And to guarantee this is the final time, I’m cutting and pasting it into the body of this article (hit “Read More” to see it)…
The C++ Interview
These 40 questions and answers will help you land the assignment
by Alex Bykov
How do you rank your C++ skills on a scale of 1 to 10?
This is often the first question you will hear on an interview for a
C++ contract. You will be tempted to rate yourself high, and you should.
This is your chance to convince the client that you are just what he is
looking for–an assertive and knowledgeable professional who will be productive
either working on a team or on your own. Naturally, though, you should
be able to support the ranking you gave yourself by doing well on the
interview. This article will help you prepare for your C++ interview.
I put together a list of 40 questions that I have had to answer during
numerous technical interviews in the past few years. You, too, will have
to answer at least some of them during an interview. Even if you use C++
on a daily basis, it pays to go through the questions. Most of us, no
matter how experienced, use only a segment of the language that we are
most comfortable with. Brief answers are included, but you can find more
information in the references listed.
Q1. Is there anything you can do in C++ that you cannot
do in C?
A1. No. There is nothing you can do in C++ that you cannot do
in C. After all you can write a C++ compiler in C.
Q2. What is the difference between C++ structure and C++
class?
A2. The default access level assigned to members of struct is
public while the default access level assigned to a class is private.
Q3. What is encapsulation?
A3. Encapsulation is welding of code and data together into objects.
Q4. What is inheritance?
A4. Inheritance is a mechanism through which a subclass inherits
the properties and behavior of its superclass.
Q5. What is polymorphism?
A5. In Greek this means "many shapes." As a consequence
of inheritance and virtual functions, a single task (for example, drawing
a geometrical shape) can be implemented using the same name (like draw())
and implemented differently (via virtual functions) as each type in object
hierarchy requires(circle.draw() or rectangle.draw()). Later, when a polymorphic
object (whose type is not known at compile time) executes the draw() virtual
function, the correct implementation is chosen and executed at run time.
Q6. What would you say if you saw "delete this" while
reviewing your peer’s code?
A6. You should never do this. Since compiler does not know whether
the object was allocated on the stack or on the heap, "delete this"
could cause a disaster.
Q7. What is the difference between public, protected, and private
members of a class?
A7. Private members are accessible only by members and friends
of the class. Protected members are accessible by members and friends
of the class and by members and friends of derived classes. Public members
are accessible by everyone.
Q8. What is the difference between non-virtual and virtual functions?
A8. The behavior of a non-virtual function is known at compile
time while the behavior of a virtual function is not known until the run
time.
Q9. What is a pure virtual function?
A9. “A pure virtual function is a function declared in a base
class that has no definition relative to the base.”
Q10. What is an abstract base class?
A10. It is a class that has one or more pure virtual functions.
Q11. What is the difference between MyClass p; and MyClass p();?
A11. MyClass p; creates an instance of class MyClass by calling
a constructor for MyClass. MyClass p(); declares function p which takes
no parameters and returns an object of class MyClass by value.
Q12. How do you know that your class needs a virtual destructor?
A12. If your class has at least one virtual function, you should
make a destructor for this class virtual. This will allow you to delete
a dynamic object through a pointer to a base class object. If the destructor
is non-virtual, then wrong destructor will be invoked during deletion
of the dynamic object.
Q13. Why were the templates introduced?
A13. Many data structures and algorithms can be defined independently
of the type of data they work with. You can increase the amount of shared
code by separating data-dependent portions from data-independent portions,
and templates were introduced to help you do that.
Q14. What is a static member of a class?
A14. Static data members exist once for the entire class, as opposed
to non-static data members, which exist individually in each instance
of a class.
Q15. What feature of C++ would you use if you wanted to
design a member function that guarantees to leave "thisÓ object unchanged?
A15. It is "const" as in: "int MyFunc (int test)
const;"
Q16. Can you overload a function based only on whether a parameter
is a value or a reference?
A16. No. Passing by value and by reference looks identical to
the caller.
Q17. What is the difference between function overloading and function
overriding?
A17. Overloading is a method that allows defining multiple member
functions with the same name but different signatures. The compiler will
pick the correct function based on the signature. Overriding is a method
that allows the derived class to redefine the behavior of member functions
which the derived class inherits from a base class. The signatures of
both base class member function and derived class member function are
the same; however, the implementation and, therefore, the behavior will
differ.
Q18. Can derived class override some but not all of a set of overloaded
virtual member functions inherited from the base class?
A18. Compiler will allow this, but it is a bad practice since
overridden member functions will hide all of the inherited overloads from
the base class. You should really override all of them.
Q19. What is the difference between assignment and initialization
in C++?
A19. Assignment changes the value of the object that has already
been constructed. Initialization constructs a new object and gives it
a value at the same time.
Q20. When are copy constructors called?
A20. Copy constructors are called in three cases: when a function
returns an object of that class by value, when the object of that class
is passed by value as an argument to a function, and, finally, when you
construct an object based on another object of the same class (Circle
c1=c2;).
Q21. Why do you have to provide your own copy constructor and assignment
operator for classes with dynamically allocated memory?
A21. If you don’t, the compiler will supply and execute the default
constructor and the assignment operator, but they will not do the job
correctly. The default assignment operator does memberwise assignment
and the default copy constructor does memberwise copy. In both cases you
will only assign and manipulate pointers to dynamic memory, which will
lead to memory leaks and other abnormalities. You should write your own
assignment operator and copy constructor, which would copy the pointer
to memory so that each object has its own copy.
Q22. Does compiler guarantee that initializers will be executed
in the same order as they appear on the initialization list?
A22. No. C++ guarantees that base class subobjects and member
objects will be destroyed in the opposite order from which they were constructed.
This means that initializers are executed in the order, which supports
the above-mentioned guarantee.
Q23. What is function’s signature?
A23. Function’s signature is its name plus the number and types
of the parameters it accepts.
Q24. What does extern "C" int func(int *, Foo) accomplish?
A24. It will turn off "name mangling" for this function
so that one can link to code compiled by C compiler.
Q25. Why do C++ compilers need name mangling?
A25. Name mangling is the rule according to which C++ changes
function’s name into function signature before passing that function to
a linker. This is how the linker differentiates between different functions
with the same name.
Q26. What is the difference between a pointer and a reference?
A26. A reference must always refer to some object and, therefore,
must always be initialized; pointers do not have such restrictions. A
pointer can be reassigned to point to different objects while a reference
always refers to an object with which it was initialized.
Q27. How can you access the static member of a class?
A27.
Q28. How are prefix and postfix versions of operator++() differentiated?
A28. The postfix version of operator++() has a dummy parameter
of type int. The prefix version does not have dummy parameter.
Q29. What functions does C++ silently write and call?
A29. Constructors, destructors, copy constructors, assignment
operators, and address-of operators.
Q30. What is the difference between new/delete and malloc/free?
A30. Malloc/free do not know about constructors and destructors.
New and delete create and destroy objects, while malloc and free allocate
and deallocate memory.
Q31. What is the difference between delete and delete[ ]?
A31. Delete deletes one object; delete[ ] deletes an array of
objects.
Q32. Name two cases where you MUST use initialization list as opposed
to assignment in constructors.
A32. Both non-static const data members and reference data members
cannot be assigned values; instead, you should use initialization list
to initialize them.
Q33. What is the difference between const char *myPointer and char
*const myPointer?
A33. Const char *myPointer is a non constant pointer to constant
data; while char *const myPointer is a constant pointer to non constant
data.
Q34. Suppose that objects A, B, and C are instances of class MyClass
(MyClass A, B, C;). How should you design an assignment operator so that
the "A=B=C;" statement would be allowed by a compiler but "(A=B)=C;"
would not be allowed by a compiler?
A34. Make operator=return a reference to a const object.
Q35. Is there any problem with the following: char *a=NULL; char&
p = *a;?
A35. The result is undefined. You should never do this. A reference
must always refer to some object.
Q36. Class B is derived from class A. Function f is A’s friend.
Is f B’s friend as well?
A36. No. Friendship cannot be inherited.
Q37. What issue do auto_ptr objects address?
A37. If you use auto_ptr objects you would not have to be concerned
with heap objects not being deleted even if the exception is thrown.
Q38. What happens when a function throws an exception that was
not specified by an exception specification for this function?
A38. Unexpected() is called, which, by default, will eventually
trigger abort().
Q39. Why should you prefer throw/catch mechanism to setjmp/longjmp?
A39. The main problem with longjmp() is that it does not destroy
local objects properly.
Q40. Can you think of a situation where your program would crash
without reaching the breakpoint which you set at the beginning of main()?
A40. C++ allows for dynamic initialization of global variables
before main() is invoked. It is possible that initialization of global
will invoke some function. If this function crashes the crash will occur
before main() is entered.
If you feel comfortable answering these questions, then rest assured
that your chances of impressing any interviewer are very high. Be prepared
to know basic computer science concepts such as data structures, search
and sort algorithms, basic database concepts, etc. The client’s needs
will determine what particular branch of computer science you have to
be familiar with, but you should always be ready to implement the stock,
the queue, and the linked list data structures with either C or C++ programming
languages. And know how to write your own version of strcpy (string copy)
in C programming language since very often they ask you to do that.
