C Plus Plus Theory Questions Part 2 - Online Article

Question 1

What is an orthogonal base class?

Answer

If two base classes have no overlapping methods or data they are said to be independent of, or orthogonal to each other. Orthogonal in the sense means that two classes operate in different dimensions and do not interfere with each other in any way. The same derived class may inherit such classes with no difficulty.

Question 2

What is a container class? What are the types of container classes?

Answer

A container class is a class that is used to hold objects in memory or external storage. A container class acts as a generic holder. A container class has a predefined behavior and a well-known interface. A container class is a supporting class whose purpose is to hide the topology used for maintaining the list of objects in memory. When a container class contains a group of mixed objects, the container is called a heterogeneous container; when the container is holding a group of objects that are all the same, the container is called a homogeneous container.

Question 3

What is a protocol class?

Answer

An abstract class is a protocol class if:

  • it neither contains nor inherits from classes that contain member data, non-virtual functions, or private (or protected) members of any kind.
  • it has a non-inline virtual destructor defined with an empty implementation,
  • all member functions other than the destructor including inherited functions, are declared pure virtual functions and left undefined.

Question 4

What is a mixin class?

Answer

A class that provides some but not all of the implementation for a virtual base class is often called mixin. Derivation done just for the purpose of redefining the virtual functions in the base classes is often called mixin inheritance. Mixin classes typically don't share common bases.

Question 5

What is a concrete class?

Answer

A concrete class is used to define a useful object that can be instantiated as an automatic variable on the program stack. The implementation of a concrete class is defined. The concrete class is not intended to be a base class and no attempt to minimize dependency on other classes in the implementation or behavior of the class.

Question 6

What is the handle class?

Answer

A handle is a class that maintains a pointer to an object that is programmatically accessible through the public interface of the handle class. Explanation: In case of abstract classes, unless one manipulates the objects of these classes through pointers and references, the benefits of the virtual functions are lost. User code may become dependent on details of implementation classes because an abstract type cannot be allocated statistically or on the stack without its size being known. Using pointers or references implies that the burden of memory management falls on the user. Another limitation of abstract class object is of fixed size. Classes however are used to represent concepts that require varying amounts of storage to implement them. A popular technique for dealing with these issues is to separate what is used as a single object in two parts: a handle providing the user interface and a representation holding all or most of the object's state. The connection between the handle and the representation is typically a pointer in the handle. Often, handles have a bit more data than the simple representation pointer, but not much more. Hence the layout of the handle is typically stable, even when the representation changes and also that handles are small enough to move around relatively freely so that the user needn't use the pointers and the references.

Question 7

What is an action class?

Answer

The simplest and most obvious way to specify an action in C++ is to write a function. However, if the action has to be delayed, has to be transmitted 'elsewhere' before being performed, requires its own data, has to be combined with other actions, etc then it often becomes attractive to provide the action in the form of a class that can execute the desired action and provide other services as well. Manipulators used with iostreams is an obvious example. Explanation: A common form of action class is a simple class containing just one virtual function.

class Action
{
	public: virtual int do_it( int ) = 0;
	virtual ~Action( );
}

Given this, we can write code say a member that can store actions for later execution without using pointers to functions, without knowing anything about the objects involved, and without even knowing the name of the operation it invokes.

Example

class write_file : public Action
{
	File& f;
	public: int do_it(int)
	{
		return fwrite( ).suceed( );
	}
};
class error_message: public Action
{
	response_box db(message.cstr( ),"Continue","Cancel","Retry");
	switch (db.getresponse( ))
	{
		case 0: 
			return 0;
		case 1:
			abort();
		case 2:
			current_operation.redo( );
			return 1;
	}
};

A user of the Action class will be completely isolated from any knowledge of derived classes such as write_file and error_message.

Question 8

When can you tell that a memory leak will occur?

Answer

A memory leak occurs when a program loses the ability to free a block of dynamically allocated memory.

Question 9

What is a parameterized type?

Answer

A template is a parameterized construct or type containing generic code that can use or manipulate any type. It is called parameterized because an actual type is a parameter of the code body. Polymorphism may be achieved through parameterized types. This type of polymorphism is called parameteric polymorphism. Parametric polymorphism is the mechanism by which the same code is used on different types passed as parameters.

Question 10

Differentiate between a deep copy and a shallow copy?

Answer

Deep copy involves using the contents of one object to create another instance of the same class. In a deep copy, the two objects may contain ht same information but the target object will have its own buffers and resources. the destruction of either object will not affect the remaining object. The overloaded assignment operator would create a deep copy of objects. Shallow copy involves copying the contents of one object into another instance of the same class thus creating a mirror image. Owing to straight copying of references and pointers, the two objects will share the same externally contained contents of the other object to be unpredictable. Explanation: Using a copy constructor we simply copy the data values member by member. This method of copying is called shallow copy. If the object is a simple class, comprised of built in types and no pointers this would be acceptable. This function would use the values and the objects and its behavior would not be altered with a shallow copy, only the addresses of pointers that are members are copied and not the value the address is pointing to. The data values of the object would then be inadvertently altered by the function. When the function goes out of scope, the copy of the object with all its data is popped off the stack. If the object has any pointers a deep copy needs to be executed. With the deep copy of an object, memory is allocated for the object in free store and the elements pointed to are copied. A deep copy is used for objects that are returned from a function.

Question 11

What is an opaque pointer?

Answer

A pointer is said to be opaque if the definition of the type to which it points to is not included in the current translation unit. A translation unit is the result of merging an implementation file with all its headers and header files.

Question 12

What is a smart pointer?

Answer

A smart pointer is an object that acts, looks and feels like a normal pointer but offers more functionality. In C++, smart pointers are implemented as template classes that encapsulate a pointer and override standard pointer operators. They have a number of advantages over regular pointers. They are guaranteed to be initialized as either null pointers or pointers to a heap object. Indirection through a null pointer is checked. No delete is ever necessary. Objects are automatically freed when the last pointer to them has gone away. One significant problem with these smart pointers is that unlike regular pointers, they don't respect inheritance. Smart pointers are unattractive for polymorphic code. Given below is an example for the implementation of smart pointers.

Example

template <class X> class smart_pointer
{
	public: smart_pointer();
	// makes a null pointer smart_pointer(const X& x)
	// makes pointer to copy of x X& operator *( );
	const X& operator*( ) const;
	X* operator->() const;
	smart_pointer(const smart_pointer <X> &);
	const smart_pointer <X> & operator = (const smart_pointer<X>&);
	~smart_pointer();
	private:
	//...
};

This class implement a smart pointer to an object of type X. The object itself is located on the heap. Here is how to use it:

smart_pointer <employee> p= employee("Harris",1333);

Like other overloaded operators, p will behave like a regular pointer,

cout<<*p; p->raise_salary(0.5);

Question 12

What is reflexive association?

Answer

The 'is-a' is called a reflexive association because the reflexive association permits classes to bear the is-a association not only with their super-classes but also with themselves. It differs from a 'specializes-from' as 'specializes-from' is usually used to describe the association between a super-class and a sub-class. For example: Printer is-a printer.

Question 13

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.

Question 14

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).

Question 15

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.

Question 16

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:

  • it declares a function without specifying its body,
  • it contains an extern specifier and no initializer or function body
  • it is the declaration of a static class data member without a class definition,
  • it is a class name definition,
  • it is a typedef declaration.

A definition is a declaration unless:

  • it defines a static class data member,
  • it defines a non-inline member function.

Question 17

What is cloning?

Answer

An object can carry out copying in two ways i.e. it can set itself to be a copy of another object, or it can return a copy of itself. The latter process is called cloning.

Question 18

Describe the main characteristics of static functions.

Answer

The main characteristics of static functions include,

  • It is without the a this pointer,
  • It can't directly access the non-static members of its class
  • It can't be declared const, volatile or virtual.
  • It doesn't need to be invoked through an object of its class, although for convenience, it may.

Question 19

Will the inline function be compiled as the inline function always? Justify.

Answer

An inline function is a request and not a command. Hence it won't be compiled as an inline function always. Explanation: Inline-expansion could fail if the inline function contains loops, the address of an inline function is used, or an inline function is called in a complex expression. The rules for inlining are compiler dependent.

Question 20

Define a way other than using the keyword inline to make a function inline.

Answer

The function must be defined inside the class.

Question 21

How can a '::' operator be used as unary operator?

Answer

The scope operator can be used to refer to members of the global namespace. Because the global namespace doesn't have a name, the notation :: member-name refers to a member of the global namespace. This can be useful for referring to members of global namespace whose names have been hidden by names declared in nested local scope. Unless we specify to the compiler in which namespace to search for a declaration, the compiler simple searches the current scope, and any scopes in which the current scope is nested, to find the declaration for the name.

Question 22

What is placement new?

Answer

When you want to call a constructor directly, you use the placement new. Sometimes you have some raw memory that's already been allocated, and you need to construct an object in the memory you have. Operator new's special version placement new allows you to do it.

class Widget { public : Widget(int widgetsize); ... Widget* Construct_widget_int_buffer(void *buffer,int widgetsize) { return new(buffer) Widget(widgetsize); } };

This function returns a pointer to a Widget object that's constructed within the buffer passed to the function. Such a function might be useful for applications using shared memory or memory-mapped I/O, because objects in such applications must be placed at specific addresses or in memory allocated by special routines.

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