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Question 21. What Is Your Reaction To This Line Of Code?

Answer :

delete this;

It’s not a good practice.

A good programmer will insist that the statement is never to be used if the class is to be used by other programmers and instantiated as static, extern, or automatic objects. That much should be obvious. The code has two built-in pitfalls. First, if it executes in a member function for an extern, static, or automatic object, the program will probably crash as soon as the delete statement executes. There is no portable way for an object to tell that it was instantiated on the heap, so the class cannot assert that its object is properly instantiated. Second, when an object commits suicide this way, the using program might not know about its demise. As far as the instantiating program is concerned, the object remains in scope and continues to exist even though the object did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster. A reader pointed out that a class can ensure that its objects are instantiated on the heap by making its destructor private. This idiom necessitates a kludgy DeleteMe kind of function because the instantiator cannot call the delete operator for objects of the class. The DeleteMe function would then use “delete this.” I got a lot of mail about this issue. Many programmers believe that delete this is a valid construct. In my experience, classes that use delete this when objects are instantiated by users usually spawn bugs related to the idiom, most often when a program dereferences a pointer to an object that has already deleted itself. 

• Question 22. What Is A Default Constructor?

Answer :

A constructor that has no arguments or one where all the arguments have default argument values.

If you don’t code a default constructor, the compiler provides one if there are no other constructors. If you are going to instantiate an array of objects of the class, the class must have a default constructor. 

• Question 23. What Is A Conversion Constructor?

Answer :

A constructor that accepts one argument of a different type.

The compiler uses this idiom as one way to infer conversion rules for a class. A constructor with more than one argument and with default argument values can be interpreted by the compiler as a conversion constructor when the compiler is looking for an object of the type and sees an object of the type of the constructor’s first argument. 

• Question 24. What Is The Difference Between A Copy Constructor And An Overloaded Assignment Operator? 

A copy constructor constructs a new object by using the content of the argument object. An overloaded assignment operator assigns the contents of an existing object to another existing object of the same class. First, you must know that a copy constructor is one that has only one argument, which is a reference to the same type as the constructor. The compiler invokes a copy constructor wherever it needs to make a copy of the object, for example to pass an argument by value. If you do not provide a copy constructor, the compiler creates a member-by-member copy constructor for you. You can write overloaded assignment operators that take arguments of other classes, but that behavior is usually implemented with implicit conversion constructors. If you do not provide an overloaded assignment operator for the class, the compiler creates a default member-by-member assignment operator. This discussion is a good place to get into why classes need copy constructors and overloaded assignment operators. By discussing the requirements with respect to data member pointers that point to dynamically allocated resources, you demonstrate a good grasp of the problem. 

• Question 25. When Should You Use Multiple Inheritance?

Answer : There are three acceptable answers: “Never,” “Rarely,” and “When the problem domain cannot be accurately modeled any other way.” There are some famous C++ pundits and luminaries who disagree with that third answer, so be careful. Let’s digress to consider this issue lest your interview turn into a religious debate. Consider an Asset class, Building class, Vehicle class, and CompanyCar class. All company cars are vehicles. Some company cars are assets because the organizations own them. Others might be leased. Not all assets are vehicles. Money accounts are assets. Real-estate holdings are assets. Some real-estate holdings are buildings. Not all buildings are assets. Ad infinitum. When you diagram these relationships, it becomes apparent that multiple inheritance is an intuitive way to model this common problem domain. You should understand, however, that multiple inheritance, like a chainsaw, is a useful tool that has its perils, needs respect, and is best avoided except when nothing else will do. Stress this understanding because your interviewer might share the common bias against multiple inheritance that many object-oriented designers hold. 

• Question 26. What Is A Virtual Destructor? 

The simple answer is that a virtual destructor is one that is declared with the virtual attribute. The behavior of a virtual destructor is what is important. If you destroy an object through a pointer or reference to a base class, and the base-class destructor is not virtual, the derived-class destructors are not executed, and the destruction might not be complete. 

• Question 27. Explain The Isa And Hasa Class Relationships. How Would You Implement Each In A Class Design? 

A specialized class “is a” specialization of another class and, therefore, has the ISA relationship with the other class. An Employee ISA Person. This relationship is best implemented with inheritance. Employee is derived from Person. A class may have an instance of another class. For example, an Employee “has a” Salary, therefore the Employee class has the HASA relationship with the Salary class. This relationship is best implemented by embedding an object of the Salary class in the Employee class. The answer to this question reveals whether you have an understanding of the fundamentals of object-oriented design, which is important to reliable class design. There are other relationships. The USESA relationship is when one class uses the services of another. The Employee class uses an object (cout) of the ostream class to display the employee’s name onscreen, for example. But if you get ISA and HASA right, you usually don’t need to go any further. 

• Question 28. When Is A Template A Better Solution Than A Base Class? 

When you are designing a generic class to contain or otherwise manage objects of other types, when the format and behavior of those other types are unimportant to their containment or management, and particularly when those other types are unknown (thus the genericity) to the designer of the container or manager class. Prior to templates, you had to use inheritance; your design might include a generic List container class and an application-specific Employee class. To put employees in a list, a ListedEmployee class is multiply derived (contrived) from the Employee and List classes. These solutions were unwieldy and error-prone. Templates solved that problem. 

• Question 29. What Is The Difference Between C And C++ ? Would You Prefer To Use One Over The Other ? 

C is based on structured programming whereas C++ supports the object-oriented programming paradigm.Due to the advantages inherent in object-oriented programs such as modularity and reuse, C++ is preferred. However almost anything that can be built using C++ can also be built using C. 

• Question 30. What Are The Access Privileges In C++ ? What Is The Default Access Level ? 

The access privileges in C++ are private, public and protected. The default access level assigned to members of a class is private. Private members of a class are accessible only within the class and by friends of the class. Protected members are accessible by the class itself and it’s sub-classes. Public members of a class can be accessed by anyone. 

• Question 31. What Is Data Encapsulation ? 

Data Encapsulation is also known as data hiding. The most important advantage of encapsulation is that it lets the programmer create an object and then provide an interface to the object that other objects can use to call the methods provided by the object. The programmer can change the internal workings of an object but this transparent to other interfacing programs as long as the interface remains unchanged. 

• Question 32. What Is Inheritance ? 

Inheritance is the process of deriving classes from other classes. In such a case, the sub-class has an ‘is-a’ relationship with the super class. For e.g. vehicle can be a super-class and car can be a sub-class derived from vehicle. In this case a car is a vehicle. The super class ‘is not a’ sub-class as the sub- class is more specialized and may contain additional members as compared to the super class. The greatest advantage of inheritance is that it promotes generic design and code reuse. 

• Question 33. What Is Multiple Inheritance ? What Are It’s Advantages And Disadvantages ? 

Multiple Inheritance is the process whereby a sub-class can be derived from more than one super class. The advantage of multiple inheritance is that it allows a class to inherit the functionality of more than one base class thus allowing for modeling of complex relationships. The disadvantage of multiple inheritance is that it can lead to a lot of confusion when two base classes implement a method with the same name. 

• Question 34. What Do The Keyword Static And Const Signify? 

When a class member is declared to be of a static type, it means that the member is not an instance variable but a class variable. Such a member is accessed using Classname.Membername (as opposed to Object.Membername). Const is a keyword used in C++ to specify that an object’s value cannot be changed. • Question 35. How Is Memory Allocated/deallocated In C ? How About C++ ? 

Memory is allocated in C using malloc() and freed using free(). In C++ the new() operator is used to allocate memory to an object and the delete() operator is used to free the memory taken up by an object. 

• Question 36. What Is Uml? 

UML refers to Unified Modeling Language. It is a language used to model OO problem spaces and solutions. 

• Question 37. What Is The Difference Between A Shallow Copy And A Deep Copy? 

A shallow copy simply creates a new object and inserts in it references to the members of the original object. A deep copy constructs a new object and then creates in it copies of each of the members of the original object

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