Object Oriented ProgrammingInheritance 3/27/2011 QAU-IT-2010-Fall 1 Treating Similar Classes Our programs model a variety of real-life objects through their data and operations The modeling of real-life entities should ideally reflect "common features" among real objects We use the class construct to capture commonality of objects when We believe that these objects can be characterized by the same sets of attributes the same patterns of behavior 3/27/2011 QAU-IT-2010-Fall 2 Treating Similar Classes Often, commonalities might exist among classes The groups of objects might have basically similar but still somewhat different sets of attributes and operations Employees Hourly employees Salaried employees Some object groups might have somewhat different sets of operations or provide additional operations Current account Saving account 3/27/2011 QAU-IT-2010-Fall 3 Merging Subclass Features in a Class Merging all these characteristics into one class will satisfy the client code requirements But it is inherently unsafe The client code might use the object incorrectly, assuming the presence of the features Merging all attributes and operations into one class to provide for all possible alternatives is a viable method of abstraction But client have to take care of the correct use of objects 3/27/2011 QAU-IT-2010-Fall 4 Merging Subclass Features in a Class You cannot prevent all coding mistakes You should prevent mistakes as many as possible This design needs to be improved Client code makes an explicit comment about the nature of each object of a class of merged 3/27/2011 QAU-IT-2010-Fall 5 a tag field.Pushing Responsibility to the Server To avoid the danger of incorrect use of a server object Add to the server class an additional attribute. Kind! It means that you are introducing subclasses to class The constructors for two different kinds of accounts have a different number of parameters Enumerations can be used for Kind to enhance readability The constructor code would include a parameter for the kind of object 3/27/2011 QAU-IT-2010-Fall 6 . Pushing Responsibility to the Server What the code designer has now expressed in code so that the designer's knowledge is transmitted to the maintainer You should always weigh conciseness against increase in coordination and decrease in understandability Merging data and operations of different subtypes in one class. each server method enforces its legal operations The server class needs additional code for type analysis For a large system with a large number of object kinds?? Client coding errors are still run-time. not compile-time 3/27/2011 QAU-IT-2010-Fall 7 . Separate Classes for Each Kind A good solution to this problem is to design a set of separate classes Each of these classes is designed from scratch e. a compile-time error is generated It does not convey the designer's knowledge to the maintainer well 3/27/2011 QAU-IT-2010-Fall 8 .g each type of account This design resolves the problem of erroneous client use beautifully Instead of a run-time error. this class represents the generalization of state and behavior of these subclasses 3/27/2011 QAU-IT-2010-Fall 9 .Using C++ Inheritance to Link Related Classes The designer of these classes knows that they have these features in common. which contains the common denominator of features common to all subtypes In terms of object-oriented analysis and design. but this knowledge is not conveyed to the maintenance programmer Inheritance is another solution to this problem You create a class. Using C++ Inheritance to Link Related Classes Reuse the common features for other specialized classes Each specialized class adds specialized features to the generalized class Protected keyword Base class Vs Derived classes (Java uses extension) Super class or parent class and Subclass or Child class Derived classes add and sometimes replace features of a more-general base class Additional data and methods in derived classes reflect the relationship of specialization among classes 3/27/2011 QAU-IT-2010-Fall 10 . Using C++ Inheritance to Link Related Classes There exist "natural" super class/subclass relationships among real world entities If relationship A Kind of exists between entities then they can be related through inheritance Inheritance can be either direct or indirect Inheritance can be used as a tool for division of labor in software development The base and derived classes can be developed by different programmers 3/27/2011 QAU-IT-2010-Fall 11 . Using C++ Inheritance A good way to enhance abstraction. the use of commonality in classes results in less code to write and in better modularization However code amount depends on different aspects as well Another popular use of inheritance is for run-time binding of methods Static and dynamic binding of methods Polymorphism is a special case of object-oriented programming 3/27/2011 QAU-IT-2010-Fall 12 . or protected What is private in the base remains private in the derived class and so on Capabilities of base classes are not lost at the bottom of the hierarchy of derived classes Inheritance enhances modularization and code reuse 3/27/2011 QAU-IT-2010-Fall 13 . private.Using C++ Inheritance Polymorphism allows you to process a list of objects Syntax of C++ Inheritance Name of base class Different Modes of Derivation from the Base Class inheritance public. Inheritance . does not have to know about derivation 3/27/2011 QAU-IT-2010-Fall 14 . base classes need neither editing nor recompilation The client code.Reuse A group of well-designed general purpose classes can be organized into a library The library classes can be customized by creating new derived classes Application programmers use the capabilities implemented in library classes and only add specific capabilities Normally. The method is absent in the base class and is added to the derived The method is present in the base class and is redefined by the derived 3/27/2011 QAU-IT-2010-Fall 15 . it can define and use objects of the base class and the derived classes Method calls for base and derived classes Objects of the base class behave in the client code as if derived classes do not exist When the target of the message is an object of a derived class Then The method is inherited from the base class as is and is not redefined in the derived class.Defining and Using Objects of Base and Derived Classes When the client code needs an object. Object Oriented Principle and C++ Notice that the use of inheritance breaks the first principle of object-oriented programming: Binding data and operations together in the class definition within the boundaries of the class scope C++ claims that an object of the derived class is an object of the base class (plus more) and hence has all data and methods defined in the base class C++ makes the scope of the derived class nested within the scope of the base class Derived class pointer can be save in Base class pointer 3/27/2011 QAU-IT-2010-Fall 16 . Public Inheritance 3/27/2011 QAU-IT-2010-Fall 17 . Protected Inheritance 3/27/2011 QAU-IT-2010-Fall 18 . Private Inheritance 3/27/2011 QAU-IT-2010-Fall 19 . Members Access Adjusting Access to Base Members in the Derived Class Default inheritance mode : private The default mode of inheritance for a C++ structure is public Keywords class and struct denote the same thing with the exception of default access rights This is consistent with the default C++ rules for access to class members 3/27/2011 QAU-IT-2010-Fall 20 . Scope Rules Under Inheritance Class scope can be viewed as nested under derivation General theory of nested scopes is applicable Derived class members are invisible in the scope of the base class Use scope resolution operator to resolve name conflicts Name Overloading and Name Hiding The function signature is not a deciding factor It does not matter for the resolution of nested inheritance scopes No function overloading Base to derived range QAU-IT-2010-Fall 21 3/27/2011 . the more we want to guard this class against change because the change can affect other derived classes Place the new method in the derived class with params 3/27/2011 QAU-IT-2010-Fall 22 .Scope Rules Under Inheritance Calling a Base Method Hidden by the Derived Class One remedy is to overload the method in the base class rather than in the derived class Both functions are inherited by the derived class and can be called through the derived class object The new method might need data that is defined in the derived class and is not available in the base class The higher a class is in the inheritance hierarchy. you could look for the ways to inherit from existing classes to support new requirements This a new way of thinking about writing code Weigh this approach against the drawbacks of creating too many small classes 3/27/2011 QAU-IT-2010-Fall 23 .Program Evolution using Inheritance Most people try to change exiting code to accommodate new changes Writing new code instead of modifying existing code Instead of looking for ways to change existing code. According to the principles of objectoriented programming You do not want the client code to create dependencies on server class data names You do not want to complicate the client code with direct operations over data You do not want the client code to know more about server design than is necessary You want the client code to call server methods whose names explain the actions You want the client code to push responsibility for lower level details to servers 3/27/2011 QAU-IT-2010-Fall 24 . the base part of the object is always created before the derived part of the object is created and before the derived class constructor is executed Calling of Base class parameterized constructors Using Initialization Lists in Derived Class Constructors Destructors Under Inheritance The order of destructor invocation is opposite to that of the order of constructor invocation.Constructors and Destructors for Derived Classes For class inheritance. 3/27/2011 QAU-IT-2010-Fall 25 . the hierarchy of classes might become a graph rather than a tree as with single inheritance Multiple inheritance allows the server class designer to mix the characteristics of diverse classes in one class 3/27/2011 QAU-IT-2010-Fall 26 . the hierarchy of classes is a tree. This phenomenon is known multiple inheritance With single inheritance. with the base class on the top of the hierarchy and derived classes below the base class With multiple inheritance.Multiple Inheritance A derived class might have more than one base class. for adding or redefining members in existing classes Each parent class contributes properties to the derived class. and iostream classes in the C++ standard library C++ does not put a limit on the number of base classes that can participate in forming a derived class 3/27/2011 QAU-IT-2010-Fall 27 . for example. Now accounts.Multiple Inheritance Multiple inheritance is good for customizing existing class libraries. the derived class is a union of the base features Examples of using multiple inheritance include graphic objects. The advantages are few and complications are many There are always ways to adequately support the client code without using multiple inheritance 3/27/2011 QAU-IT-2010-Fall 28 .Multiple Inheritance It is hard to come up with examples of multiple inheritance with three or four base classes so that they make good sense Use multiple inheritance with caution. and the missing capabilities are nowhere to come from 3/27/2011 QAU-IT-2010-Fall 29 . or private Conversions Between Classes Conversion from a base class to the derived class is not allowed A base object has only part of the data and capabilities that a derived object has.Multiple Inheritance Access Rules A derived class inherits all members of all bases Access rules for multiple inheritance are the same as for single : public. protected. not in the order of the initialization list of the derived class constructor In the member initialization list.Multiple Inheritance Constructors and Destructors All base class constructors are called before the derived class constructor is called They are called in the order in which the base classes are listed in the derived class declaration. the Derived class constructor calls base constructors using class names in a comma-separated sequence of constructor calls Then the base class destructors are called in an order that is the reverse of constructor invocations 3/27/2011 QAU-IT-2010-Fall 30 . Multiple Inheritance Ambiguities If two (or more) base classes have a data member with the same name (of the same or different types). and this results in ambiguity Conflicts between names of data members should be resolved by the derived class to avoid ambiguities and to protect the client code. The scope operator has to be used Everything that can be done with the use of multiple inheritance can be achieved as a combination of inheritance and composition 3/27/2011 QAU-IT-2010-Fall 31 . the derived class object has both copies. Understandability Impact of change Designer s knowledge transfer to maintainer Server and client concept: Commons things among them Which to choose? Basic decision making: a part of and a kind of In case of ambiguity Pluses and Minuses of Inheritance & Composition 3/27/2011 QAU-IT-2010-Fall 32 . Maintainability.Deciding between Inheritance and Composition Composition Vs Inheritance Reusability. Virtual Functions The most common relationship is the relationship of containment The most general relationship is that of association Client class contains a pointer or a reference to an object of the server class Degree of visibility of server object Being class member Being local variable in the function Being passed as function parameter Bering a pointer / reference of server class Inheritance 3/27/2011 QAU-IT-2010-Fall 33 . Advanced Inheritance The goal of utilizing inheritance is making the job of the server class designer easier The goal of inheritance is making the job of the client programmer easier by streamlining the client code When the client code deals with collections of similar objects that undergo similar processing "Similar processing" means that the client code treats different objects basically the same way Depending on the type of object. some things should be done somewhat differently Client code have to use switch for differentiating objects 3/27/2011 QAU-IT-2010-Fall 34 . most C++ code is (and should be) written without the use of inheritance However. programming with virtual functions is definitely is useful It is one of the most complex topics in C++ 3/27/2011 QAU-IT-2010-Fall 35 .Virtual functions Programming with virtual functions and abstract classes is often presented as the essence of object-oriented programming Most of C++ code deals with cooperating objects and does not need virtual functions Actually. assignments and function arguments objects used as targets of messages Conversion constructors Casts Between Pointers (or References) The rules for implicit conversions apply only to values and not to references or to pointers: Type casting is required Conversion operators 3/27/2011 QAU-IT-2010-Fall 36 .Conversions : Nonrelated Classes C++ supports the concept of strong typing Two different classes are nonrelated if neither of them serves as a direct or indirect base class for another class expressions . Safe and Unsafe Conversions Moving data from larger to smaller variable Moving data from smaller to larger variable C++ defers to the programmer in evaluating the situation Assigning Base class object to derived class object Assigning derived class object to Base class object Derived class object can not be assigned to base class object Base class object can be assigned to derived class object Use conversion constructor & assignment op overloaded Type casting uses constructor 3/27/2011 QAU-IT-2010-Fall 37 . Safe and Unsafe Conversions 3/27/2011 QAU-IT-2010-Fall 38 . Safe and Unsafe Conversions 3/27/2011 QAU-IT-2010-Fall 39 . Conversions of Pointers and References to Objects In this topic everything about pointers applies to references as well The conversion from a Base pointer (or reference) to a Derived pointer (reference) is not safe Derived class pointer can be assigned to base class Using cast operator. unsafe conversion can be performed Pointer of Derived in pointer of Base is restrictive Pointers of a specific class must not point to objects of classes not related to this one by inheritance 3/27/2011 QAU-IT-2010-Fall 40 . conversion is possible if provided But for pointers and references. type casting is needed Derived class pointer can be passed where Base class pointer is required Base class pointer can not be passed where Derived class pointer is required 3/27/2011 QAU-IT-2010-Fall 41 .Conversions of Pointer and Reference Arguments If formal and actual parameters are not related through inheritance. syntax error will be result If parameters are passed by value. the function identifier.Virtual Functions: Another New Idea Object name and type association concepts Compile time associations Early or Static binding The name that the compiler knows is a concatenation of the name of the class to which the function belongs. the return type. each function name is actually unique for the compiler This technique is known as name mangling 3/27/2011 QAU-IT-2010-Fall 42 . and the types of its parameters As a result. Virtual Functions-Binding time The class name of the object is known at compile time and cannot change during execution Static binding is standard in modern languages such as C++. Pascal It was introduced to improve performance Static binding could be successfully used to enforce type checking at compile time Dynamic or late binding What possible advantages might it bring? 3/27/2011 QAU-IT-2010-Fall 43 . Java. C. Ada. This is why it is called late rather than early binding This binding to allows the same function name to take on different meanings depending on the nature of the object used. This is why it is called run-time binding We want this binding to take place later than during the compile time.Virtual Functions-Binding terms The compiler binds the function name to a particular function We want this binding to take place at run time. So it is called dynamic binding 3/27/2011 QAU-IT-2010-Fall 44 . Polymorphism The ability of the function name in a function call to take different meanings is called polymorphism("many forms") Assigning the meaning to the method call at run time depending on the actual type of the object that is the target of the message Dynamic binding is not an issue specific to objectoriented programming Non-Object Oriented approach 3/27/2011 QAU-IT-2010-Fall 45 . Polymorphism Object oriented approach Define a Base class with one data member kind Code a Write method in both derived classes Code a Read method in both derived classes Use kind to call proper method by type-casting Data and operations are bound together Work is pushed to server classes The source code is longer in object-oriented solution 3/27/2011 QAU-IT-2010-Fall 46 . Dynamic Binding: Virtual Functions Keyword virtual creates the run-time type resolution property for a message sent to a derived type object Implement the function with the same name in the base class and in each of the derived classes Pointer of the derived class will be stored in Base class pointer The type of the object will be decided on run time If virtual function is called on object then there is static binding Function signatures should be same in base and derived 3/27/2011 QAU-IT-2010-Fall 47 . virtual or not) from the derived class pointer 3/27/2011 QAU-IT-2010-Fall 48 . only those methods defined in the base class can be called A derived pointer pointing to a derived object can reach methods defined in the derived class and methods inherited from the base class Methods redefined in the derived class hide methods defined in the base class (with the same or different signature.Dynamic and Static Binding For the base pointer pointing to a base object. not the base class function 3/27/2011 QAU-IT-2010-Fall 49 .Virtual function call A base pointer pointing to a derived object can reach only those methods defined in the base class with one exception: If the function is redefined in the derived class as virtual it is the derived class function that the base pointer invokes using dynamic binding. Pure Virtual Functions Base virtual functions may have no job to do because they have no meaning within the application (mostly) Their job is just to define the interface as a standard for its derived classes This is why virtual functions are introduced in the first place Some time it is possible that Base class can not create meaningful objects C++ makes it possible through the use of pure virtual functions and abstract classes 3/27/2011 QAU-IT-2010-Fall 50 . including virtual functions QAU-IT-2010-Fall 51 3/27/2011 . it becomes abstract as well An abstract class is a C++ class in all regards: it can have data members regular non-pure functions. is used after function declaration A pure virtual function has no implementation Derived class must implement the pure virtual functions If it does not implement.Pure Virtual Functions An abstract class is a class with at least one pure virtual function No C++ keywords for pure virtual functions and abstract classes: =0. the base class destructor is called So make destructor of the Base class as virtual This will prevent memory leaks and all constructor will be called 3/27/2011 QAU-IT-2010-Fall 52 .Virtual Functions: Destructors When the delete operator is invoked. the destructor is called and the object is destroyed Which destructor is called? When a base pointer points to a derived class object.