1. Constructor & Destructor
Q: Is it possible to have Virtual Constructor? If yes, how? If not, Why not possible?
A: There is nothing like Virtual Constructor. The Constructor can’t be virtual as the constructor
is a code which is responsible for creating an instance of a class and it can’t be delegated to
any other object by virtual keyword means.
Q: What about Virtual Destructor?
A: Yes there is a Virtual Destructor. A destructor can be virtual as it is possible as at runtime
depending on the type of object caller is calling to, proper destructor will be called.
Q: What is copy constructor?
A: Constructor which initializes the it's object member variables ( by
shallow copying) with another object of the same class. If you don't implement one in your class
then compiler implements one for you. for example:
(a) Boo Obj1(10); // calling Boo constructor
(b) Boo Obj2(Obj1); // calling boo copy constructor
(c) Boo Obj2 = Obj1;// calling boo copy constructor
Q: When are copy constructors called?
A: Copy constructors are called in following cases:
(a) when a function returns an object of that
class by value
(b) when the object of that class is passed by
value as an argument to a function
(c) when you construct an object based on another
object of the same class
(d) When compiler generates a temporary object
2. Virtual
Q: What is virtual function?
A: When derived class overrides the base class method by redefining the same function, then if
client wants to access redefined the method from derived class through a pointer from base class
object, then you must define this function in base class as virtual function.
class parent {
void Show( ) {
cout << "i'm parent" << endl;
}
};
class child : public parent {
void Show( ) {
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls parent->show()
now we goto virtual world...
class parent {
virtual void Show( ) {
cout << "i'm parent" << endl;
}
};
class child : public parent {
void Show( ) {
cout << "i'm child" << endl;
}
};
parent * parent_object_ptr = new child;
parent_object_ptr->show() // calls child->show()
Q: What is a "pure virtual" member function?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.
Q: How virtual functions are implemented C++?
A: Virtual functions are implemented using a table of function pointers, called the vtable. There
is one entry in the table per virtual function in the class. This table is created by the constructor
of the class. When a derived class is constructed, its base class is constructed _rst which creates
the vtable. If the derived class overrides any of the base classes virtual functions, those entries in
the vtable are overwritten by the derived class constructor. This is why you should never call
virtual functions from a constructor: because the vtable entries for the object may not have
been set up by the derived class constructor yet, so you might end up calling base class
implementations of those virtual functions
Q: What is pure virtual function? or what is abstract class?
A: When you de_ne only function prototype in a base class without implementation and do the
complete implementation in derived class. This base class is called abstract class and client won't
able to instantiate an object using this base class. You can make a pure virtual function or
abstract class this way..
class Boo {
void foo() = 0;
}
Boo MyBoo; // compilation error
Q: What is Pure Virtual Function? Why and when it is used?
A: The abstract class whose pure virtual method has to be implemented by all the classes which
derive on these. Otherwise it would result in a compilation error. This construct should be used
when one wants to ensure that all the derived classes implement the method defined as pure
virtual in base class.
3. Inheritance
Q: What is inheritance?
A: Inheritance allows one class to reuse the state and behavior of another class. The derived class
inherits the properties and method implementations of the base class and extends it by overriding
methods and adding additional properties and methods.
Q: What is multiple inheritance(virtual inheritance)? What are its advantages and disadvantages?
A: Multiple Inheritance is the process whereby a child can be derived from more than one parent
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(ambiguity)
when two base classes implement a method with the same name.
4. Polymorphism
Q: What is Polymorphism?
A: Polymorphism allows a client to treat different objects in the same way even if they were
created from different classes and exhibit different behaviors. You can use implementation
inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's
pointer can invoke methods in derived class objects. You can also achieve polymorphism in
C++ by function overloading and operator overloading.
5. Class
Q: What are the differences between a C++ struct and C++ class?
A: The default member and base class access specifies are different. This is one of the
commonly misunderstood aspects of C++. Believe it or not, many programmers think that a C++
struct is just like a C struct, while a C++ class has inheritance, access specifes, member
functions, overloaded operators, and so on. Actually, the C++ struct has all the features of the
class. The only differences are that a struct defaults to public member access and public base
class inheritance, and a class defaults to the private access specified and private base-class
inheritance.
Q: What is encapsulation?
A: Containing and hiding Information about an object, such as internal data structures and code.
Encapsulation isolates the internal complexity of an object's operation from the rest of the
application. For example, a client component asking for net revenue from a business object need
not know the data's origin.
Q: What is Overriding?
A: To override a method, a subclass of the class that originally declared the method must declare
a method with the same name, return type (or a subclass of that return type), and same parameter list.
The definition of the method overriding is:
· Must have same method name.
· Must have same data type.
· Must have same argument list.
Overriding a method means that replacing a method functionality in child class. To imply
overriding functionality we need parent and child classes. In the child class you define the same
method signature as one defined in the parent class.
6. Memory Allocation/ Deallocation
Q: What is the difference between new/delete and malloc/free?
A: malloc allocates memory for object in heap but doesn't invoke object's constructor to initialize
the object. new allocates memory and also invokes constructor to initialize the object. malloc()
and free() do not support object semantics, does not construct and destruct objects
Eg. string * ptr = (string *)(malloc (sizeof(string))) Are not safe, and does not calculate the size
of the objects that it construct
The following return a pointer to void
int *p = (int *) (malloc(sizeof(int)));
int *p = new int;
Are not extensible
new and delete can be overloaded in a class
"delete" first calls the object's termination routine (i.e. its destructor) and then releases the space
the object occupied on the heap memory. If an array of objects was created using new, then
delete must be told that it is dealing with an array by preceding the name with an empty []:-
Int_t *my_ints = new Int_t[10];
...
delete []my_ints;
Q: What is the difference between delete and delete[]?
A: Whenever you allocate memory with new[], you have to free the memory using delete[].
When you allocate memory with 'new', then use 'delete' without the brackets. You use new[] to
allocate an array of values (always starting at the index 0).
7. Pointer
Q: What is the difference between a pointer and a reference?
A: 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.
Q: When should I use references, and when should I use pointers?
A: Use references when you can, and pointers when you have to.
References are usually preferred over pointers whenever you don't need "reseating". This usually
means that references are most useful in a class's public interface. References typically appear on
the skin of an object, and pointers on the inside.
The exception to the above is where a function's parameter or return value needs a "sentinel"
reference a reference that does not refer to an object. This is usually best done by
returning/taking a pointer, and giving the NULL pointer this special significance (references
should always alias objects, not a dereferenced NULL pointer).
Note: Old line C programmers sometimes don't like references since they provide reference
semantics that isn't explicit in the caller's code. After some C++ experience, however, one
quickly realizes this is a form of information hiding, which is an asset rather than a liability. E.g.,
programmers should write code in the language of the problem rather than the language of the
machine.
Q: What is a smart pointer?
A: A smart pointer is a C++ class that mimics a regular pointer in syntax and some semantics,
but it does more. Because smart pointers to different types of objects tend to have a lot of code in common, almost all good-quality smart pointers in existence are templated by the pointee type.
Q: What is auto pointer?
A: The simplest example of a smart pointer is auto_ptr, which is included in the standard C++
library. Auto Pointer only takes care of Memory leak and does nothing about dangling pointers
issue. You can find it in the header . Here is part of auto_ptr's implementation.
8. Overload
Q: What is overloading??
A: With the C++ language, you can overload functions and operators. Overloading is the
practice of supplying more than one definition for a given function name in the same scope.
- Any two functions in a set of overloaded functions must have different argument lists.
- Overloading functions with argument lists of the same types, based on return type alone, is an
error.
