MFC Info

Qt 4.5 with Visual Studio 2008 (VC++ Express)

noreply@blogger.com (Anil) — Thu, 23 Jul 2009 03:05:00 -0700

Qt is a cross platform GUI toolkit from Nokia. Its SDK is available under an LPGL license, it means we can use it free of charge to develop proprietary, commercial, closed-source software (thanks to Nokia). However, the current LPGL release leaves some gaps in using Qt from within Visual Studio 2008. This is somewhat understandable given that Visual Studio integration was a commercial-only offering and Nokia has Qt Creator (and not Visual Studio). My problem with Qt Creator is it uses MingGW (gcc) compiler which is nowhere near as good as VC++ 2008. Also for Visual Studio background like me, who is not familiar with Qt creator, learning Qt Creator will take time. Thanks to Wiria and AK who brought Qt to my table for a review.

Qt GUI Toolkit is very useful to develop portable application (single source codes) for *NIX, MAC and Windows (including Windows 98 but not being tested for Windows 7). In this post, I will show you how to start using Qt SDK only with Visual C++ 2008, you can use FREE Visual C++ 2008 express too.

Build VC++ Version of Qt

- Download and install qt-sdk-win-opensource-2009.01.1.
- Install the SDK to default directory, in my case C:\qt\2009.01. I didn;t install MingGW and Qt Creator as I don’t need.

C/C++/VC++ Interview Questions: Part 5

noreply@blogger.com (Anil) — Tue, 14 Apr 2009 02:13:00 -0700

Q : What is a heap ?

Ans : Heap is a chunk of memory. When in a program memory is allocated dynamically, the C run-time library gets the memory from a collection of unused memory called the heap. The heap resides in a program's data segment. Therefore, the amount of heap space available to the program is fixed, and can vary from one program to another.

Q : How to obtain a path of the given file?

Ans: The function searchpath( ) searches for the specified file in the subdirectories of the current path. Following program shows how to make use of the searchpath( ) function.

#include "dir.h"

void main ( int argc, char *argv[] )

{

char *path ;

if ( path = searchpath ( argv[ 1 ] ) )

printf ( "Pathname : %s\n", path ) ;

else

printf ( "File not found\n" ) ;

}

Q : Can we get the process identification number of the current program?

Ans: Yes! The macro getpid( ) gives us the process identification number of the program currently running. The process id. uniquely identifies a program. Under DOS, the getpid( ) returns the Program Segment Prefix as the process id. Following program illustrates the use of this macro.

#include

void main( )

{

printf ( "The process identification number of this program is %X\n",

getpid( ) ) ;

}

Q : How do I write a function that takes variable number of arguments?

Ans: The following program demonstrates this.

#include

void main( )

{

int i = 10 ;

float f = 2.5 ;

char *str = "Hello!" ;

vfpf ( "%d %f %s\n", i, f, str ) ;

vfpf ( "%s %s", str, "Hi!" ) ;

}

void vfpf ( char *fmt, ... )

{

va_list argptr ;

va_start ( argptr, fmt ) ;

vfprintf ( stdout, fmt, argptr ) ;

va_end ( argptr ) ;

}

Here, the function vfpf( ) has called vfprintf( ) that take variable argument lists. va_list is an array that holds information required for the macros va_start and va_end. The macros va_start and va_end provide a portable way to access the variable argument lists. va_start would set up a pointer argptr to point to the first of the variable arguments being passed to the function. The macro va_end helps the called function to perform a normal return.

Q : Can we change the system date to some other date?

Ans: Yes, We can! The function stime( ) sets the system date to the specified date. It also sets the system time. The time and date is measured in seconds from the 00:00:00 GMT, January 1, 1970. The following program shows how to use this function.

#include

void main( )

{

time_t tm ;

int d ;

tm = time ( NULL ) ;

printf ( "The System Date : %s", ctime ( &tm ) ) ;

printf ( "\nHow many days ahead you want to set the date : " ) ;

scanf ( "%d", &d ) ;

tm += ( 24L * d ) * 60L * 60L ;

stime ( &tm ) ;

printf ( "\nNow the new date is : %s", ctime ( &tm ) ) ;

}

In this program we have used function ctime( ) in addition to function stime( ). The ctime( ) function converts time value to a 26-character long string that contains date and time.

Q : How to use function strdup( ) in a program?

Ans : The string function strdup( ) copies the given string to a new location. The function uses malloc( ) function to allocate space required for the duplicated string. It takes one argument a pointer to the string to be duplicated. The total number of characters present in the given string plus one bytes get allocated for the new string. As this function uses malloc( ) to allocate memory, it is the programmer’s responsibility to deallocate the memory using free( ).

#include

void main( )

{

char *str1, *str2 = "double";

str1 = strdup ( str2 ) ;

printf ( "%s\n", str1 ) ;

free ( str1 ) ;

}

Q : On including a file twice I get errors reporting redefinition of function.

How can I avoid duplicate inclusion?

Ans: Redefinition errors can be avoided by using the following macro definition. Include this definition in the header file.

#if !defined filename_h

#define filename_h

/* function definitions */

#endif

Replace filename_h with the actual header file name. For example, if name of file to be included is 'goto.h' then replace filename_h

Q : How to write a swap( ) function which swaps the values of the variables using bitwise operators.

Ans: Here is the swap( ) function.

swap ( int *x, int *y )

{

*x ^= *y ;

*y ^= *x ;

*x ^= *y ;

}

The swap( ) function uses the bitwise XOR operator and does not require any temporary variable for swapping.

C/C++/VC++ Interview Questions: Part 4

noreply@blogger.com (Anil) — Tue, 14 Apr 2009 02:11:00 -0700

Q. Name some pure object oriented languages.

Answer:

 Smalltalk,

 Java,

 Eiffel,

 Sather.

Q. Name the operators that cannot be overloaded.

Answer:

sizeof . .* .-> :: ?:

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q.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.

Q. 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/C++/VC++ Interview Questions: Part 3

noreply@blogger.com (Anil) — Tue, 14 Apr 2009 02:11:00 -0700

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. What is a Null object?

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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);

........

C/C++/VC++ Interview Questions: Part 2

noreply@blogger.com (Anil) — Tue, 14 Apr 2009 01:54:00 -0700

Q. 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’.

Q. 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

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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.

Q. 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).

C/C++/VC++ Interview Questions: Part 1

noreply@blogger.com (Anil) — Tue, 14 Apr 2009 01:27:00 -0700

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 is Constructor ?

A: Constructor creates an object and initializes it. It also creates V-Table for virtual functions. It is different from other methods in a class.

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 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.

Q: Can a constructor throws an exception? How to handle the error when the constructor fails?

A: The constructor never throws an error.

Q: What is default constructor?

A: Constructor with no arguments or all the arguments has default values.

Q: What is default (Implicit) copy constructor?

A: Constructor which initializes 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.

Examples:

(a) Boo Obj1(10); // calling Boo constructor

(b) Boo Obj2(Obj1); // calling boo copy constructor

Q: When is 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

(d) When compiler generates a temporary object

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

A: No. It is specified in the definition of the copy constructor itself. It should generate an error if a programmer specifies a copy constructor with a first argument that is an object and not a reference.

Q: What is conversion constructor?

A: constructor with a single argument makes that constructor as conversion ctor and it can be used for type conversion.

Example:

class Boo

{

public:

Boo( int i );

};

Boo BooObject = 10 ; // assigning int 10 Boo object

Q:What is conversion operator??

A:class can have a public method for specific data type conversions.

For example:

class Boo

{

double value;

public:

Boo(int i )

operator double()

{

return value;

}

};

Boo BooObject;

double i = BooObject; // assigning object to variable i of type double. now conversion operator gets called to assign the value.

Q: How can I handle a constructor that fails?

A: throw an exception. Constructors don't have a return type, so it's not possible to use return codes. The best way to signal constructor failure is therefore to throw an exception.

Q: How can I handle a destructor that fails?

A: Write a message to a log-_le. But do not throw an exception. The C++ rule is that you must never throw an exception from a destructor that is being called during the "stack unwinding" process of another exception. For example, if someone says throw Foo(), the stack will be unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get popped. This is called stack unwinding. During stack unwinding, all the local objects in all those stack frames are destructed. If one of those destructors throws an exception (say it throws a Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a } catch (Bare) { handler? There is no good answer:either choice loses information. So the C++ language guarantees that it will call terminate() at this point, and terminate() kills the process. Bang you're dead.

Q: What is Virtual Destructor?

A: Using virtual destructors, you can destroy objects without knowing their type - the correct destructor for the object is invoked using the virtual function mechanism. Note that destructors can also be declared as pure virtual functions for abstract classes. if someone will derive from your class, and if someone will say "new Derived", where "Derived" is derived from your class, and if someone will say delete p, where the actual object's type is "Derived" but the pointer p's type is your class.

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference of the object?

Q: What's the order that local objects are destructed?

A: In reverse order of construction: First constructed, last destructed. In the following example, b's destructor will be executed first, then a's destructor:

void userCode()

{

Fred a;

Fred b;

...

}

Q: What's the order that objects in an array are destructed?

A: In reverse order of construction: First constructed, last destructed. In the following example, the order for destructors will be a[9], a[8], ..., a[1], a[0]:

void userCode()

{

Fred a[10];

...

}

C++ Portability Rules - 1

noreply@blogger.com (Anil) — Wed, 8 Apr 2009 04:58:00 -0700

8. Use the common denominator between members of a C/C++ compiler family.

For many of the compiler families we use, the implementation of the C and C++ compilers are completely different, sometimes this means that there are things you can do in the C language, that you cannot do in the C++ language on the same machine. One example is the 'long long' type. On some systems (IBM's compiler used to be one, but I think it's better now), the C compiler supports long long, while the C++ compiler does not. This can make porting a pain, as often times these types are in header files shared between C and C++ files. The only thing you can do is to go with the common denominator that both compilers support. In the special case of long long, we developed a set of macros for supporting 64 bit integers when the long long type is not available. We have to use these macros if either the C or the C++ compiler does not support the special 64 bit type.

9. Don't put C++ comments in C code.

The quickest way to raise the blood pressure of a Netscape Unix engineer is to put C++ comments (// comments) into C files. Yes, this might work on your Microsoft Visual C compiler, but it's wrong, and is not supported by the vast majority of C compilers in the world. Just do not go there.

Many header files will be included by C files and included by C++ files. We think it's a good idea to apply this same rule to those headers. Don't put C++ comments in header files included in C files. You might argue that you could use C++ style comments inside #ifdef __cplusplus blocks, but we are not convinced that is always going to work (some compilers have weird interactions between comment stripping and pre-processing), and it hardly seems worth the effort. Just stick to C style /**/ comments for any header file that is ever likely to be included by a C file.

10. Don't put carriage returns in XP code.

While this is not specific to C++, we have seen this as more of an issue with C++ compilers, see Use the common denominator between members of a C/C++ compiler family. In particular, it causes bustage on at least the IRIX native compiler.

On unix systems, the standard end of line character is line feed ('\n'). The standard on many PC editors is carriage return and line feed ("\r\n"), while the standard on Mac is carriage return ("\r"). The PC compilers seem to be happy either way, but some Unix compilers just choke when they see a carriage return (they do not recognize the character as white space). So, we have a rule that you cannot check in carriage returns into any cross platform code. This rule is not enforced on the Windows front end code, as that code is only ever compiled on a PC. The Mac compilers seem to be happy either way, but the same rule applies as for the PC - no carriage returns in cross platform code.

MacCVS, WinCVS, and cygwin CVS when configured to use DOS line endings automatically convert to and from platform line endings, so you don't need to worry. However, if you use cygwin CVS configured for Unix line endings, or command line cvs on Mac OS X, you need to be somewhat careful.

11. Put a new line at end-of-file.

Not having a new-line char at the end of file breaks .h files with the Sun WorkShop compiler and it breaks .cpp files on HP.

12. Don't put extra top-level semi-colons in code.

Non-portable example:

int

A::foo()

{

};

This is another problem that seems to show up more on C++ than C code. This problem is really a bit of a drag. That extra little semi-colon at the end of the function is ignored by most compilers, but it is not permitted by the standard and it makes some compilers report errors (in particular, gcc 3.4, and also perhaps old versions of the AIX native compiler). Don't do it.

Portable example:

int

A::foo()

{

}

13. C++ filename extension is .cpp.

This one is another plain annoying problem. What's the name of a C++ file? file.cpp, file.cc, file.C, file.cxx, file.c++, file.C++? Most compilers could care less, but some are very particular. We have not been able to find one file extension which we can use on all the platforms we have ported Mozilla code to. For no great reason, we've settled on file.cpp, probably because the first C++ code in Mozilla code was checked in with that extension. Well, it's done. The extension we use is .cpp. This extension seems to make most compilers happy, but there are some which do not like it. On those systems we have to create a wrapper for the compiler (see STRICT_CPLUSPLUS_SUFFIX in ns/config/rules.mk and ns/build/*), which actually copies the file.cpp file to another file with the correct extension, compiles the new file, then deletes it. If in porting to a new system, you have to do something like this, make sure you use the #line directive so that the compiler generates debug information relative to the original .cpp file.

14. Don't mix varargs and inlines.

Non-portable example:

class FooBar {

void va_inline(char* p, ...) {

// something

}

};

The subject says it all, varargs and inline functions do not seem to mix very well. If you must use varargs (which can cause portability problems on their own), then ensure that the vararg member function is a non-inline function.

Portable example:

// foobar.h

class FooBar {

void

va_non_inline(char* p, ...);

};

// foobar.cpp

void

FooBar::va_non_inline(char* p, ...)

{

// something

}

15. Don't use initializer lists with objects.

Non-portable example:

FooClass myFoo = {10, 20};

Some compilers won't allow this syntax for objects (HP-UX won't), actually only some will allow it. So don't do it. Again, use a wrapper function, see Don't use static constructors.

Always have a default constructor.

Always have a default constructor, even if it doesn't make sense in terms of the object structure/hierarchy. HP-UX will barf on statically initialized objects that don't have default constructors.

C++ Portability Rules

noreply@blogger.com (Anil) — Wed, 8 Apr 2009 04:41:00 -0700

1. Be very careful when writing C++ templates.

Don't use C++ templates unless you do only things already known to be portable because they are already used in Mozilla (such as patterns used by nsCOMPtr or CallQueryInterface) or are willing to test your code carefully on all of the compilers we support and be willing to back it out if it breaks.

2. Don't use static constructors.

Non-portable example:

FooBarClass static_object(87, 92);

void

bar()

{

if (static_object.count > 15) {

...

}

Static constructors don't work reliably either. A static initialized object is an object which is instanciated at startup time (just before main() is called). Usually there are two components to these objects. First there is the data segment which is static data loaded into the global data segment of the program. The second part is a initializer function that is called by the loader before main() is called. We've found that many compilers do not reliably implement the initializer function. So you get the object data, but it is never initialized. One workaround for this limitation is to write a wrapper function that creates a single instance of an object, and replace all references to the static initialized object with a call to the wrapper function:

Portable example:

static FooBarClass* static_object;

FooBarClass*

getStaticObject()

{

if (!static_object)

static_object =

new FooBarClass(87, 92);

return static_object;

}

void

bar()

{

if (getStaticObject()->count > 15) {

...

}

3. Don't use exceptions.

Exceptions are another C++ feature which is not very widely implemented, and as such, their use is not portable C++ code. Don't use them. Unfortunately, there is no good workaround that produces similar functionality.

One exception to this rule (don't say it) is that it's probably ok, and may be necessary to use exceptions in some machine specific code. If you do use exceptions in machine specific code you must catch all exceptions there because you can't throw the exception across XP (cross platform) code.

4. Don't use Run-time Type Information.

Run-time type information (RTTI) is a relatively new C++ feature, and not supported in many compilers. Don't use it.

If you need runtime typing, you can achieve a similar result by adding a classOf() virtual member function to the base class of your hierarchy and overriding that member function in each subclass. If classOf() returns a unique value for each class in the hierarchy, you'll be able to do type comparisons at runtime.

5. Don't use C++ standard library features, including iostream

Using C++ standard library features involves significant portability problems because newer compilers require the use of namespaces and of headers without .h, whereas older compilers require the opposite. This includes iostream features, such as cin and cout.

Furthermore, using the C++ standard library imposes difficulties on those attempting to use Mozilla on small devices.

There is one exception to this rule: it is acceptable to use placement new. To use it, include the standard header by writing #include NEW_H.

6. Don't use namespace facility.

Support of namespaces (through the namespace and using keywords) is a relatively new C++ feature, and not supported in many compilers. Don't use it.

7. main() must be in a C++ file.

The first C++ compiler, Cfront, was in fact a very fancy preprocessor for a C compiler. Cfront reads the C++ code, and generates C code that would do the same thing. C++ and C startup sequences are different (for example static constructor functions must be called for C++), and Cfront implements this special startup by noticing the function called "main()", converting it to something else (like "__cpp__main()"), adding another main() that does the special C++ startup things and then calls the original function. Of course for all this to work, Cfront needs to see the main() function, hence main() must be in a C++ file. Most compilers lifted this restriction years ago, and deal with the C++ special initialization duties as a linker issue. But there are a few commercial compilers shipping that are still based on Cfront: HP, and SCO, are examples.

So the workaround is quite simple. Make sure that main() is in a C++ file. On the Unix version of Mozilla, we did this by adding a new C++ file which has only a few lines of code, and calls the main main() which is actually in a C file.

C optimisations

noreply@blogger.com (Anil) — Tue, 7 Apr 2009 04:22:00 -0700

Using array indices

If you wished to set a variable to a particular character, depending upon the value of something, you might do this :

switch ( queue ) {

case 0 : letter = 'W';

break;

case 1 : letter = 'S';

break;

case 2 : letter = 'U';

break;

}

or maybe

if ( queue == 0 )

letter = 'W';

else if ( queue == 1 )

letter = 'S';

else

letter = 'U';

A neater ( and quicker ) method is to simply use the value as an index into a character array, eg.

static char *classes="WSU";

letter = classes[queue];

--------------------------------------------------------------------------------

Aliases

Consider the following:

void func1( int *data )

{

int i;

for(i=0; i<10;>

{

somefunc2( *data, i);

}

Even though "*data" may never change, the compiler does not know that somefunc2() did not alter it, and so the program must read it from memory each time it is used - it may be an alias for some other variable that is altered elsewhere. If you know it won't be altered, you could code it like this instead:

void func1( int *data )

{

int i;

int localdata;

localdata = *data;

for(i=0; i<10;>

{

somefunc2( localdata, i);

}

This gives the compiler better opportunity for optimisation.

--------------------------------------------------------------------------------

Integers

Use unsigned ints instead of ints if you know the value will never be negative. Some processors can handle unsigned integer arithmetic considerably faster than signed ( this is also good practise, and helps make for self-documenting code).

So the best declaration for an int variable in a tight loop would be

(although it is not guaranteed that the compiler will take any notice of "register", and "unsigned" may make no difference to the processor.)

Remember, integer arithmetic is much faster than floating-point arithmetic, as it can be usually be done directly by the processor, rather than relying on external FPUs or floating point maths libraries. If you only need to be accurate to two decimal places (e.g. in a simple accounting package), scale everything up by 100, and convert it back to floating point as late as possible.

Loop jamming

Never use two loops where one will suffice:
for(i=0; i<100; i++)
{
stuff();
}

for(i=0; i<100; i++)
{
morestuff();
}
It would be better to do:
for(i=0; i<100; i++)
{
stuff();
morestuff();
}
Note, however, that if you do a lot of work in the loop, it might not fit into your processor's instruction cache. In this case, two separate loops may actually be faster as each one can run completely in the cache.

--------------------------------------------------------------------------------

Faster for() loops

Simple but effective.
Ordinarily, you would code a simple for() loop like this:
for( i=0; i<10; i++){ ... }
i loops through the values 0,1,2,3,4,5,6,7,8,9
If you don't care about the order of the loop counter, you can do this instead:

for( i=10; i--; ) { ... }
Using this code, i loops through the values 9,8,7,6,5,4,3,2,1,0, and the loop should be faster.
This works because it is quicker to process "i--" as the test condition, which says "is i non-zero? If so, decrement it and continue.".
For the original code, the processor has to calculate "subtract i from 10. Is the result non-zero? if so, increment i and continue.". In tight loops, this make a considerable difference.
The syntax is a little strange, put is perfectly legal. The third statement in the loop is optional (an infinite loop would be written as "for( ; ; )" ). The same effect could also be gained by coding:

for(i=10; i; i--){}
or (to expand it further)
for(i=10; i!=0; i--){}
The only things you have to be careful of are remembering that the loop stops at 0 (so if you wanted to loop from 50-80, this wouldn't work), and the loop counter goes backwards.It's easy to get caught out if your code relies on an ascending loop counter.

--------------------------------------------------------------------------------

switch() instead of if...else...

For large decisions involving if...else...else..., like this:
if( val == 1)
dostuff1();
else if (val == 2)
dostuff2();
else if (val == 3)
dostuff3();
it may be faster to use a switch:
switch( val )
{
case 1: dostuff1(); break;

case 2: dostuff2(); break;

case 3: dostuff3(); break;
}
In the if() statement, if the last case is required, all the previous ones will be tested first. The switch lets us cut out this extra work. If you have to use a big if..else.. statement, test the most likely cases first.

MFC Interview Questions & Answers - 3

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:49:00 -0700

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.

MFC Interview Questions & Answers - 2

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:48:00 -0700

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( ));

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 - 1

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:46:00 -0700

Q:- What is model and modeless dialog box ? Give some examples?

Ans:- When we create Modal Dialog Box we can't move to other windows until this dialog is closed. For eg: MessageBox, where we can't move to the other window until we press ok or cancel.

When we create Modeless Dilaog Box we can swap to the other windows. For eg: like a conventional window.

Q:- How to create open & save dialogs ?

Ans:- In CommonDialogs class we have to use CFileDialog class where the first parameter TRUE for open dialog and FALSE for Save dialog.

For file open:

CFileDialog SampleDlg(TRUE,NULL,NULL,OFN_OVERWRITEPROMPT,"Text Files (*.txt)|*.txt|Comma Separated Values(*.csv)|*.csv||");

int iRet = SampleDlg.DoModal();

Q:- What is CSingleDocTemplate?

Ans:- It’s a document template class used to create single document interface SDI applications. Only one document can be opened at a time. It identifies the document class used to manage the application's data, the frame window class that encloses views of that data, and the view class used to draw visual representations of the data. The document template also stores a resource ID that the framework uses to load menus, accelerators, and other resources that shape the application's user interface.

Q:- What is the difference between hinstance and hprevinstance in WinMain function?

Ans:- hInstance is used for things like loading resources and any other task which is performed on a per-module basis. A module is either the EXE or a DLL loaded into your program. hPrevInstance used to be the handle to the previously run instance of your program (if any) in Win16. It is always NULL for Win32 programs.

Q:- Explain about MDI and CMultiDocTemplate ?

Ans:- MDI applications are designed using the doc-view architectures in which there could be many views associated with a single document object and an application can open multiple docuements at the same time for eg:WORD.

In MDI terms, your main window is called the Frame, this is probably the only window you would have in a SDI (Single Document Interface) program. In MDI there is an additional window, called the MDI Client Window which is a child of your Frame window. CMultiDocTemplate is the document template class used to create MDI applications..The document template also stores a resource ID that the framework uses to load menus, accelerators, and other resources that shape the application's user interface.

Q:- Which are the different controls in MFC ?

Ans:- CAnimateCtrl, CButton, CEdit, CListBox, CComboBox, CRichEditCtrl, CStatic, TreeCtrl, ToolTipCtrl, IPAddressCtrl, TabCtrl,CDa teTimeCtrl, HeaderCtrl, ListCtrl, MonthCalCtrl, OleCtrl, ProgressCtrl, ScrollBar, SliderCtrl, StatusBarCtrl, TollBarCtrl etc.,

Q:- What is the use of OnInitDialog ?

Ans:- This message is sent to the dialog box during the Create, CreateIndirect, or DoModal calls, which occur immediately before the dialog box is displayed. This can be used to intialize the dialog controls or show/hide the controls etc.,

Q:- What is the functioning of UpdateData() funciton ?

Ans:- This is to initialize data in a dialog box, or to retrieve and validate dialog data.

The framework automatically calls UpdateData with bSaveAndValidate set to FALSE when a modal dialog box is created in the default implementation of CDialog::OnInitDialog. The call occurs before the dialog box is visible. The default implementation of CDialog::OnOK calls this member function with bSaveAndValidate set to TRUE to retrieve the data, and if successful, will close the dialog box. If the Cancel button is clicked in the dialog box, the dialog box is closed without the data being retrieved.

Q:- How to update all the views whenever document got updated ?

Ans:- call UpdateAllViews()- which updates all views associated with the document by calling OnUpdate() function of all the views.

Q:- How to handle RTTI in MFC ?

Ans:- Run-Time Type Information is a mechanism that allows the type of an object to be determined during the program execution. 3 main elements to RTTI in MFC are

1. Dynamic_cast operator Used for conversion of polymorphic types.

2. typeid - used for identifying the exact type of an object

3. type_info classused to hold the type information returned by typeid.

Q:- What is serialization ?which function is responsible for serializing data ?

Ans:- Searialization is the process of streaming the object data to or from a persistent storage medium. It's useful in Doc-View Architecture. CObject :: Serialize() function is used to do serialization.

Q:- what is the use of Mutex and critical section ?

Ans:- Mutex as the name suggest allows a mutullay exclusive access to a shared resource among the threads. Critical section is a piece of code that can be executed safely to be accessed by two or more threads. Criticalsection provides synchronization means for one process only, while mutexes allow data synchronization across processes. Means two or more threads can share the common resources among more than one application or process boundaries in mutex.

MFC Interview Questions & Answers - 0

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:44:00 -0700

Q:- What is socket?

Ans:- A "socket" is an endpoint of communication: an object through which your application communicates with other Windows Sockets applications across a network.The two MFC Windows Sockets programming models are supported by the following classes: CAsyncSocket and CSocket

Q:- What is the difference between Synchronous sockets and asynchronous sockets?

Ans:- Consider a server application that is listening on a specific port to get data from clients. In synchronous receiving, while the server is waiting to receive data from a client, if the stream is empty the main thread will block until the request for data is satisfied. Hence, the server cannot do anything else until it receives data from the client. If another client attempts to connect to the server at that time, the server cannot process that request because it is blocked on the first client. This behavior is not acceptable for a real-world application where we need to support multiple clients at the same time.

In asynchronous communication, while the server is listening or receiving data from a client, it can still process connection requests from other clients as well as receive data from those clients. When a server is receiving asynchronously, a separate thread (at the OS level) listens on the socket and will invoke a callback function when a socket event occurs. This callback function in turn will respond and process that socket event.

Q:- Have you ever used win32 APIs ?

Ans:- MFC is a wrapper around win32 API, It provides classes which uses the win32 API, Some of the API's we usually work with are : GetDlgItemInt,GetDlgItemText,GetWindowTextA,Messag eBoxA,CreateFile,CreateMutex,CreateEvent,WaitForSi ngleObject,CreateWindow,ShowWindow etc.,

Q:- What is the difference between ANSI Code and UNICODE ?

Ans:- ANSI code represents 8bytes data where UNICODE represents 16bytes data for supporting universal languages. One major draw back to ASCII was you could only have 256 different characters. However, languages such as Japanese and Arabic have thousands of characters. Thus ASCII would not work in these situations. The result was Unicode which allowed for up to 65,536 different character

Q:- What is the difference between regular dlls and extended dlls?

Ans:- Regular dlls wraps only the c/c++ functions. Where extention dlls include c++ interfaces where we can create the objects of it and use in our classes. Extended dlls support object oriented concepts.Regural dlls uses mfc internally and exported functions can be used by any mfc or non-mfc applications.Extention dlls implements reusable classes derived from mfc library,built using dll version of mfc.Only mfc executables(applications/dll-shared version of mfc) can use extention dlls.extention dlls used for passing mfc derived objects b/w applications and dlls. Regulardlls linked both statically and dynamically but extended dlls links dynamically.

Q:- What is a message map, and what is the advantage of a message map over virtual function ?

Ans:- MessageMap is a logical table that maps the windows messages to the member functions of the class. We use message maps over virtual function because of lots of overhead. If every windows message had a virtual function associated with it , there would be several hundred bytes per window class of vtable. Message maps means we only pay for the messages we use.

Q:- Given two processes, how can they share memory?

Ans:- Processes and thread are very similar but they differ in the way they share their resources. Processes are independent and have its own address space. If two independent processes want to communicate they do this by using the following techniques 1.Message Passing 2.Sockets 3. named pipes

Use memory-mapped files to share data memory between processes

Q:- How to restrict only one instance of a class object to be created?

Ans:- Create a Named Mutex.

HANDLE hMutex=CreateMutex(TRUE,_T(“NamedMutex”))

And check the Mutex existence for each of your instance launch and dont allow it to launch if it exists.

Q:- How do I dynamically change the mainframe menu?

Ans:- CMenu newMenu;

newMenu.LoadMenu (IDR_MENU1);

AfxGetMainWnd()->SetMenu( &newMenu );

AfxGetMainWnd()->DrawMenuBar();

newMenu.Detach ()

C++ Interview Questions & Answers

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:38:00 -0700

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 is constructor or ctor?

A: Constructor creates an object and initializes it. It also creates vtable for virtual functions. It is

different from other methods in a class.

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 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.

Q: Can a constructor throws an exception? How to handle the error when the constructor fails?

A:The constructor never throws an error.

Q: What is default constructor?

A: Constructor with no arguments or all the arguments has default values.

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

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

object of the same class

(d) When compiler generates a temporary object

Q: Can a copy constructor accept an object of the same class as parameter, instead of reference

of the object?

A: No. It is specified in the definition of the copy constructor itself. It should generate an error if

a programmer specifies a copy constructor with a first argument that is an object and not a

reference.

Q: What is conversion constructor?

A: constructor with a single argument makes that constructor as conversion ctor and it can be

used for type conversion.

for example:

class Boo

{

public:

Boo( int i );

};

Boo BooObject = 10 ; // assigning int 10 Boo object

Q:What is conversion operator??

A:class can have a public method for specific data type conversions.

for example:

class Boo

{

double value;

public:

Boo(int i )

operator double()

{

return value;

}

};

Boo BooObject;

double i = BooObject; // assigning object to variable i of type double.

now conversion operator gets called to assign the value.

Q: How can I handle a constructor that fails?

A: throw an exception. Constructors don't have a return type, so it's not possible to use return

codes. The best way to signal constructor failure is therefore to throw an exception.

Q: How can I handle a destructor that fails?

A: Write a message to a log-_le. But do not throw an exception. The C++ rule is that you must

never throw an exception from a destructor that is being called during the "stack unwinding"

process of another exception. For example, if someone says throw Foo(), the stack will be

unwound so all the stack frames between the throw Foo() and the } catch (Foo e) { will get

popped. This is called stack unwinding. During stack unwinding, all the local objects in all those stack frames are destructed. If one of those destructors throws an exception (say it throws a

Bar object), the C++ runtime system is in a no-win situation: should it ignore the Bar and end up

in the } catch (Foo e) { where it was originally headed? Should it ignore the Foo and look for a }

catch (Bare) { handler? There is no good answer:either choice loses information. So the C++ language guarantees that it will call terminate() at this point, and

terminate() kills the process. Bang you're dead.

Encapsulation

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:34:00 -0700

Encapsulation

Think of encapsulation as a black box; data is sent to a method, a lot of work goes on using the data, of which you don't know or care about. An output is returned to the caller. That is the process of encapsulation, or information hiding. Let's take a look at the following class:

public class MultiLineString
{
private CollectionBase<string> _lines = null;
private bool _allowEmptyLines = false;
private static object _lock = new object();
public bool AllowEmptyLines
{
get { return _allowEmptyLines; }
set { _allowEmptyLines = value; }
}
public int LineCount
{
get { return _lines.Count; }
}
public string this[int index]
{
get { return _lines[index]; }
}
public MultiLineString(string text) : this(text, false) { }
public MultiLineString(string text, bool allowEmptyLines)
{
_allowEmptyLines = allowEmptyLines;
_lines = new CollectionBase<string>();
if (!allowEmptyLines)
{
string[] lines = text.Split('\n');
foreach (string line in lines)
{
if (line != null)
this.Add(line);
else if (line == '\r' && this.AllowEmptyLines)
this.Add(string.Empty);
}
}
}
public MultiLineString(StringBuilder builder) : this(builder, false) { }
public MultiLineString(
StringBuilder builder,
bool allowEmptyLines) : this(builder.ToString(), allowEmptyLines) { }
public void AddLine(string line)
{
if (!this.AllowEmptyLines && string.IsNullOrEmpty(line))
throw new ArgumentNullException("line");
lock (_lock)
{
_lines.Add(this.TrimLine(line));
}
}
public void RemoveLine(int index)
{
lock (_lock)
{
_lines.RemoveAt(index);
}
}
public override string ToString()
{
string output = string.Empty;
foreach (string line in lines)
output += line + "\n";
return output;
}
private string TrimLine(string line)
{
return line.Trim(new char[] { ' ', '\t', '\r' });
}
}

From this class, we see a class that works with multi-line text, ensuring that the new line text is separated by line and retrieved by an index value. The constructor takes astring and parses the initial text. It also allows the adding and removing of lines. However, notice that the base object that stores the lines, the CollectionBase object, is never exposed. Only certain object properties are exposed, but the list isn't directly exposed. This is information hiding, in that we don't need to expose the source object for subscriber to use it, but the subscriber can only use the exposed methods/properties to interact with it.

Why is encapsulation important? Using the above example, it would be really easy to change the underlying source to use a database, XML file, text file, or another data store, without breaking any code that the developer may have created. If the class exposed the list directly, any code that used the list would have to change, replacing it with database code. That makes using this class much harder to maintain and more likely that someone would stay away from using the class. But with the implementation above, nothing is known about the source and so switching the underlying data source is much easier.

This concept is prevalent throughout the .NET framework and any object-oriented development project, as it is important to hide certain information to the consumer of an object, because it aids in maintenance, development, and general use of the object.

Polymorphism

noreply@blogger.com (Anil) — Fri, 3 Apr 2009 05:22:00 -0700

Polymorphism

Polymorphism is a simple concept that you understand right now, as it is prevalent through life. You are a person, which person is a base type and you are a more specific representation of that type. There are many people that inherit that type, of which you are a concrete implementation of that type: you have your own beliefs, attributes, and qualities that go beyond just a general person. For instance, we all have a blood type, shoe size, arm length, and many other properties. But we each add our own implementation to the person interface.

You drive an automobile, which this type has properties like wheel size, engine size, gas tank size, and other properties. The automobile you drive is a concrete implementation of the automobile interface. It has additional features, like sliding doors, logo type, cd changer slot count, moon roof lever location, or other various properties that are specific to the make/model of the car. In addition, automobile may have certain behaviors like open/close door, open trunk, turn wheel, and other behaviors that would be specific to an automobile.

In OO programming, using the automobile example, Automobile would be the base class, and each automobile manufacturer would have its own implementation. For instance, Honda has V-Tec technology, which is in its own implementation. Volvo uses diesel engines, which is the TDI technology. More importantly, you may add an added level of detail between automobile and the make/model implementation, such as Car, Truck, or Suv supertypes, to provide more relevant information.

Let's take a look at the automobile scenario. Here is an example automobile class:

public abstract class Automobile
{
//Properties
public int DoorCount { .. }
public string EngineType { .. }
public float Height { .. }
public float GasTankSize { .. }
public float WheelBase { .. }
public float WheelSize { .. }
//Abstract Properties
public abstract string BodyType { get; }
//Methods
public float CalculateFuelEfficiency(int lastMileage);
}

The Automobile class is the interface, in that it defines an interface that classes will derive from. You can't directly instantiate it through new Automobile() because it's not an implementation; however, you can cast derived types to the Automobile type. In this sense, it defines a common interface across many derived implementations.

It's important to note the BodyType property. This property is declared abstract, meaning that any derived implementation must override it and return some relevant string (we will see more of this with the next class definitions). This is similar to the Template Method pattern in that it exposes properties/methods that derived classes must override, but which Automobile doesn't need to know anything about. Automobile wants the derived class to return something relevant to it, but doesn't care what.

Now, from the example above, let's say we want a higher level of abstraction, in that there are varying types of cars and we want to target that. It is possible for classes to inherit from Automobile, but to also be an interface in that it can't be directly instantiated, but any class can inherit additional properties/methods from it. Below are two such class definitions, for Car and Suv.

public abstract class Car : Automobile
{
//Properties
public bool HasSpoiler { .. }
public bool IsHatchback { .. }
//Overridden Properties (abstract)
public override string BodyType { get { return "Car"; } }
//Abstract properties
public abstract bool IsFast { get; }
}
public abstract class Suv : Automobile
{
public bool CanOffroad { .. }
public int TowingCapacity { .. }
public override string BodyType { get { return "SUV"; } }
}

Notice the BodyType method is overridden to return a string that represents the type of body that's appropriate to the supertype. Also notice that Car and Suv also expose their own properties/methods, even abstractly in terms of the Car.IsFast property. Lastly, let's create an implementation of the Car class:

public class SubaruWrx : Car
{
public override bool IsFast
{
get { return true; }
}
}

The SubaruWrx class can use any of the properties defined in Automobile and Car. It has DoorCount, EngineType, etc. properties that are available, because it inherits from Car, which Car inherits from Automobile. You can cast SubaruWrx to Car or to Automobile, because it inherits from these base classes, and adds its own properties/methods. However, say you cast SubaruWrx to Automobile; none of the properties/methods defined in SubaruWrx or Car will be available, because all this object knows is that it's of type Automobile. To gain access to those properties/methods, you must upcast to the appropriate type.

There are several ways that polymorphism can take place. The first is through class inheritance, which you see above. Class inheritance exposes the interface of the base class to all of the derived classes. Another option is interface inheritance. The interface declares a contract that the class must follow to implement the interface. In addition to class inheritance, an object can inherit from one or more interfaces, which adds properties/methods to the body of the class that it must provide implementation for. This is another common option that can be used in .NET development. Some languages offer the ability to inherit from multiple base classes; however, .NET is not one of those languages.

Examples of Polymorphism

There are many examples of Polymorphism in the .NET framework. One of them is the Membership provider. When you call Membership.GetUser or any other method, it calls the default provider, which is defined as a MembershipProvider class. Any derivatives (SqlMembershipProvider or other custom providers) expose the MembershipProvider interface to create a concrete implementation. You can easily switch the underlying data store without having to change any code for the Membership object.

When you work with a typed dataset, the base objects for the tables/rows are inherited from DataTable and DataRow. These objects can be downcast to this base type. When downcast to this type, only the properties/methods defined in DataTable/DataRow can be used, eliminating the strong-typing that comes from a typed dataset.

The GridView and DetailsView web controls allow you to create custom fields, by creating a class that inherits from DataControlField. Because of this, you do not have to create your own custom GridView/DetailsView classes to expose new field types; you simply create a new field that inherits from DataControlField, and create a custom implementation by overriding the base class's ExtractValuesFromCell and InitializeCell methods.

Type Char Expected Input - Format of output

noreply@blogger.com (Anil) — Tue, 3 Mar 2009 20:27:00 -0800

d Integer signed decimal integer
i Integer signed decimal integer
o Integer unsigned octal integer
u Integer unsigned decimal integer
x Integer unsigned hexadecimal int (with a, b, c, d, e, f)
X Integer unsigned hexadecimal int (with A, B, C, D, E, F)
f Floating point signed value of the form [-]dddd.dddd.

e Floating point signed value of the form [-]d.dddd or e
[+/-]ddd
g Floating point signed value in either e or f form, based on given value and precision. Trailing zeros and the decimal point are printed if necessary.
E Floating point Same as e; with E for exponent.
G Floating point Same as g; with E for exponent if e format used

Characters
c Character Single character
s String pointer Prints characters until a null-terminator is pressed or precision is reached
% None Prints the % character

Pointers
n Pointer to int Stores (in the location pointed to by the input argument) a count of the chars written so far.
p Pointer Prints the input argument as a pointer; format depends on which memory model was used. It will be either XXXX:YYYY or YYYY (offset only).

Arrays & Pointers

noreply@blogger.com (Anil) — Sun, 1 Mar 2009 22:12:00 -0800

Arrays

An array is a collection of objects of the same data type, allocated contiguously in memory. Individual objects in an array, called elements, are accessed by their position in the array. The subscripting operator ([]) provides the mechanics for creating an index to array elements. This form of access is called indexing or subscripting. An array facilitates the coding of repetitive tasks by allowing the statements executed on each element to be put into a loop that iterates through each element in the array.

Pointers

A pointer type variable holds the address of a data object or a function. A pointer can refer to an object of any one data type; it cannot refer to a bit field or a reference.

Some common uses for pointers are:

To access dynamic data structures such as linked lists, trees, and queues.
To access elements of an array or members of a structure or C++ class.
To access an array of characters as a string.
To pass the address of a variable to a function. (In C++, you can also use a reference to do this.) By referencing a variable through its address, a function can change the contents of that variable.

Note that the placement of the type qualifiers volatile and const affects the semantics of a pointer declaration. If either of the qualifiers appears before the *, the declarator describes a pointer to a type-qualified object. If either of the qualifiers appears between the * and the identifier, the declarator describes a type-qualifed pointer.

The following table provides examples of pointer declarations.

Table 1. Pointer declarations
Declaration	Description
`long *pcoat;`	`pcoat` is a pointer to an object having type `long`
`extern short * const pvolt;`	`pvolt` is a constant pointer to an object having type `short`
`extern int volatile *pnut;`	`pnut` is a pointer to an `int` object having the `volatile` qualifier
`float * volatile psoup;`	`psoup` is a `volatile` pointer to an object having type `float`
`enum bird *pfowl;`	`pfowl` is a pointer to an enumeration object of type `bird`
`char (*pvish)(void);`	`pvish` is a pointer to a function that takes no parameters and returns a `char`

Difference:

- Arrays automatically allocate space which is fixed in size and location; pointers are dynamic.

- Arrays are refered directly to the elements. But in pointers we refers to address of the elements

but not elements . indirect calling of the elements.

- They're related but not totally comparable.

A pointer is an address in memory where a variable is located.

An array is a conceptual data representation consisting of a list of more than one item of a particular scalar type (int, float, char, structure, etc.) where each element is accessed by its index. The index can be thought of as a counter or enumerator telling you how many elements you have to skip over to get to the one you're interested in. Here's where addresses come in ... the location of a particular element in memory is offset from the so-called base address (i.e. the address of the starting element) by the value

(sizeof(one element) * index #)

The C compiler is smart enough to take the sizeof() into account when you do pointer arithmetic, so you can find the address of the i-th element as (address of base) + i, rather than having to do the multiplication explicitly.

The idea of element address as offset also accounts for C's use of zero-relative array definitions. Since the first element is offset by 0 positions from the first element its index in C is 0, not 1. The second (ordinal) element is offset from the first by 1 position so its index is 1, and so on.

Member access

noreply@blogger.com (Anil) — Thu, 26 Feb 2009 01:37:00 -0800

Member access determines if a class member is accessible in an expression or declaration. Suppose x is a member of class A. Class member x can be declared to have one of the following levels of accessibility:

public: x can be used anywhere without the access restrictions defined by private or protected.
private: x can be used only by the members and friends of class A.
protected: x can be used only by the members and friends of class A, and the members and friends of classes derived from class A.

Members of classes declared with the keyword class are private by default. Members of classes declared with the keyword struct or union are public by default.

To control the access of a class member, you use one of the access specifiers public, private, or protected as a label in a class member list. The following example demonstrates these access specifiers:

struct A {

  friend class C; private:   int a; public:   int b; protected:   int c;

};

struct B : A {

  void f() {     // a = 1;     b = 2;     c = 3;   }

};

struct C {

  void f(A x) {     x.a = 4;     x.b = 5;     x.c = 6;

};

int main() {

  A y; //  y.a = 7;   y.b = 8; //  y.c = 9;    B z; //  z.a = 10;   z.b = 11; //  z.c = 12;

The following table lists the access of data members A::a A::b, and A::c in various scopes of the above example.

Scope	`A::a`	`A::b`	`A::c`
function `B::f()`	No access. Member `A::a` is private.	Access. Member `A::b` is public.	Access. Class `B` inherits from `A`.
function `C::f()`	Access. Class `C` is a friend of `A`.	Access. Member `A::b` is public.	Access. Class `C` is a friend of `A`.
object `y` in main()	No access. Member `y.a` is private.	Access. Member `y.a` is public.	No access. Member `y.c` is protected.
object `z` in `main()`	No access. Member `z.a` is private.	Access. Member `z.a` is public.	No access. Member `z.c` is protected.

An access specifier specifies the accessibility of members that follow it until the next access specifier or until the end of the class definition. You can use any number of access specifiers in any order. If you later define a class member within its class definition, its access specification must be the same as its declaration. The following example demonstrates this:

class A {

    class B;   public:     class B { };

};

The compiler will not allow the definition of class B because this class has already been declared as private.

A class member has the same access control regardless whether it has been defined within its class or outside its class.

Access control applies to names. In particular, if you add access control to a typedef name, it affects only the typedef name. The following example demonstrates this:

class A {

    class B { };   public:     typedef B C;

};

int main() {

  A::C x; //  A::B y;

The compiler will allow the declaration A::C x because the typedef name A::C is public. The compiler would not allow the declaration A::B y because A::B is private.

Note that accessibility and visibility are independent. Visibility is based on the scoping rules of C++. A class member can be visible and inaccessible at the same time.

Abstract class

noreply@blogger.com (Anil) — Thu, 26 Feb 2009 01:00:00 -0800

An abstract class is a class that is designed to be specifically used as a base class. An abstract class contains at least one pure virtual function. You declare a pure virtual function by using a pure specifier (= 0) in the declaration of a virtual member function in the class declaration.

The following is an example of an abstract class:

class AB {

public:

virtual void f() = 0;

};

Virtual Function

noreply@blogger.com (Anil) — Wed, 25 Feb 2009 22:45:00 -0800

A function call with the correct function definition at compile time. This is called static binding. You can specify that the compiler match a function call with the correct function definition at run time; this is called dynamic binding. You declare a function with the keyword virtual if you want the compiler to use dynamic binding for that specific function.

A virtual function is a member function you may redefine for other derived classes, and can ensure that the compiler will call the redefined virtual function for an object of the corresponding derived class, even if you call that function with a pointer or reference to a base class of the object.

A class that declares or inherits a virtual function is called a polymorphic class.

The following example demonstrates this:

#include  using namespace std;

struct A {    virtual void f() { cout << "Class A" <<>

struct B: A {    void f(int) { cout << "Class B" <<>

struct C: B {    void f() { cout << "Class C" <<>

int main() {

   B b; C c;

   A* pa1 = &b;

   A* pa2 = &c;

   pa1->f();

   pa2->f();

The following is the output of the above example:

Class A

Class C

The function B::f is not virtual. It hides A::f. Thus the compiler will not allow the function call b.f(). The function C::f is virtual; it overrides A::f even though A::f is not visible in C.

If you declare a base class destructor as virtual, a derived class destructor will override that base class destructor, even though destructors are not inherited.

Inheritance

noreply@blogger.com (Anil) — Wed, 25 Feb 2009 21:59:00 -0800

Inheritance is a mechanism of reusing and extending existing classes without modifying them, thus producing hierarchical relationships between them.

Multiple inheritance allows you to create a derived class that inherits properties from more than one base class. Because a derived class inherits members from all its base classes, ambiguities can result. For example, if two base classes have a member with the same name, the derived class cannot implicitly differentiate between the two members. Note that, when you are using multiple inheritance, the access to names of base classes may be ambiguous.

A direct base class is a base class that appears directly as a base specifier in the declaration of its derived class.

An indirect base class is a base class that does not appear directly in the declaration of the derived class but is available to the derived class through one of its base classes. For a given class, all base classes that are not direct base classes are indirect base classes.

Constructor & Destructor

noreply@blogger.com (Anil) — Wed, 25 Feb 2009 20:45:00 -0800

Constructor :-

Constructor creates an object and initializes it. It also creates V-Table for virtual functions. It is different from other methods in a class.

Constructors are used to create, and can initialize, objects of their class type.

You cannot declare a constructor as virtual or static, nor can you declare a constructor as const, volatile, or const volatile.

You do not specify a return type for a constructor. A return statement in the body of a constructor cannot have a return value.

Virtual Constructor :-

It is not possible to have 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.

Destructor :-

Destructors are usually used to deallocate memory and do other cleanup for a class object and its class members when the object is destroyed. A destructor is called for a class object when that object passes out of scope or is explicitly deleted.

Virtual Destructor :-

A Virtual 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.

pseudo-destructor :-

A pseudo-destructor is a destructor of a nonclass type.

The following example calls the pseudo destructor for an integer type:

typedef int I;

int main() {

I x = 10;

x.I::~I();

x = 20;

}

The call to the pseudo destructor, x.I::~I(), has no effect at all. Object x has not been destroyed; the assignment x = 20 is still valid. Because pseudo destructors require the syntax for explicitly calling a destructor for a nonclass type to be valid, you can write code without having to know whether or not a destructor exists for a given type.