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Q: What is your reaction to this line of code?
A: 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.
Q: What is a default constructor?
A: 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.
Q: What is a conversion constructor?
A: 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.
Q: What is the difference between a copy constructor and an overloaded assignment operator?
A: 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.
Q: When should you use multiple inheritance?
A: 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.
Q: What is a virtual destructor?
A: 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.
Q: Explain the ISA and HASA class relationships. How would you implement each in a class design?
A: 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.
Q: When is a template a better solution than a base class?
A: 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.
Questions for ANSI-Knowledgeable Applicants
There are six questions for those who profess knowledge of the progress of the ANSI committee. If you claim to have that much interest in the language, you should know the answers to all these questions.
Q: What is a mutable member?
A: One that can be modified by the class even when the object of the class or the member function doing the modification is const.
Understanding this requirement implies an understanding of C++ const, which many programmers do not have. I have seen large class designs that do not employ the const qualifier anywhere. Some of those designs are my own early C++ efforts. One author suggests that some programmers find const to be such a bother that it is easier to ignore const than to try to use it meaningfully. No wonder many programmers don’t understand the power and implications of const. Someone who claims to have enough interest in the language and its evolution to keep pace with the ANSI deliberations should not be ignorant of const, however.
Q: What is an explicit constructor?
A: A conversion constructor declared with the explicit keyword. The compiler does not use an explicit constructor to implement an implied conversion of types. Its purpose is reserved explicitly for construction.
Q: What is the Standard Template Library?
A: A library of container templates approved by the ANSI committee for inclusion in the standard C++ specification.
An applicant who then launches into a discussion of the generic programming model, iterators, allocators, algorithms, and such, has a higher than average understanding of the new technology that STL brings to C++ programming.
Q: Describe run-time type identification.
A: The ability to determine at run time the type of an object by using the typeid operator or the dynamic_cast operator.
Q: What problem does the namespace feature solve?
A: Multiple providers of libraries might use common global identifiers causing a name collision when an application tries to link with two or more such libraries. The name-space feature surrounds a library’s external declarations with a unique namespace that eliminates the potential for those collisions.
This solution assumes that two library vendors don’t use the same namespace, of course.
Q: Are there any new intrinsic (built-in) data types?
A: Yes. The ANSI committee added the bool intrinsic type and its true and false value keywords and the wchar_t data type to support character sets wider than eight bits.
Other apparent new types (string, complex, and so forth) are implemented as classes in the Standard C++ Library rather than as intrinsic types.
In my original column, I left out wchar_t even though I should have known about it. Several readers wrote to correct me. I tell you this now to emphasize that even I would not have scored 100 percent on my own test.