles and links is the proof: Normally, a pure virtual function in a base class would cause the derived class to be abstract unless it (and all other pure virtual functions) is given a definition. To use polymorphism and thus, object-oriented techniques effectively in your programs you must expand your view of programming to include not just members and messages of an individual class, but also the commonality among classes and their relationships with each other. Virtual Function vs Pure Virtual Function. However, since an upcast is always safe and its a common thing to do, I consider anexplicit cast for upcasting to be cluttering and unnecessary. However, theyre also the turning point in the understanding of object-oriented programming. Youll typically want a situation thats more deterministic than in the example above before usingstatic_castrather thandynamic_cast(and, again, you want to carefully examine your design before usingdynamic_cast). Remember that the starting address corresponds to the value ofthis, andthisis quietly pushed on the stack as an argument before every member function call, so the member function knows which particular object it is working on. Thus, the compiler guarantees the purity of the abstract class. At this point in the function, the registersi(part of the Intel X86 processor architecture) contains the address ofi. This is usually the case with mathematical components (for which you often overload operators). By making a class abstract, you can ensure that a pointer or reference is always used during upcasting to that class. public abstract void MyFunction();. Connecting a function call to a function body is calledbinding. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions. There is only one VPTR for each object when using simple inheritance like this. One way or another, for every object that can be created (that is, its class has no pure virtuals) theres always a full set of function addresses in the VTABLE, so youll never be able to make a call to an address that isnt there (which would be disastrous). Abstract classes cannot be instantiated. When you make a virtual function call through a base-class pointer (that is, when you make a polymorphic call), the compiler quietly inserts code to fetch the VPTR and look up the function address in the VTABLE, thus calling the correct function and causing late binding to take place. And as described in Chapter 14, if you dont explicitly create a constructor for a class, the compiler will synthesize one for you. If the class has virtual functions, the synthesized constructor will include the proper VPTR initialization code. Normally, the action of the destructor is quite adequate. This is another reason why theconstructors are called in order from base to most-derived. Creating containers that holdObjects is not an unreasonable approach ifyou have a singly-rooted hierarchy (enforced either by the language or by the requirement that every class inherit fromObject). In all other functions, onlythatfunction will be called (and not base-class versions), whether its virtual or not. A diagram will help you visualize whats happening. The other benefit to this feature is that it allows you to change from an ordinary virtual to a pure virtual without disturbing the existing code. Derived classes can override virtual functions. (You can see this in theadjustentry in theBrassVTABLE.) But notice that in this design the process is effectively the same as usingdynamic_cast, and the client programmer must do some testing to discover the cast that was actually successful. One solution was presented in the exampleC14:InheritStack.cpp, in which theStackwas inherited into a new class that accepted and produced onlystringpointers. This means that if an object of a class inherited fromPetis passed todescribe(), the compiler accepts it, but it copies only thePetportion of the object. Because of the tight class modularization in C++, it isnt a large problem when this occurs because changes you make in one part of a system tend not to propagate to other parts of the system as they do in C. How can late bindinghappen? By making a class abstract, you ensure that the client programmer cannot misuse it. In Chapter 14, you saw that redefining an overloaded function in the base classhides all of the other base-class versions of that function. In addition, with constructors that must have hidden code inserted by the compiler, a preprocessor macro wouldnt work at all. . When you run the following example, you can see that the pure virtual function body is called after the derived class version, just as with any other destructor: As a guideline, any time you have a virtual function in a class, you should immediately add a virtual destructor (even if it does nothing). https://msdn.microsoft.com/en-us/library/aa664435. As you might guess, because the constructor has the job of bringing an object into existence, it is also the constructors job to set up the VPTR. Linkobjects now hold pointers toObjectrather thanvoidpointers, and theStackwill only accept and returnObjectpointers. Inmain(), you can see this happening as aWindobject is passed totune(), with nocast necessary. A special thing of pure virtual functions is: if a class contains at least one pure virtual function, of does not override
Polymorphism completes the object-oriented features of the language, but there are two more major featuresin C++: templates (which are introduced in Chapter 16 and covered in much more detail in Volume 2), and exception handling (which is covered in Volume 2). However, even if you dont discover the proper base class interfaces and virtual functions at the initial creation of the program, youll often discover them later, even much later, when you set out to extend or otherwise maintain the program. But understanding this war involves recognizing some realities that the intellectual left finds uncomfortable: NATO is not the aggressor, Russia is; Ukrainians are driven by a form of nationalism; and there is . Inside the constructor, however, you must be able to assume that all members that you use have been built. The VPTR contained inbxpoints to the starting address of the VTABLE, but the function pointer to call isnt at location zero of the VTABLE, but instead at location two (because its the third function in the list). If there had been no data members, the C++ compiler would have forced the objects to be a nonzero sizebecause each object must have a distinct address. Also, the fact thatsomedestruction is occurring can further mask the problem. Just because one pure virtual function prevents the VTABLE from being completed doesnt mean that you dont want function bodies for some of the others. The main difference between virtual function and pure virtual function is that virtual function has its definition in the parent class and the child class which is inheriting it can redefine the definition whereas a pure virtual function will not have any definition and all the inherited child classes must give a new definition. The virtual function allows one type to express its distinction from another, similar type, as long as theyre both derived from the same base type. If some member function vf is declared as virtual in a class Base, and some class Derived, which is derived, directly or indirectly, from Base, has a declaration for member function with the same. The only way to guarantee this is for the base-class constructor to be called first. This makes sense because the constructor has a special job: to see that the object is built properly. This was a nice solution, but it requires you to inherit a new container class for each type that you want to hold in the container. If you want to call a function that only exists in that subclass, then you must cast the pointer. If you make a lot of inline constructor calls, your code size can grow without any benefits in speed. The problem is that you want the container to hold more than one type, but you dont want to usevoidpointers. What happens if youre inside a constructor and you call a virtual function? As a procedural programmer, you have no referent with which to think about virtual functions, as you do with almost every other feature in the language. You should keep in mind that constructors and destructors are the only places where this hierarchy of calls must happen (and thus the proper hierarchy is automatically generated by the compiler). It doesn't have a derived class. Of course, you probably wont make all tiny constructors non-inline right away, because theyre much easier to write as inlines. To do this, the containers have heldvoidpointers and so they havent known the type of object theyve held. This is true, but the type information is hidden. Youre still telling the compiler not to allow objects of that abstract base class, and the pure virtual functions must still be defined in derived classes in order to create objects. You dont ever want to be able to make a call to a virtual function before the VPTR is properly initialized. In C++, it was thought that the enforced use of this common base class would cause too much overhead, so it was left out. And do JAVA and C# have pure virtual function too? That is, you dont want anyone to actually create an object of the base class, only to upcast to it so that its interface can be used. It is much better to catch the problem at compile time. This means you must be able to assume that all the members of the base class are valid when youre in the derived class. The rule shown in Chapter 14 still works: if you override one of the overloaded member functions in the base class, the other overloaded versions become hidden in the derived class. When you usedynamic_castto try to cast down to a particular type, the return value will be a pointer to the desired type only if the cast is proper and successful, otherwise it will return zero to indicate that this was not the correct type. It's as if that private virtual function is not even there. class, so instance creation is impossible. No implementation need be provided for the pure virtual function in the base class but it must be overridden in a derived class if you want to create objects of that type. This is acceptable; the interface inInstrumentmust exist inWind, becauseWindis publicly inherited fromInstrument. The reason forinlinefunctions is to reduce the calling overhead for small functions. This kind of mechanical organization makes ready sense to someone with a procedural programming background. But destructors are called from the outside in (from the most-derived destructor down to the base destructor), so the actual function called would rely on portions of an object that havealready been destroyed! In that case, everything is guaranteed to be anObjectand so its not very complicated to use the containers. When an object containing virtual functions is created, its VPTRmust be initialized to point to the proper VTABLE. When an abstract class is inherited, all pure virtual functions must be implemented, or the inherited class becomes abstract as well. A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class if the derived class is not abstract. Then it places the VPTR(discovered inSizes.cpp) into the class. Even though the destructor, like the constructor, is an exceptional function, it is possible for the destructor to be virtual because the object already knows what type it is (whereas it doesnt during construction). It provides another dimension of separation of interface from implementation, to decouplewhatfromhow. In all of the instrument examples, the functions in the base classInstrumentwere always dummy functions. Heres a simple example: The classPetcontains a two virtual functions:speak()andname(). Each destructor knows what its class is derivedfrom, but not what is derived from it. . Sign in|Recent Site Activity|Report Abuse|Print Page|Powered By Google Sites, virtual function and pure virtual function, // if Base::f() is virtual and Derived overrides it, Derived::f() will be called. Why not just upcast all the return types toPetFood*, just asBird::eats()did? That is,Bird::eats()upcasts theBirdFoodto aPetFood. Withplay()defined asvirtualin the base class, you can add as many new types as you want without changing thetune()function. ACircleobject is created and the address is upcast to aShapepointer; the second version of the expression shows how you can usestatic_castto be more explicit about the upcast. Inmain(), you might expect the first call to produce This is Alfred, and the second to produce Fluffy likes to sleep. In fact, both calls use the base-class version ofdescription(). Pure Virtual Function in c++ The virtual function that is only declared but not defined in the base class is called the pure virtual functions. This is typically a good solution, but at the end ofmain(), you see the difference:Cat::eats()can return the exact type ofPetFood, whereas the return value ofBird::eats()must be downcast to the exact type. Instead, the compiler resolves the calls at compile-time and calls only the local version of the function. Under all interpretations, the object truly becomes aPetduring slicing. HeresInstrument4.cppmodified to use pure virtual functions. Dont be fooled; if it isnt late binding, it isnt polymorphism. The redefinition of avirtualfunction in a derived class is usually calledoverriding. Because of this, upcasting into an object is not done often; in fact, its usually something to watch out for and prevent. Theres something that happens during destruction that you might not immediately expect. With C++, you have the aid of the compiler. The second step is object-based C++. Theres no explicit copy-constructor here, so the compiler synthesizes one. Fortunately, the compiler takes care of all the bookkeeping for you and ensures that all the function pointers in all the VTABLEs of a particular class hierarchy occur in the same order, regardless of the order that you may override them in derived classes. (What that's good for is debatable.). The fact that the return type is inherited from the return type of the base-class function is the only reason this compiles. As explained before, this is the goal when using polymorphism code that manipulates a base type can transparently manipulate derived-type objects as well. Virtual activity is the activity of a foundation phase member that can be redefined in acquired classes. The pure virtual function in c++ simply tells the compiler that the function is pure. You need only one table because all the virtual function addresses are contained in that single table. The only distinction occurs when you have a class that only has a single pure virtual function: the destructor. For efficiencys sake, most compilers will perform early bindingwhen they are making a call to a virtual function for an object because they know the exact type. The function definition takes place in the derived class. Now the actual virtual function call must be performed. The order of the constructor calls is important. Dogadds a third virtual function calledsit(), as well as overriding the meaning ofspeak(). To get the desired behavior fromInstrument2.cpp, simply add thevirtualkeyword in the base class beforeplay(): This file is identical toInstrument2.cppexcept for the addition of thevirtualkeyword, and yet the behavior is significantly different: Now the output isWind::play. A class with such a function becomes abstract and cant be made an instance of. name It is only used as a placeholder and does not contain any function definition (do-nothing function). Such a program isextensiblebecause you can add new functionality by inheriting new data types from the common base class. You only need to look for functions that can be made non-virtual when searching for ways to speed up your code (and there are usually much bigger gains to be had in other areas a good profiler will do a better job of finding bottlenecks than you will by making guesses). If you upcast to an object instead of a pointer or reference, something will happen that may surprise you: the object is sliced until all that remains is the subobject that corresponds to the destination type of your cast. You cannot use thevirtualkeyword withconstructors, but destructors can and often must be virtual. For example: ThePet::eats()member function returns a pointer to aPetFood. When the compiler generates code for that constructor, it generates code for a constructor of that class, not a base class and not a class derived from it (because a class cant know who inherits it). Its also easy to feel like youre creating data types you make classes and objects, you send messages to those objects, and everything is nice and neat. Because the virtual function is intimately bound with the concept of type, and type is at the core of object-oriented programming, there is no analog to the virtual function in a traditional procedural language. Although this requires significant effort, its a worthy struggle, because the results are faster program development, better code organization, extensible programs, and easier code maintenance. If it uses the virtual mechanism for function calls, it will produce only a call through its own VTABLE, not the most-derived VTABLE (as would be the case afterallthe constructors were called). One of the main reasons for the invention of C++ was to make C programmers more efficient. With a single virtual function inOneVirtual, the size of the object is the size ofNoVirtualplus the size of avoidpointer. A pure virtual function makes it so the base class can not be instantiated, and the derived classes are forced to define these functions before they can be instantiated. This way, you ensure against any surprises later. You can get stuck at the object-based level because you can quickly get there and you get a lot of benefit without much mental effort. Castinguphappens automatically, with no coercion, because its completely safe. That is, when you upcast you are always clearly derived from an ancestor class (typically only one, except in the case of multiple inheritance) but when you downcast there are usually several possibilities that you could cast to. This is calledmultiple inheritanceand it is the subject of an entire chapter in Volume 2 of this book (downloadable fromwww.BruceEckel.com). At this point you may have a question: If this technique is so important, and if it makes the right function call all the time, why is it an option? These are mentioned as pure as implementation would differ based on derived classes but are a must. In this situation, however, everything is simple enough that we dont trip across any multiple inheritance pitfalls: Although this is similar to the previous version of the test program forStack, youll notice that only 10 elements are popped from the stack, which means there are probably some objects remaining. That is, the dynamic type of the object is used instead of its static type: Derived d; Base& rb = d; // if Base::f () is virtual and Derived overrides it, Derived::f () will be called. Youve seen in this chapter that its impossible to understand, or even create, an example of polymorphism without using data abstraction and inheritance. Here, the base class contains a virtual destructor and that suffices. This is a tool that allows you to enforce a particular design. Once an object has been constructed, its VPTR is initialized, so virtual function calls can take place. If you stop here, youre missing out on the greatest part of the language, which is the jump to true object-oriented programming. That pointer might point to some other type, which doesnt have asit()function. The problem in the program above is caused by early binding because the compiler cannot know the correct function to call when it has only anInstrumentaddress. People are often confused by other, non-object-oriented features of C++, like overloading and default arguments, which are sometimes presented as object-oriented. The problem when using containers is that they need to be flexible enough to hold different types of objects. That is, the destructor starts at the most-derived class and works its way down to the base class. The goal is to be able to multiply any twoMathobjects and produce the desired result and note that multiplying a matrix by a vector is a very different operation than multiplying a vector by a matrix. Then it would be possible for the virtual call to resolve to a function that was farther out (more derived) on the inheritance hierarchy than the current destructor. Yes I missed ; at the end of class definition. Why is this? Because the VPTR determines the virtual function behavior of the object, you can see how its critical that the VPTR always be pointing to the proper VTABLE. This seems counterintuitive; how can a virtual function be pure if it needs a function body? But, for starters, you are missing semi-colons at the end of your class definitions. You can imagine what happens when you perform inheritance and override some of the virtual functions. Virtual Function. It turns out that there are many other benefits to using a singly-rooted hierarchy; in fact, every other object-oriented language but C++ enforces the use of such a hierarchy when you create a class, you are automatically inheriting it directly or indirectly from a common base class, a base class that was established by the creators of the language. Thus, instead of saying, Call the function at the absolute locationInstrument::adjust (early binding; the wrong action), it generates code that says, in effect, Call the function at VPTR+2. Because the fetching of the VPTR and the determination of the actual function address occur at runtime, you get the desired late binding. fetches the word thatsi(that is,this)points to, which is the VPTR. So a virtual function is basically a member function of a class that is declared within the base class. Some object-oriented languages have taken the approach that late binding is so intrinsic to object-oriented programming that it should always take place, that it should not be an option, and the user shouldnt have to know about it. rb.f(); A pure virtual function is a virtual function whose . This has several implications. In C++, however, you cannot expect this from every class, so youre bound to trip over multiple inheritance if you take this approach. So whats the difference between a regular virtual destructor and a pure virtual destructor? More specifically, aCircleis a type ofShape(thats the upcast), but if you try to downcast aShapeit could be aCircle,Square,Triangle, etc. Each time you create a class that contains virtual functions, or you derive from a class that contains virtual functions, the compiler creates a unique VTABLEfor that class, seen on the right of the diagram. When designing your classes, however, you shouldnt be worrying about efficiency tuning. In Chapter 14, you saw how inheritance allows the treatment of an object as its own typeorits base type. If you dont override a function that was declared virtual in the base class, the compiler uses the address of the base-class version in the derived class. Unlike an ordinary pure virtual function, youmustgive it a function body. If the pointer is to the base class, the compiler can only know to call the base-class version of the destructor duringdelete. On the other hand, by making a base class virtual function private, only gives the derived class the ability to override the function, but I see no benefit of this. This requires both code space and execution time. You can do this only with virtual functions. InBird, this member function is overloaded exactly as in the base class, including the return type. At the same time, a pure virtual function forces inherited classes to provide a definition for it. Forgetting to make a destructorvirtualis an insidious bug because it often doesnt directly affect the behavior of your program, but it can quietly introduce a memory leak. If youre inside an ordinary member function and you call a virtual function, that function is called using the late-binding mechanism. Deleting avoidpointer doesnt call the destructor, so the container couldnt be responsible for cleaning up its objects. When you refer to a derived class object using a pointer or a reference to the base class, you can call a virtual function for that object and execute the derived class's version of the function. Instead, it should automatically install all the mechanisms necessary to perform late binding. In that table it places the addresses of all the functions that are declared virtual in this class or in the base class. It will call the default constructor if you dont explicitly call a base-class constructor in the constructor initializer list. Youre saved from disaster because the object is being passed by value. To solve the ownership problem, we can create an extremely simpleObjectfor the base class, which contains only a virtual destructor. Why do I even need to know about it?. An abstract method in Java can be considered as a pure virtual function. Virtual Function It has its own definition inside the class. In addition, inline functionswould not be possible, because virtual functions must have an address to put into the VTABLE. All base-class constructors are always called in the constructor for an inherited class. The base class can override a virtual function. a pure virtual function (or abstract function) in c++ is a virtual function for which we can have implementation, but we must override that function in the derived class, otherwise the derived class will also become abstract class (for more info about where we provide implementation for such functions refer to this However, the compiler doesnt know that they are anything more thanInstrumentobjects, so left to its own devices it would normally call the base-class versions of all the functions. The hierarchy of constructor calls brings up an interesting dilemma. What is the difference between virtual function and pure virtual function in C++? Also, you should not assume that the pointer will be exactly the same, because sometimes pointer adjustments take place during upcasting and downcasting (in particular, with multiple inheritance). Upcasting fromWindtoInstrumentmay narrow that interface, but never less than the full interface toInstrument. For that matter, any object of a class derived fromInstrumentshould have its version ofplay()used, regardless of the situation. at least one inherited pure virtual function, then it cannot be instantiated: http://en.cppreference.com/w/cpp/language/abstract_class, https://msdn.microsoft.com/en-us/library/aa664435. Heres a drawing of the array of pointersA[ ]inInstrument4.cpp: The array ofInstrumentpointers has no specific type information; they each point to an object of typeInstrument.Wind,Percussion,Stringed, andBrassall fit into this category because they are derived fromInstrument(and thus have the same interface asInstrument, and can respond to the same messages), so their addresses can also be placed into the array. A pure virtual function is a member of an "abstract" base class. Polymorphism implemented in C++ with virtual functions means different forms. In object-oriented programming, you have the same face (the common interface in the base class) and different forms using that face: the different versions of the virtual functions. Virtual Function in Java A virtual function or virtual method in an OOP language is a function or method used to override the behavior of the function in an inherited class with the same signature to achieve the polymorphism. To accomplish this, the typical compiler[54]creates a single table (called the VTABLE) for each class that containsvirtualfunctions. Polymorphism (implemented in C++ withvirtualfunctions) is the third essential feature of an object-oriented programming language, after data abstraction and inheritance. In the following example you can see what happens when an object is sliced: The functiondescribe()is passed an object of typePetby value. The constructorhas the special job of putting an object together piece-by-piece, first by calling the base constructor, then the more derived constructors in order of inheritance (it must also call member-object constructors along the way). The behavior ofInstrument2.cppis not surprising, given Cs approach to functions. Both of virtual functions and pure virtual functions can have a body in base classes and can be overridden in derived classes. Pure virtual work is the work of a basic class member whose proclamation is in the foundation phase and implementation is in the acquired phase. This means that you easily see the code organization benefits of grouping a data structure together with the functions that act upon it, the value of constructors and destructors, and perhaps some simple inheritance. (Although this seems tedious now, it will actually work quite well in Chapter 16, when templates are introduced.). A virtual function is a defined function in the base class in the case of a virtual function, while in the pure virtual function it is not declared within the class. It calls a function in a derived class. Inmain()the code that testsDerived4shows that this happens even if the new version off()isnt actually overriding an existing virtual function interface both of the base-class versions off()are hidden byf(int). For this compiler the VPTR is inserted at the beginning of the object, so the contents ofthiscorrespond to the VPTR. In addition, many compilers recognize that a virtual function call is being made inside a constructor, and perform early binding because they know that late-binding will produce a call only to the local function. Thats why the compiler enforces a constructor call for every portion of a derived class. Itslicesthe derived portion off of the object, like this: Now you may wonder about the virtual function call. Virtual functions are not different from polymorphism, its a tool for it. Then when youre in the derived-class constructor, all the members you can access in the base class have been initialized. By the time this second call is made, youve determined both types of object, and can perform the proper activity. If youcouldleave off the definition for a pure virtual destructor, what function body would be called during destruction? So in general the pattern is to implement the public non-virtual function and the private or protected virtual function, it looks like you have researched this already, but this is generally known as the Template Method Pattern, with more explanations by Herb Sutter here and here.. In some cases you may know something special during downcasting that allows you to say for sure what type youre dealing with, in which case the extra overhead of thedynamic_castbecomes unnecessary, and you can use astatic_castinstead. How could it possibly know that you are working with aDogobject, if it has only a pointer to a base-class object? If the compiler has an object, it knows the exact type and therefore (in C++) will not use late binding for any function calls or at least, the compiler doesntneedto use late binding. However, there may be a common piece of code that you want some or all of the derived class definitions to call rather than duplicating that code in every function. During the destructor call,Derived::f()isnotcalled, even thoughf()is virtual. Dublin, Aug. 11, 2020 (GLOBE NEWSWIRE) -- The "Next Generation Network OSS and BSS Market by Infrastructure, Components, Applications, and Services 2020 - 2025" report has been added to . Inside an ordinary member function you can imagine what will happen the virtual call is resolved at runtime because the object cannot know whether it belongs to the class the member function is in, or some class derived from it. A pure virtual function in C++, also known as the do-nothing function, is a virtual function that does not perform any task. Try commenting out theint ain all the classes in the example above to see this. The first concerns efficiency. This is generally true, but there is a special case in which you can slightly modify the return type. I understood that pure virtual function of C++ is abstract method of C#. But upcasting is easy since as you move up an inheritance hierarchy the classes always converge to more general classes. If you could do this inside a constructor, youd be calling a function that might manipulate members that hadnt been initialized yet, a sure recipe for disaster. virtual function vs pure virtual function|difference between virtual and pure virtual function If you follow this practice, you can assume that all base class membersandmember objects of the current object have been initialized. Thus, its absolutely necessary that the compiler and linker enforce the existence of a function body for a pure virtual destructor. When you read that chapter, youll see that multiple inheritance can be fraught with complexity, and is a feature you should use sparingly. If you think polymorphically, this doesnt seem necessary. If a class hierarchy has novirtualfunctions (which is a questionable design) or if you have other information that allows you to safely downcast, its a tiny bit faster to do the downcaststatically than withdynamic_cast. When the type information is inserted because of thevirtualkeyword, this takes the place of the dummy member. Heres what a call toadjust()for aBrassobject looks like, if made through anInstrumentpointer (AnInstrumentreference produces the same result): The compiler begins with theInstrumentpointer, which points to the starting address of the object. Heres the output from one compiler for the call. Object slicing actually removes part of the existing object as it copies it into the new object, rather than simply changing the meaning of an address as when using a pointer or reference. Virtual functions called through base class pointers/references will be resolved at run-time. Here, thetypeidkeyword (declared in the header file
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