C++ Interview questions - part2

When C++ creates a default constructor ?

- C++ create a default constructor, if an explicit constructor is not defined by the programmer. A default constructor is a constructor that either has no parameters, or if it has parameters, all the parameters have default values.

- default constructor for a class as a constructor that can be called with no arguments (this includes a constructor whose parameters all have default arguments)

- The compiler will implicitly define a default constructor if no constructors are explicitly defined for a class.

- This implicitly-declared default constructor is equivalent to a default constructor defined with a blank body.

- if some constructors are defined, but they are all non-default, the compiler will not implicitly define a default constructor. This means that a default constructor may not exist for a class.

- When an object value is declared with no argument list, e.g. MyClass x;; or allocated dynamically with no argument list, e.g. new MyClass; the default constructor is used to initialize the object

- When an array of objects is declared, e.g. MyClass x[10];; or allocated dynamically, e.g. new MyClass [10]; the default constructor is used to initialize all the elements

- When a derived class constructor does not explicitly call the base class constructor in its initializer list, the default constructor for the base class is called

- When a class constructor does not explicitly call the constructor of one of its object-valued fields in its initializer list, the default constructor for the field's class is called

- In the standard library, certain containers "fill in" values using the default constructor when the value is not given explicitly, e.g. vector(10); initializes the vector with 10 elements, which are filled with the default-constructed value of our type.

Link : http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fcplr376.htm

Whats the difference between copy constrctor and overloaded assignment operator ?

- If a new object has to be created before the copying can occur, the copy constructor is used.

- If a new object does not have to be created before the copying can occur, the assignment operator is used.

Eg. base a;

base b;

a=b;

There are three general cases where the copy constructor is called instead of the assignment operator:

1. When instantiating one object and initializing it with values from another object.

Eg. base b=a;

2. When passing an object by value.

3. When an object is returned from a function by value.

Link : http://www.learncpp.com/cpp-tutorial/911-the-copy-constructor-and-overloading-the-assignment-operator/

Why an empty class have size of one byte.?

- The reason is, the language standard states that all classes must have a memory size of at least 1 byte so that the class doesn't occupy the same memory space with another class. It'll be more clear if you think of two empty classes(class with no members).

- Each instance of the class/struct must have its own unique address , compiler compiler just adds a byte for padding. where as in case, if you have a member variable say "int a" inside the class/struct, it will be 4 bytes , not 5 bytes !!!

What is initialization list ?

Link : http://www.cprogramming.com/tutorial/initialization-lists-c++.html

How C++ implemented map container ? whats the data-structure it uses?

- Most probably a balanced binary search tree is used to implement map container.