C++ Interview questions Part 11

C++ interview question:What is slicing?

Answer:Slicing means that the data added by a subclass are discarded when an object of the subclass is passed or returned by value or from a function expecting a base class object. 

Explanation:

Consider the following class declaration:

class base

{

...

base& operator =(const base&);

base (const base&);

}

void fun( )

{

base e=m;

e=m;

} 

As base copy functions don't know anything about the derived only the base part of the derived is copied. This is commonly referred to as slicing. One reason to pass objects of classes in a hierarchy is to avoid slicing. Other reasons are to preserve polymorphic behavior and to gain efficiency. 

C++ interview question:What is name mangling?

Answer:Name mangling is the process through which your c++ compilers give each function in your program a unique name. In C++, all programs have at-least a few functions with the same name. Name mangling is a concession to the fact that linker always insists on all function names being unique.

Example:

In general, member names are made unique by concatenating the name of the member with that of the class e.g. given the declaration:

class Bar

{

public: 

int ival;

...

};

ival becomes something like:

// a possible member name mangling

ival__3Bar

Consider this derivation:

class Foo : public Bar 

{ 

public:

int ival;

...

}

The internal representation of a Foo object is the concatenation of its base and derived class members.

// Pseudo C++ code

// Internal representation of Foo

class Foo

{

public:

int ival__3Bar;

int ival__3Foo;

...

};

Unambiguous access of either ival members is achieved through name mangling. Member functions, because they can be overloaded, require an extensive mangling to provide each with a unique name. Here the compiler generates the same name for the two overloaded instances(Their argument lists make their instances unique). 

C++ interview question:What are proxy objects?

Answer:Objects that points to other objects are called proxy objects or surrogates. Its an object that provides the same interface as its server object but does not have any functionality. During a method invocation, it routes data to the true server object and sends back the return value to the object. 

C++ interview question:Differentiate between declaration and definition in C++.

Answer:A declaration introduces a name into the program; a definition provides a unique description of an entity (e.g. type, instance, and function). Declarations can be repeated in a given scope, it introduces a name in a given scope. There must be exactly one definition of every object, function or class used in a C++ program. 

A declaration is a definition unless:

Ø it declares a function without specifying its body,

Ø it contains an extern specifier and no initializer or function body,

Ø it is the declaration of a static class data member without a class definition,

Ø it is a class name definition,

Ø it is a typedef declaration.

A definition is a declaration unless:

Ø it defines a static class data member,

Ø it defines a non-inline member function.