RTV Studios: Advanced c++ interview questions and answers 1

Let me assemble the c++ questions and answers so that I don't have to go elsewhere searching for questions for my next interview!

1. Can I call constructor from another constructor in the same class?

SHOW/HIDE ANSWER

No you can not do this. So the below given code snippet won't work.

    1 class Test
    2 {
    3 private:
    4     int mI;
    5 
    6 public:
    7     Test()
    8      : Test(0)
    9     {}
   10 
   11     Test(int i)
   12      : mI(i)
   13     {}
   14 };

However you can very much do this in c++11.

So is there a way to achieve it? Well yes. Its by using placement new, modified code is shown below.

    1 class Test
    2 {
    3 private:
    4     int mI;
    5 
    6 public:
    7     Test()
    8     {
    9         new (this) Test(100);
   10     }
   11     Test(int i)
   12      : mI(i)
   13     {}
   14 };

2. Do we need to call the base class operator= function from derived class, and how to do it?

SHOW/HIDE ANSWER

Yes you would need to call the base class assignment operator function, otherwise base class part of the variable won't set correctly. How to do it! see below,

    1 #include <iostream>
    2 
    3 class Base
    4 {
    5 private:
    6     int mBI;
    7 
    8 public:
    9     Base()
   10      : mBI(100)
   11     {}
   12 
   13     void setBValue(int i)
   14     {
   15         mBI = i;
   16     }
   17 
   18     void print() 
   19     {
   20         std::cout << "Base I " << mBI << std::endl;
   21     }
   22 };
   23 
   24 class Derived: public Base
   25 {
   26 private:
   27     int mDI;
   28 
   29 public:
   30     Derived()
   31      : mDI(1000)
   32     {}
   33 
   34     Derived & operator=(Derived const & o)
   35     {
   36         if (this != &o) {
   37             static_cast<Base &>(*this) = o;
   38             mDI = o.mDI;
   39         }
   40 
   41  return *this;
   42     }
   43 
   44     void setDValue(int i)
   45     {
   46         mDI = i;
   47     }
   48 
   49     void print() 
   50     {
   51         Base::print();
   52         std::cout << "Derived I " << mDI << std::endl;
   53     }
   54 };
   55 
   56 int main(int argc, char **argv)
   57 {
   58     Derived d1;
   59     Derived d2;
   60 
   61     d1.setBValue(1);
   62     d1.setDValue(1);
   63 
   64     d2 = d1;
   65 
   66     d1.print();
   67     d2.print();
   68 
   69     return 0;
   70 }

3. Why do I need to return *this in an assignment operator function?

SHOW/HIDE ANSWER

To make assignment such as (obj3 = obj2) = obj1; to work.

4. How to declare and use pointer to member variable?

SHOW/HIDE ANSWER

    1 #include <iostream>
    2 
    3 class Test
    4 {
    5 public:
    6     int mI;
    7 };
    8 
    9 int main(int argc, char **argv)
   10 {
   11     Test d1;
   12     d1.mI = 100;
   13 
   14     int Test::*pmI = &Test::mI;
   15     d1.*pmI = 20;
   16 
   17     std::cout << "I " << d1.mI << std::endl;
   18 
   19     return 0;
   20 }

5. How to declare and use pointer to member function?

SHOW/HIDE ANSWER

    1 #include <iostream>
    2 
    3 class Test
    4 {
    5 public:
    6     int print() {
    7         std::cout << "Test::print" << std::endl;
    8     }
    9 };
   10 
   11 int main(int argc, char **argv)
   12 {
   13     Test d;
   14 
   15     int (Test::*pPrint)();
   16 
   17     pPrint = &Test::print;
   18     (d.*pPrint)();
   19 
   20     return 0;
   21 }

6. What is the issue with the given program?

    1 #include <iostream>
    2 
    3 class Base
    4 {
    5 public:
    6     Base()
    7     {
    8         print();
    9     }
   10 
   11     virtual ~Base()
   12     {
   13         print();
   14     }
   15 
   16     virtual void print() = 0;
   17 };
   18 
   19 class Derived: public Base
   20 {
   21 public:
   22     Derived()
   23     {}
   24 
   25     void print()
   26     {
   27         std::cout << "Derived " << std::endl;
   28     }
   29 };
   30 
   31 int main(int argc, char **argv)
   32 {
   33     Derived d;
   34     return 0;
   35 }

SHOW/HIDE ANSWER

Pure virtual function called from constructor and destructor!

7. What is the output of the following program?

    1 #include <iostream>
    2 
    3 class Test
    4 {
    5 private:
    6     int m1;
    7     int m2;
    8 
    9 public:
   10     Test()
   11      : m2(1),
   12        m1(m2)
   13     {}
   14 
   15     void print() const
   16     {
   17         std::cout << m1 << ", " << m2 << std::endl;
   18     }
   19 };
   20 
   21 
   22 int main(int argc, char **argv)
   23 {
   24     Test t;
   25     
   26     t.print();
   27 
   28     return 0;
   29 }

SHOW/HIDE ANSWER

Garbage value, 1
The problem is with the order of initialization list

8.How to initialize constant and reference member variable?

SHOW/HIDE ANSWER

Using initialization list.

9. What is constant in a const function?

SHOW/HIDE ANSWER

Variable 'this'.

10. How to modify member variable from a const funciton?

SHOW/HIDE ANSWER

Declare the member variables as mutable or use const_cast as shown below.

    1 #include <iostream>
    2 
    3 class Test
    4 {
    5 private:
    6     int m1;
    7     int m2; 
    8 
    9 public:
   10 
   11     void print() const
   12     {
   13         const_cast<Test *>(this)->m1 = 100;
   14         const_cast<Test *>(this)->m2 = 200;
   15         std::cout << m1 << ", " << m2 << std::endl;
   16     }
   17 };
   18 
   19 
   20 int main(int argc, char **argv)
   21 {
   22     Test t;
   23     t.print();
   24 
   25     return 0;
   26 }

11. What is the issue in the following program?

    1 #include <iostream>
    2 
    3 int main(int argc, char **argv)
    4 {
    5     const int & r1 = 100;
    6     int v = 200;
    7     int &r2 = v;
    8     int & r3 = 200;
    9     return 0;
   10 }

SHOW/HIDE ANSWER

Issue is in the initialization of r3 at line 8, rvalue should be a variable.

12. Can the destructor be pure virtual function?

SHOW/HIDE ANSWER

Yes, but you still have to define it!

13. What is the output of the following program?

    1 #include <iostream>
    2 
    3 class Base
    4 {
    5 public:
    6     Base()
    7     {
    8         print();
    9     }
   10 
   11     virtual ~Base()
   12     {
   13         print();
   14     }
   15 
   16     virtual void print() = 0;
   17 };
   18 
   19 class Derived: public Base
   20 {
   21 public:
   22     Derived()
   23     {}
   24 
   25     void print()
   26     {
   27         std::cout << "Derived" << std::endl;
   28     }
   29 };
   30 
   31 void Base::print()
   32 {
   33     std::cout << "Base" << std::endl;
   34 }
   35 
   36 int main(int argc, char **argv)
   37 {
   38     Derived *d = new Derived();
   39     d->print();
   40     delete d;
   41 
   42     return 0;
   43 }

SHOW/HIDE ANSWER

Base
Derived
Base

Some compiler will throw some warning on calling pure virtual function from constructor and destructor.

14. What is the output of the following program?

    1 #include <iostream>
    2 
    3 class Base
    4 {
    5 };
    6 
    7 class Derived: public Base
    8 {
    9 };
   10 
   11 int main(int argc, char **argv)
   12 {
   13     try {
   14         throw Derived();
   15     } catch (Base const & d) {
   16         std::cout << "Base" << std::endl;
   17     } catch (Derived const & d) {
   18         std::cout << "Derived" << std::endl;
   19     }
   20 
   21     return 0;
   22 }

SHOW/HIDE ANSWER

Base

Exception will be handled by the first possible catch caluse.

15. What is the output of the following program?

    1 #include <iostream>
    2 
    3 class Exception
    4 {
    5 public:
    6     Exception()
    7     {
    8         std::cout << "Exception" << std::endl;
    9     }
   10     Exception(const Exception & o)
   11     {
   12         std::cout << "Exception Copy " << std::endl;
   13     }
   14 
   15     Exception & operator=(Exception const & o)
   16     {
   17         std::cout << "Exception Assign" << std::endl;
   18     }
   19 };
   20 
   21 void func(int i) {
   22     try
   23     {
   24         throw Exception();
   25     } catch (Exception const & e) {
   26         if (i) 
   27             throw e;
   28         else
   29             throw;
   30     }
   31 }
   32 
   33 int main(int argc, char **argv)
   34 {
   35     for (int i=0; i<2; ++i) {
   36         try {
   37             func(i);
   38         } catch (Exception const & d) {
   39             std::cout << "Caught Exception" << std::endl;
   40         }
   41     }
   42     return 0;
   43 }

SHOW/HIDE ANSWER

Exception
Caught Exception
Exception
Exception Copy
Caught Exception

16. What is the memory structure of an object?

SHOW/HIDE ANSWER

Usually C++ objects are made by concatenating member variables.
For example;

    1 class Test
    2 {
    3  int i;
    4  float j;
    5 };

is represented by an int followed by a float.

    1 class TestSub: public Test
    2 {
    3  int k;
    4 };

The above class is represented by Test and then an int(for int k). So finally it will be int, float and int.

In addition to this each object will have the vptr(virtual pointer) if the class has virtual function, usually as the first element in a class.