Part - 2 MFC Interview Questions & Answers

Q:- What is thread & process?

Ans:- Threads are similar to processes, but differ in the way that they share resources. Threads are distinguished from processes in that processes are typically independent, carry considerable state information and have separate address spaces. Threads typically share the memory belonging to their parent process.

Q:- what is the use of AFX_MANAGE_STATE ?

Ans:- By default, MFC uses the resource handle of the main application to load the resource template. If you have an exported function in a DLL, such as one that launches a dialog box in the DLL, this template is actually stored in the DLL module. You need to switch the module state for the correct handle to be used. You can do this by adding the following code to the beginning of the function:

AFX_MANAGE_STATE(AfxGetStaticModuleState( ));

This swaps the current module state with the state returned from AfxGetStaticModuleState until the end of the current scope. If all your resources lies in the single DLL you can even change the default handle to the DLL handle with the help of AfxSetResourceHandle function.

Q:- Why wizards generate enum IDD for dialogs?

Ans:- It's good programming practice to do it this way, as from the client code you can always refer to the CMyDlg::IDD without worrying what the actual constant is.

Q:- What is CArchive class does?

Ans:- The CArchive class allows you to save a complex network of objects in a permanent binary form (usually disk storage) that persists after those objects are deleted. Later you can load the objects from persistent storage, reconstituting them in memory. This process of making data persistent is called “serialization.”

Q:- How to handle command line arguements from simple MFC application ?

Ans:- m_lpCmdLine Corresponds to the lpCmdLine parameter passed by Windows to WinMain. Points to a null-terminated string that specifies the command line for the application. Use m_lpCmdLine to access any command-line arguments the user entered when the application was started. m_lpCmdLine is a public variable of type LPTSTR.

BOOL CMyApp::InitInstance()

{

// ...

if (m_lpCmdLine[0] == _T('\0'))

{

// Create a new (empty) document.

OnFileNew();

}

else

{

// Open a file passed as the first command line parameter.

OpenDocumentFile(m_lpCmdLine);

}

// ...

}

Q:- What is the base class for MFC Framework ?

Ans:-CObject

Q:- If I derive a new class from CObject what are the basic features my derived will get?

Ans:-Searialization, Debugging support, Runtime time class information, compatibility with collection classes.

Q:- What is the use of CCmdTarget ?

Ans:- It is the base class for the MFC library message map architecture.Which maps commands/messages to the member functions to handle them. Classes derived from this are CWnd,CWinApp,CFrameWnd,CView, CDocument

Q:- What is document-view architecture ? Give me one real time example for SDI ?

Ans:- Document/view architecture, which defines a program structure that relies on document objects to hold an application's data and on view objects to render views of that data. MFC provides the infrastructure for documents and views in the classes CDocument and CView. example of SDI is a wordpad application

Q:- Can you explain the relashionship between document,frame and view ?

Ans:- The frame window is the application's top-level window. It's normally a WS_OVERLAPPEDWINDOW-style window with a resizing border, a title bar, a system menu, and minimize, maximize, and close buttons. 

The view is a child window sized to fit the frame window so that it becomes the frame window's client area. 

The application's data is stored in the document object, a visible representation of which appears in the view. 

For an SDI application, the frame window class is derived from CFrameWnd, the document class is derived from CDocument, and the view class is derived from CView or a related class such as CScrollView.

Q:- How to access document object from view ?

Ans:- Using GetDocument() function within a CView class.

Q:- What is the entry point for window based applications ?

Ans:- WinMain() is the entry point for window based applications.

Q:- Explain the flow for a simple win32 based application ?

Ans:- Starting point for win32 based applications is WinMain()

WinMain begins by calling the API function RegisterClass to register a window class. The window class(WNDCLASS) defines important characteristics of a window such as its window procedure address, its default background color, and its icon. Once the WNDCLASS is registered, WinMain calls the all-important CreateWindow function to create the application's window. WinMain follows CreateWindow with calls to ShowWindow and UpdateWindow, which make the window visible and ensure that its WM_PAINT handler is called immediately. Next comes the message loop. In order to retrieve and dispatch messages, WinMain executes a simple while loop that calls the GetMessage, TranslateMessage, and DispatchMessage API functions repeatedly. GetMessage checks the message queue. If a message is available, it is removed from the queue and copied to msg; TranslateMessage converts a keyboard message denoting a character key to an easier-to-use WM_CHAR message, and DispatchMessage dispatches the message to the window procedure. The message loop executes until GetMessage returns 0, which happens only when a WM_QUIT message is retrieved from the message queue. When this occurs, WinMain ends and the program terminates.

MFC Interview Questions & Answers

Que: What is the difference between GetMessage and PeekMessage ?

Ans: GetMessage function waits for a message to be placed in the queue before returning where as PeekMessage function does not wait for a message to be placed in the queue before returning.

Que: What’s the difference between PostMessage and SendMessage?

Ans: The PostMessage function places (posts) a message in the message queue associated with the thread that created the specified window and then returns without waiting for the thread to process the message.

The SendMessage function sends the specified message to a window or windows. The function calls the window procedure for the specified window and does not return until the window procedure has processed the message.

Que: How to create a model less dialog?

Ans: m_pModeless is a variable of type CDialog or any of its descendants.

m_pModeless->Create(IDD_DIALOG1, this);

m_pModeless->ShowWindow(SW_SHOW); 

this pointer as a paramter suggest we are creating a child dialog of the current dialog/window.

Que: How to setup a timer?

Ans: Use the SetTimer function

UINT_PTR SetTimer( HWND hWnd, UINT_PTR nIDEvent, UINT uElapse, 

TIMERPROC lpTimerFunc );

To kill the timer use

BOOL KillTimer(int nIDEvent);

Where the nIDEvent is the ID returned by the SetTimer Function.

Que: Name the Synchronization objects ?

Ans: Following are the synchronization objects 

1) Critical Section 2) Event 3) Mutex 4) Semaphore

Classes provided for above synchronization objects are:

1) CCriticalSection 2) CEvent 3) CMutex 4) CSemaphore

Que: What Is CMutex ?

Ans: An object of class CMutex represents a “mutex” — a synchronization object that allows one thread mutually exclusive access to a resource. Mutexes are useful when only one thread at a time can be allowed to modify data or some other controlled resource. For example, adding nodes to a linked list is a process that should only be allowed by one thread at a time. By using a CMutex object to control the linked list, only one thread at a time can gain access to the list.

To use a CMutex object, construct the CMutex object when it is needed. Specify the name of the mutex you wish to wait on, and that your application should initially own it. You can then access the mutex when the constructor returns. Call CSyncObject::Unlock when you are done accessing the controlled resource.

An alternative method for using CMutex objects is to add a variable of type CMutex as a data member to the class you wish to control. During construction of the controlled object, call the constructor of the CMutex data member specifying if the mutex is initially owned, the name of the mutex (if it will be used across process boundaries), and desired security attributes.

To access resources controlled by CMutex objects in this manner, first create a variable of either type CSingleLock or type CMultiLock in your resource’s access member function. Then call the lock object’s Lock member function (for example, CSingleLock::Lock). At this point, your thread will either gain access to the resource, wait for the resource to be released and gain access, or wait for the resource to be released and time out, failing to gain access to the resource. In any case, your resource has been accessed in a thread-safe manner. To release the resource, use the lock object’s Unlock member function (for example, CSingleLock::Unlock), or allow the lock object to fall out of scope

Que: What is thread & process?

Ans: Threads are similar to processes, but differ in the way that they share resources. Threads are distinguished from processes in that processes are typically independent, carry considerable state information and have separate address spaces. Threads typically share the memory belonging to their parent process.

Que: what is the use of AFX_MANAGE_STATE ?

Ans: By default, MFC uses the resource handle of the main application to load the resource template. If you have an exported function in a DLL, such as one that launches a dialog box in the DLL, this template is actually stored in the DLL module. You need to switch the module state for the correct handle to be used. You can do this by adding the following code to the beginning of the function:

AFX_MANAGE_STATE(AfxGetStaticModuleState( ));

This swaps the current module state with the state returned from AfxGetStaticModuleState until the end of the current scope. If all your resources lies in the single DLL you can even change the default handle to the DLL handle with the help of AfxSetResourceHandle function.

C Inteview Question

Question :: Read below code and answer the following questions.

 

LINE	Contains
50	char * b, q, *r;
200	b=getbuf();
201	q = *b;
212	r= anotherfunction(b);
213-300	/* we want to use ‘q’ and  ‘r’ here*/
2000	char * getbuf()
2001	{
2002	char buff[8];
2003-2050	/* unspecified, buff defined here *./
2051	return (char *) buff;
2052	}

1. What will be in variable ‘q’ after line 201 is executed? Under what conditions might this not be so?
Answer:


2. Is there an alternative, but equivalent, way to write line 2000? If so, what is it?
Answer:



3. Is getbuf() a reasonable function?
Answer:



4. Will getbuf() execute at all?
Answer:



5. Please comment on line 2051.
Answer:



6. Is getbuf() good practice, and why?
Answer:



7. What line not given should be provided for compilation?
Answer :

(Suggestions and comments are welcome)

Interview FAQs : C++ (Answers End of Post)

1) Which is the parameter that is added to every non-static member function when it is called?
---------------------------------------------------------------------------------------
2)
class base
{
public:
int bval;
base(){ bval=0;}
};
class deri:public base
{
public:
int dval;
deri(){ dval=1;}
};
void SomeFunc(base *arr,int size)
{
for(int i=0; i <>bval;
cout << endl; } int main() { base BaseArr[5]; SomeFunc(BaseArr,5); deri DeriArr[5]; SomeFunc(DeriArr,5); }
---------------------------------------------------------------------------------------
3)
class base { public: void baseFun(){ cout << "from base" <<>baseFun();
}
int main()
{
base baseObject;
SomeFunc(&baseObject);
Deri deriObject;
SomeFunc(&deriObject);
}
---------------------------------------------------------------------------------------
4)
class base
{
public:
virtual void baseFun(){ cout << "from base" <<>baseFun();
}
int main()
{
base baseObject;
SomeFunc(&baseObject);
Deri deriObject;
SomeFunc(&deriObject);
}
---------------------------------------------------------------------------------------
5)
class some
{
public:
~some()
{
cout << "some's destructor" << endl; } }; void main() { some s; s.~some(); } ---------------------------------------------------------------------------------------
6) #include class fig2d { int dim1; int dim2; public: fig2d() { dim1=5; dim2=6;} virtual void operator<<(ostream & rhs); }; void fig2d::operator<<(ostream &rhs) { rhs <<>dim1 <<" "<<<" "; } /*class fig3d : public fig2d { int dim3; public: fig3d() { dim3=7;} virtual void operator << (ostream &rhs); }; void fig3d::operator << (ostream &rhs) { fig2d::operator <<(rhs); rhs <<>dim3;
}
*/

void main()
{
fig2d obj1;
// fig3d obj2;
obj1 << cout; // obj2 << cout; }
---------------------------------------------------------------------------------------
7) Class opOverload { Public: bool operator==(opOverload temp); }; bool opOverload::operator==(opOverload temp) { if(*this == temp ) { cout << "The both are same objects\n"; return true; } else { cout << "The both are different\n"; return false; } } void main() { opOverload a1, a2; a1= =a2; }
---------------------------------------------------------------------------------------
8) Class complex { double re; double im; public: complex() : re(1),im(0.5) {} bool operator==(complex &rhs); operator int(){} }; bool complex::operator == (complex &rhs) { if((this->re == rhs.re) && (this->im == rhs.im))
return true;
else
return false;
}
int main()
{
complex c1;
cout << c1;
}
---------------------------------------------------------------------------------------
9)
Class complex
{
double re;
double im;
public:
complex() : re(0),im(0) {}
complex(double n) { re=n,im=n;};
complex(int m,int n) { re=m,im=n;}
void print() { cout << re; cout << im;}
};
void main()
{
complex c3;
double i=5;
c3 = i;
c3.print();
}
---------------------------------------------------------------------------------------
C++ Answer
---------------------------------------------------------------------------------------
1. ‘this’ pointer

00000

2. 01010

3. from base
from base

4. from base
from Derived

5. some's destructor
some's destructor

6. 5 6

7. Runtime Error: Stack Overflow

8. Garbage value

9. 5,5

Interview FAQs : C (Answers at end of post)

1. Base class has some virtual method and derived class has a method with the same name. If we initialize the base class pointer with derived
object, Calling of that virtual method will result in which method being called?

a. Base method
b. Derived method...
-----------------------------------------------------------------------------------------------
2. For the following C program

#define AREA(x)(3.14*x*x)
main()
{float r1=6.25,r2=2.5,a;
a=AREA(r1);
printf("\n Area of the circle is %f", a);
a=AREA(r2);
printf("\n Area of the circle is %f", a);
}

What is the output?
-----------------------------------------------------------------------------------------------
3.
void main()
{
int d=5;
printf("%f",d);
}
-----------------------------------------------------------------------------------------------
4.
void main()
{
int i;
for(i=1;i<4,i++) s="\12345s\n" i="1;" k=" -1"> k=255; */
signed j=-1; /* char k= -1 => k=65535 */
/* unsigned or signed int k= -1 =>k=65535 */
if(ij)
printf("greater");
else
if(i==j)
printf("equal");
}
-----------------------------------------------------------------------------------------------
5.
void main()
{
char *s="\12345s\n";
printf("%d",sizeof(s));
}
-----------------------------------------------------------------------------------------------
6.
void main()
{
unsigned i=1; /* unsigned char k= -1 => k=255; */
signed j=-1; /* char k= -1 => k=65535 */
/* unsigned or signed int k= -1 =>k=65535 */
if(ij)
printf("greater");
else
if(i==j)
printf("equal");
}
-----------------------------------------------------------------------------------------------
7.
void main()
{
float j;
j=1000*1000;
printf("%f",j);
}

1. 1000000
2. Overflow
3. Error
4. None
-----------------------------------------------------------------------------------------------
8. How do you declare an array of N pointers to functions returning
pointers to functions returning pointers to characters?
-----------------------------------------------------------------------------------------------
9.
int f()
void main()
{
f(1);
f(1,2);
f(1,2,3);
}
f(int i,int j,int k)
{
printf("%d %d %d",i,j,k);
}

What are the number of syntax errors in the above?
-----------------------------------------------------------------------------------------------
10.
void main()
{
int i=7;
printf("%d",i++*i++);
}
-----------------------------------------------------------------------------------------------
11.
#define one 0
#ifdef one
printf("one is defined ");
#ifndef one
printf("one is not defined ");
-----------------------------------------------------------------------------------------------
12.
void main()
{
int count=10,*temp,sum=0;
temp=&count;
*temp=20;
temp=∑
*temp=count;
printf("%d %d %d ",count,*temp,sum);
}

-----------------------------------------------------------------------------------------------
13. what is alloca()
-----------------------------------------------------------------------------------------------
14.
main()
{
static i=3;
printf("%d",i--);
return i>0 ? main():0;
}
-----------------------------------------------------------------------------------------------
15.
char *foo()
{
char result[100]);
strcpy(result,"anything is good");
return(result);
}
void main()
{
char *j;
j=foo()
printf("%s",j);
}
-----------------------------------------------------------------------------------------------
16.
void main()
{
char *s[]={ "dharma","hewlett-packard","siemens","ibm"};
char **p;
p=s;
printf("%s",++*p);
printf("%s",*p++);
printf("%s",++*p);
}
-----------------------------------------------------------------------------------------------
17. Output of the following program is
main()
{int i=0;
for(i=0;i<20;i++) c="-64;" i="-32" u ="-16;">i)
{printf("pass1,");
if(c<<2,x>>2);
}
-----------------------------------------------------------------------------------------------
18. What is the ouptut in the following program
main()
{char c=-64;
int i=-32
unsigned int u =-16;
if(c>i)
{printf("pass1,");
if(c<<2,x>>2);
}
-----------------------------------------------------------------------------------------------
25. Find the output for the following C program

#define swap1(a,b) a=a+b;b=a-b;a=a-b;
main()
{
int x=5,y=10;
swap1(x,y);
printf("%d %d\n",x,y);
swap2(x,y);
printf("%d %d\n",x,y);
}
int swap2(int a,int b)
{
int temp;
temp=a;
b=a;
a=temp;
return;
}
-----------------------------------------------------------------------------------------------
26. Find the output for the following C program
main()
{
char *ptr = "Ramco Systems";
(*ptr)++;
printf("%s\n",ptr);
ptr++;
printf("%s\n",ptr);
}
-----------------------------------------------------------------------------------------------
27. Find the output for the following C program
#include
main()
{
char *p1;
char *p2;
p1=(char *) malloc(25);
p2=(char *) malloc(25);
strcpy(p1,"Ramco");
strcpy(p2,"Systems");
strcat(p1,p2);
printf("%s",p1);
}
-----------------------------------------------------------------------------------------------
28. Find the output for the following C program
# define TRUE 0
some code
while(TRUE)
{
some code
}
-----------------------------------------------------------------------------------------------
29. struct list{
int x;
struct list *next;
}*head;

the struct head.x =100

Is the above assignment to pointer is correct or wrong ?
-----------------------------------------------------------------------------------------------
30.What is the output of the following ?
int i;
i=1;
i=i+2*i++;
printf(%d,i);
-----------------------------------------------------------------------------------------------
31. FILE *fp1,*fp2;
fp1=fopen("one","w")
fp2=fopen("one","w")
fputc('A',fp1)
fputc('B',fp2)
fclose(fp1)
fclose(fp2)
}
Find the Error, If Any?
-----------------------------------------------------------------------------------------------
32. #define MAN(x,y) (x)>(y)?(x):(y)
{int i=10;
j=5;
k=0;
k=MAX(i++,++j);
printf(%d %d %d %d,i,j,k);
}
-----------------------------------------------------------------------------------------------
33.void main()
{
int i=7;
printf("%d",i++*i++);
}
-----------------------------------------------------------------------------------------------
34.How will u terminate the statement?
-----------------------------------------------------------------------------------------------
35.select the wrong one
a.a+=1;
b.a*=2;
c.a**=1;
d.a>>=1;
-----------------------------------------------------------------------------------------------
36.pick the odd one
a.malloc
b.calloc
c.new
-----------------------------------------------------------------------------------------------
37.main()
{
char **p=="Hello";
printf("%s",**p);
}
-----------------------------------------------------------------------------------------------
38.main()
{
printf("%d%c\n");
printf("%d%c\n");
}
-----------------------------------------------------------------------------------------------
39.main()
{
int x==5;
printf("%d%d",x++,++x);
}
-----------------------------------------------------------------------------------------------
40.main()
{
int x==4;
printf("%d",printf(" %d %d ",x,x) );
}
-----------------------------------------------------------------------------------------------
41.main()
{
union
{
int i;
char p;
struct
{
int t;
char e;
char o;
};
};
printf("%d\n",sizeof(l) );
}
-----------------------------------------------------------------------------------------------
42.main()
{
int i==0,n==6;
while(n--0);
i+==n;
printf("%d\n",i);
}
-----------------------------------------------------------------------------------------------
43.a=3,b=2,c=1;
What's the value of k?
k= a< a="=" a=" 1;" i="=" i="=" a="e" s="'R';" i="=" a1="=" a2="=" t="¡;" a1="¢;" a2=" t;" s1="=" s2="s1;" temp=" s1;" a1= "new" a2= "dictionary" t="¡;" a1="¢;" a2="t;" s1=" s2;" s2=" s1;" temp=" s1;" p="=">add(dharma) && (*p)->harma)
"harma" (after printing, p->add(hewlett-packard) &&(*p)->harma)
"ewlett-packard"
-----------------------------------------------------------------------------------------------

17. (d)

18. (c)

19. (b)

20. (a)

21. (b)

22. An empty string

23. 57 94

24. 5 20 1

25. 10 5

26. Samco Systems

27. RamcoSystems

28. This won't go into the loop as TRUE is defined as 0

29. Wrong

30. 4

31. no error. But It will over writes on same file.

32. 10 5 0

33. 56

34. ;

35. C

36. C

37. Garbage or nothing

38. Garbage Value

39. 6 6

40. 4 4 5

41. 4

42. -1

43. 0

44. 3

45. 16 21

46. UPPER CASE

47. 5 4 3 2 1

48. (newdictionary)-(dictionarynew)

49. (newdictionary)-(dictionarynew)

50. 50

C++ Interview Questions: Part 9

31. 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.

32.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. Parameteric polymorphism is the mechanism by which the same code is used on different types passed as parameters.

33. 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.

34. 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.

35. 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 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 &);
const smart_pointer & operator =(const smart_pointer&);
~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 p= employee("Harris",1333);
Like other overloaded operators, p will behave like a regular pointer,
cout<<*p; p->raise_salary(0.5);

36. 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.

37. 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.

38. 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).

39. 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.

40. 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.

41. 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.

42. 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.

43. 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.

44. Define a way other than using the keyword inline to make a function inline.
Answer:
The function must be defined inside the class.

45. 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.

46. 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.

C++ Interview Questions: Part 8

1. What is a modifier?
Answer: A modifier, also called a modifying function is a member function that changes the value of at least one data member. In other words, an operation that modifies the state of an object. Modifiers are also known as ‘mutators’.

2. What is an accessor?
Answer: An accessor is a class operation that does not modify the state of an object. The accessor functions need to be declared as const operations

3. Differentiate between a template class and class template.
Answer:

Template class: A generic definition or a parameterized class not instantiated until the client provides the needed information. It’s jargon for plain templates.

Class template: A class template specifies how individual classes can be constructed much like the way a class specifies how individual objects can be constructed. It’s jargon for plain classes.

4. When does a name clash occur?
Answer: A name clash occurs when a name is defined in more than one place. For example., two different class libraries could give two different classes the same name. If you try to use many class libraries at the same time, there is a fair chance that you will be unable to compile or link the program because of name clashes.

5. Define namespace.
Answer: It is a feature in c++ to minimize name collisions in the global name space. This namespace keyword assigns a distinct name to a library that allows other libraries to use the same identifier names without creating any name collisions. Furthermore, the compiler uses the namespace signature for differentiating the definitions.

6. What is the use of ‘using’ declaration.
Answer: A using declaration makes it possible to use a name from a namespace without the scope operator.

7. What is an Iterator class?
Answer: A class that is used to traverse through the objects maintained by a container class.

There are five categories of iterators:

 input iterators,
 output iterators,
 forward iterators,
 bidirectional iterators,
 random access.

An iterator is an entity that gives access to the contents of a container object without violating encapsulation constraints. Access to the contents is granted on a one-at-a-time basis in order. The order can be storage order (as in lists and queues) or some arbitrary order (as in array indices) or according to some ordering relation (as in an ordered binary tree). The iterator is a construct, which provides an interface that, when called, yields either the next element in the container, or some value denoting the fact that there are no more elements to examine. Iterators hide the details of access to and update of the elements of a container class.
The simplest and safest iterators are those that permit read-only access to the contents of a container class. The following code fragment shows how an iterator might appear in code:
cont_iter:=new cont_iterator();
x:=cont_iter.next();
while x/=none do
...
s(x);
...
x:=cont_iter.next();
end;
In this example, cont_iter is the name of the iterator. It is created on the first line by instantiation of cont_iterator class, an iterator class defined to iterate over some container class, cont. Succesive elements from the container are carried to x. The loop terminates when x is bound to some empty value. (Here, none)In the middle of the loop, there is s(x) an operation on x, the current element from the container. The next element of the container is obtained at the bottom of the loop.

9. List out some of the OODBMS available.
Answer:

 GEMSTONE/OPAL of Gemstone systems.
 ONTOS of Ontos.
 Objectivity of Objectivity inc.
 Versant of Versant object technology.
 Object store of Object Design.
 ARDENT of ARDENT software.
 POET of POET software.

10. List out some of the object-oriented methodologies.
Answer:

 Object Oriented Development (OOD) (Booch 1991,1994).
 Object Oriented Analysis and Design (OOA/D) (Coad and Yourdon 1991).
 Object Modelling Techniques (OMT) (Rumbaugh 1991).
 Object Oriented Software Engineering (Objectory) (Jacobson 1992).
 Object Oriented Analysis (OOA) (Shlaer and Mellor 1992).
 The Fusion Method (Coleman 1991).

11. What is an incomplete type?
Answer: Incomplete types refers to pointers in which there is non availability of the implementation of the referenced location or it points to some location whose value is not available for modification.
Example:
int *i=0x400 // i points to address 400
*i=0; //set the value of memory location pointed by i.
Incomplete types are otherwise called uninitialized pointers.

12. What is a dangling pointer?
Answer: A dangling pointer arises when you use the address of an object after its lifetime is over. This may occur in situations like returning addresses of the automatic variables from a function or using the address of the memory block after it is freed.

13. Differentiate between the message and method.
Answer:

Message Method
Objects communicate by sending messages Provides response to a message.
to each other.
A message is sent to invoke a method. It is an implementation of an operation.

14. What is an adaptor class or Wrapper class?
Answer: A class that has no functionality of its own. Its member functions hide the use of a third party software component or an object with the non-compatible interface or a non- object- oriented implementation.

15. What is a Null object?
Answer: It is an object of some class whose purpose is to indicate that a real object of that class does not exist. One common use for a null object is a return value from a member function that is supposed to return an object with some specified properties but cannot find such an object.

15. What is a Null object?
Answer: It is an object of some class whose purpose is to indicate that a real object of that class does not exist. One common use for a null object is a return value from a member function that is supposed to return an object with some specified properties but cannot find such an object.

16. What is class invariant?
Answer: A class invariant is a condition that defines all valid states for an object. It is a logical condition to ensure the correct working of a class. Class invariants must hold when an object is created, and they must be preserved under all operations of the class. In particular all class invariants are both preconditions and post-conditions for all operations or member functions of the class.

17. What do you mean by Stack unwinding?
Answer: It is a process during exception handling when the destructor is called for all local objects between the place where the exception was thrown and where it is caught.

18. Define precondition and post-condition to a member function.
Answer:

Precondition: A precondition is a condition that must be true on entry to a member function. A class is used correctly if preconditions are never false. An operation is not responsible for doing anything sensible if its precondition fails to hold.
For example, the interface invariants of stack class say nothing about pushing yet another element on a stack that is already full. We say that isful() is a precondition of the push operation.

Post-condition: A post-condition is a condition that must be true on exit from a member function if the precondition was valid on entry to that function. A class is implemented correctly if post-conditions are never false.
For example, after pushing an element on the stack, we know that isempty() must necessarily hold. This is a post-condition of the push operation.

19. What are the conditions that have to be met for a condition to be an invariant of the class?
Answer:

 The condition should hold at the end of every constructor.
 The condition should hold at the end of every mutator(non-const) operation.

20. What are proxy objects?
Answer:

Objects that stand for other objects are called proxy objects or surrogates.

Example:
template
class Array2D
{
public:
class Array1D
{
public:
T& operator[] (int index);
const T& operator[] (int index) const;
...
};
Array1D operator[] (int index);
const Array1D operator[] (int index) const;
...
};

The following then becomes legal:
Array2Ddata(10,20);
........
cout<

21. Name some pure object oriented languages.
Answer:

 Smalltalk,
 Java,
 Eiffel,
 Sather.

22. Name the operators that cannot be overloaded.
Answer:
sizeof . .* .-> :: ?:

23. What is a node class?
Answer:

A node class is a class that,
 relies on the base class for services and implementation,
 provides a wider interface to te users than its base class,
 relies primarily on virtual functions in its public interface
 depends on all its direct and indirect base class
 can be understood only in the context of the base class
 can be used as base for further derivation
 can be used to create objects.
A node class is a class that has added new services or functionality beyond the services inherited from its base class.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

29.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.

30. 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. For 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.

C++ Interview Questions: Part 7

Q: What is a default constructor?

A: A constructor that has no arguments.
If you don't code one, the compiler provides one if there are no other constructors. If you are going to instantiate an array of objects of the class, the class must have a default constructor.
linkage specification statement turns that feature off during compilation so that the linker properly links function calls to C functions. Another acceptable answer is "I don't know. We never had to do that." Merely describing what a linker does indicates that the programmer does not understand the issue that underlies the question.

Q: Explain the scope resolution operator.

A: It permits a program to reference an identifier in the global scope that has been hidden by another identifier with the same name in the local scope.
The answer can get complicated. However, it should start with "::". If the programmer is well into the design or use of classes that employ inheritance you might hear a lot about overriding member function overrides to explicitly call a function higher in the hierarchy. That's good to know, but ask specifically about global scope resolution. You're looking for a description of C++'s ability to override the particular C behavior where identifiers in the global scope are always hidden by like identifiers in a local scope.

Q: What are the differences between a C++ struct and C++ class?

A: The default member and base-class access specifiers 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 specifiers, member functions, overloaded operators, and so on. Some of them have even written books about C++. 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 specifier and private base-class inheritance. Getting this question wrong does not necessarily disqualify an applicant. Getting it right is a definite plus.
Saying, "I don't know" is definitely the wrong answer. I advance an unusual position about this. C++ programmers should at least believe that they know the differences, even when they are wrong about them. Getting it wrong is, therefore, right. You can explain the true difference in the interview and advance the programmer's knowledge. If they disagree vociferously, you have an opportunity to observe how they handle contentious debate when they are wrong and don't know it yet.

Q: How many ways are there to initialize an int with a constant?

A: Two.
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the traditional C notation. The second format uses constructor notation.
int foo = 123;
int bar (123);
It's acceptable when a programmer does not know about the second notation, although they should certainly know about the first one. Many old-timer C programmers who made the switch to C++ never use the second idiom, although some wise heads of C++ profess to prefer it. If your applicant is quick with the right answer, that's a good sign.

Q: How does throwing and catching exceptions differ from using setjmp and longjmp?

A: The throw operation calls the destructors for automatic objects instantiated since entry to the try block.
Exceptions are in the mainstream of C++ now, so most programmers, if they are familiar with setjmp and longjmp, should know the difference. Both idioms return a program from the nested depths of multiple function calls to a defined position higher in the program. The program stack is "unwound" so that the state of the program, with respect to function calls and pushed arguments, is restored as if the calls had not been made. C++ exception handling adds to that behavior the orderly calls to the destructors of automatic objects that were instantiated as the program proceeded from within the try block toward where the throw expression is evaluated.
Applicants might think you want to hear about the notational differences between the two idioms. Let them proceed to explain the syntax of try blocks, catch exception handlers, and throw expressions. Then ask them specifically what happens in a throw that does not happen in a longjmp. Their answer should reflect an understanding of the behavior described in the previous answer.
One valid reason for not knowing about exception handling is that the applicant's experience is exclusively with older C++ compilers that do not implement exception handling. I would prefer that they have at least heard of exception handling, though. Another marginally acceptable reason is that their former supervisors and designers did not mandate and specify the use of exception handling in programs. In that case get the names of those supervisors and designers so that you can decline their applications if they should come a'knocking.
It is not unusual for C and C++ programmers to be unfamiliar with setjmp / longjmp. Those constructs are not particularly intuitive. A C programmer who has written recursive descent parsing algorithms will certainly be familiar with setjmp/longjmp. Others might not, and that's acceptable. In that case, they won't be able to discuss how setjmp/longjmp differs from C++ exception handling, but let the interview turn into a discussion of C++ exception handling in general. That conversation will reveal a lot about a programmer's understanding of C++.

Q: What is your reaction to this line of code?
delete this;

A: It's not a good practice.
Many applicants will look at you like you are nuts. They've never heard of this usage, and it's never occurred to them. That's a very good answer. Perhaps they will try to explain the behavior of the statement. Ask them to contemplate its consequences. Two quite acceptable reactions are, "Don't do it," and "Don't do it unless you really know what you are doing and you are a masochist."
A good programmer will insist that you should absolutely never use the statement if the class is to be used by other programmers and instantiated as static, extern, or automatic objects. That much should be obvious.
The code has two built-in pitfalls. First, if it executes in a member function for an extern, static, or automatic object, the program will probably crash as soon as the delete statement executes. There is no portable way for an object to tell that it was instantiated on the heap, so the class cannot assert that its object is properly instantiated. Second, when an object commits suicide this way, the using program might not know about its demise. As far as the instantiating program is concerned, the object remains in scope and continues to exist even though the object did itself in. Subsequent dereferencing of the pointer can and usually does lead to disaster. I think that the language rules should disallow the idiom, but that's another matter.
In More Effective C++ (Addison-Wesley, 1996), Scott Meyers devotes one of his items to "delete this," implying that there are valid applications for the idiom and advancing contrived code kludges to make it seem to work better. A programmer who has read this otherwise very good book might think that the practice is acceptable. Experience leads me to disagree.


Q: What is a conversion constructor?

A: A constructor that accepts one argument of a different type.
The compiler uses this idiom as one way to infer conversion rules for your class. A constructor with more than one argument and with default argument values can be interpreted by the compiler as a conversion constructor when the compiler is looking for an object of your constructor's type and sees an object of the type of the constructor's first argument.

Q: What is the difference between a copy constructor and an overloaded assignment operator?

A: A copy constructor constructs a new object by using the content of the argument object. An overloaded assignment operator assigns the contents of an existing object to another existing object of the same class.
First, the applicant must know that a copy constructor is one that has only one argument of the same type as the constructor. The compiler invokes a copy constructor wherever it needs to make a copy of the object, for example to pass an argument by value. If you do not provide a copy constructor, the compiler creates a member- by-member copy constructor for you.
You can write overloaded assignment operators that take arguments of other classes, but that behavior is usually implemented with implicit conversion constructors. If you do not provide an overloaded assignment operator for the class, the compiler creates a default member- by-member assignment operator.
This discussion is a good place to get into why classes need copy constructors and overloaded assignment operators. If the applicant discusses these with respect to data member pointers that point to dynamically allocated resources, the applicant probably has a good grasp of the problem.

Q: When should you use multiple inheritance?

A: There are three acceptable answers: "Never," "Rarely," and "When the problem domain cannot be accurately modeled any other way."
There are some famous C++ pundits and luminaries who disagree with that third answer, but I will accept it.
Let's digress to consider this issue lest your interview turn into a religious debate. Consider an Asset class, Building class, Vehicle class, and CompanyCar class. All company cars are vehicles. Some company cars are assets because the organizations own them. Others might be leased. Not all assets are vehicles. Money accounts are assets. Real estate holdings are assets. Some real estate holdings are buildings. Not all buildings are assets. Ad infinitum. When you diagram these relationships, it becomes apparent that multiple inheritance is a likely and intuitive way to model this common problem domain. The applicant should understand, however, that multiple inheritance, like a chainsaw, is a useful tool that has its perils, needs respect, and is best avoided except when nothing else will do.

Q: What is a virtual destructor?
A: The simple answer is that a virtual destructor is one that is declared with the virtual attribute.
The behavior of a virtual destructor is what is important. If you destroy an object through a pointer or reference to a base class, and the base-class destructor is not virtual, the derived-class destructors are not executed, and the destruction might not be complete.

Q: Explain the ISA and HASA class relationships. How would you implement each in a class design?

A: A specialized class "is" a specialization of another class and, therefore, has the ISA relationship with the other class. An Employee ISA Person. This relationship is best implemented with inheritance. Employee is derived from Person. A class may have an instance of another class. For example, an employee "has" a salary, therefore the Employee class has the HASA relationship with the Salary class. This relationship is best implemented by embedding an object of the Salary class in the Employee class.
The answer to this question reveals whether the applicant has an understanding of the fundamentals of object- oriented design, which is important to reliable class design.
There are other relationships. The USESA relationship is when one class uses the services of another. The Employee class uses an object (cout) of the ostream class to display the employee's name on the screen, for example. But if the applicant gets ISA and HASA right, you don't need to go any further.

Q: When is a template a better solution than a base class?

A: When you are designing a generic class to contain or otherwise manage objects of other types, when the format and behavior of those other types are unimportant to their containment or management, and particularly when those other types are unknown (thus, the genericity) to the designer of the container or manager class.
Prior to templates, you had to use inheritance; your design might include a generic List container class and an application-specific Employee class. To put employees in a list, a ListedEmployee class is multiply derived (contrived) from the Employee and List classes. These solutions were unwieldy and error-prone. Templates solved that problem.

Q: What is a mutable member?

A: One that can be modified by the class even when the object of the class or the member function doing the modification is const.
Understanding this requirement implies an understanding of C++ const, which many programmers do not have. I have seen large class designs that do not employ the const qualifier anywhere. Some of those designs are my own early C++ efforts. One author suggests that some programmers find const to be such a bother that it is easier to ignore const than to try to use it meaningfully. No wonder many programmers don't understand the power and implications of const. Someone who claims to have enough interest in the language and its evolution to keep pace with the ANSI deliberations should not be ignorant of const, however.

Q: What is an explicit constructor?

A: A conversion constructor declared with the explicit keyword. The compiler does not use an explicit constructor to implement an implied conversion of types. It's purpose is reserved explicitly for construction.

Q: What is the Standard Template Library?

A: A library of container templates approved by the ANSI committee for inclusion in the standard C++ specification.
A programmer who then launches into a discussion of the generic programming model, iterators, allocators, algorithms, and such, has a higher than average understanding of the new technology that STL brings to C++ programming.

Q: Describe run-time type identification.

A: The ability to determine at run time the type of an object by using the typeid operator or the dynamic_cast operator.

Q: What problem does the namespace feature solve?

A: Multiple providers of libraries might use common global identifiers causing a name collision when an application tries to link with two or more such libraries. The namespace feature surrounds a library's external declarations with a unique namespace that eliminates the potential for those collisions.
This solution assumes that two library vendors don't use the same namespace identifier, of course.

Q: Are there any new intrinsic (built-in) data types?

A: Yes. The ANSI committee added the bool intrinsic type and its true and false value keywords.
Other apparent new types (string, complex, and so on) are implemented as classes in the Standard C++ Library rather than as intrinsic types.