Top 25 Ad Sense Alternatives for 2015

Having trouble with getting approved for Google AdSense, or not making the revenue you were expecting? Then you will need alternative avenues of revenue to either supplement your AdSense earnings or replace them altogether. In this article, we’re going to look at the top 25 Google AdSense alternatives for 2014. They consist of great display advertising, affiliate marketing, and content monetization networks that will help you earn more revenue.

Just starting out, or have a medium-sized audience? These display networks beyond AdSense can help you monetize.

1. Advertising.com — Advertising.com combines the industry’s most advanced technology, precise targeting and a quality network to deliver results for advertisers & publishers. Publishers using this network include Boston.com, Laptopmag.com, and DIYnetwork.com. 
2. BuySellAds — BuySellAds allows publishers to list their ad inventory on their network to be discovered by advertiser in their niche. Publishers using this network include 9to5google.com, RunBlogger.com, and 1keydata.com. 
3. Chitika — Chitika sets itself apart with their pledge to only show ads to users when and where they want to see them. Publishers using this network include Used-Cars.info, Freebies4mom.com, and Binatani.com. Click here to read MonetizePros’ in-depth review of Chitika.
4. IndustryBrains — IndustryBrains allows publishers to leverage their premium online content to generate additional revenue from contextually targeted ads that are consistent with your brand. Publishers using this network include Telecomsemea.net.
5. Kontera — Kontera provides publishers with incremental advertising revenues and premium content syndication for their mobile or PC web pages. Publishers using this network include MajorGeeks.com, PracticalMachinist.com, and EventGuide.com.
6. Media.net — Media.net brings contextual ads program from Yahoo! and Bing to publishers so they can create and customize ad units that display relevant text ads consisting of sponsored links and ad topics. This network is currently invite only. 
7. PulsePoint — PulsePoint owns one of the largest independent and fully transparent ad exchanges in the industry, allowing quality publishers to reveal and monetize the true value of their inventory. Publishers using this network include Gameshok.com, Salary.com, and FlashDriven.com.
8. Conversant — Conversant (formerly ValueClick Media) helps publishers maximize profits while keeping their costs down. Publishers using this network include Phys.org, AccessAtlanta.com, and Huffingtonpost.co.uk. 
9. Adblade — Adblade’s full suite of monetization solutions will help you grow revenue by attracting top advertisers, as well as help monetize inventory that’s not currently generating revenue. Publishers using this network include LAtimes.com, Audible.com, and AutoAnything.com. 
10. Tribal Fusion — Publishers who meet Tribal Fusion’s rigorous standards can expect to discover the benefits of partnering with them and maximize the value of their highly targeted content. Publishers using their network include Ranker.com, Cheapoair.com, and CDW.com.
11. Vibrant Media — Increase incremental revenue with Vibrant’s native advertising placements. Publishers using their network include SparkPeople.com, NBCMiami.com, and Computing.net.
12. Disqus — Disqus, the popular commenting platform, is working on Promoted Discovery, which will allow publishers using their comment platform to monetize their content as well.
13. Outbrain — For publishers with high amounts of traffic, Outbrain allows you to generate revenue with links to high quality third-party content below the fold. Publishers using this network include CNN.com, DailyCurrant.com, and LAweekly.com.
14. Tabooola — Similar to Outbrain, Taboola targets publishers with high traffic to monetize with third-party links to high-quality content. Publishers using this network include Dumpaday.com, WickedLocal.com, and HeraldNews.com.
15. VigLink — VigLink automatically pays you when people buy the brands and products you write about by automatically converting regular links into paid ones. Publishers using this network include Makeuseof.com, Rumorfix.com, and HypeLifeMagazine.com.
16. LiveIntent — LiveIntent allows you to monetize your valuable newsletter inventory with offers and content in real-time. Publishers using this network include The Weather Channel, AOL, and The Wall Street Journal. 
17. NewsletterDirectory.co — Membership in NewsletterDirectory.co can help you monetize your email list by finding the right sponsors.
18. Adap.TV — Adap.tv allows you to open your inventory up to new sources of revenue by ensuring the highest possible CPMs for every impression.
19. YouTube Partner Program — YouTube video creators can monetize their videos by joining the YouTube Partner Program, which offers CPC and CPM monetization with pre-roll video ads as well as in-video overlays. 
20. Videology — The Videology Addressable Media Platform enhances the yield on a publisher’s ad inventory by connecting the world’s leading marketers with your audience. Publishers using this network include Fool.com, Activebeat.co, and Gamewise.co.
21. Amazon Associates — Amazon Associates allows you to earn up to 10% commission by simply referring people to products you recommend on Amazon.
22. ClickBank — As an affiliate on Clickbank, you will be able to promote downloadable products you’ve picked out from their extensive marketplace for your audience based on their interests.
23. CJ Affiliate by Conversant (Formerly Commission Junction) — Whether you are looking to make money online by monetizing your website or want to grow your existing affiliate business, CJ’s robust technology platform and proven expertise drive results.
24. Rakuten LinkShare — Regardless of what type of site you have in mind (or already have in place!) as you build a loyal following you have an opportunity to monetize your website and start earning commissions.
25. ShareASale — ShareASale hosts 3,900+ affiliate programs across 40 different countries in a wide range of categories of products beyond online or downloadable only.

You want to get Google adsense approval within 3 days? No problem, it’s 100% possible to get approval within 3 days. You might be wondering, why does Google Adsense disapprove my request? You might have tried many times to apply again, to no avail. There are many reasons for Google Adsense to disapprove your request. You should know the basic requirements before for applying to Google AdSense. Even if you have a good blog, traffic, rank, it’s not must your account should be approved. Find out what makes your site to stand out an get an AdSense approval soon. Below are few important points you need to keep in mind before applying Google Adsense.

Get Google Adsense approval within 3 days:

Before applying for Google Adsense, make sure to follow these points. Everything counts, including your blog/site layout, content, number of posts, SEO metrics, back-links, other required metrics and etc. Every single one of them will play a important role in getting Adsense approved. Check out the most important requirements one should follow before applying for Adsense.

Minimum Posts/Articles/Pages:

If you are applying for Adsense for a WordPress blog, you should maintain a minimum of 40 quality posts/articles. And if it’s any other website (html, Joomla), try to maintain a minimum of 30 pages. Every article is considered to be a page in your site. So consider this as your major requirement and proceed implementing further requirements.

Google Analytics Code:

You need to install a Google Analytics code on your website. To do this, just login to Google Analytics and click on Admin where you can see “+ New Account” under Accounts tab. Click on New Account and enter your website name, url and other details. Click on Get Tracking ID and paste it in your site.

Google Webmasters Verification Page:

Next you need to apply for a Google Verification Page. To do this, login to Google Webmaster Tools and click on “Add a Site” and Enter your site URL. Verify your ownership by any one of the following methods and get a conformation.

Bing Webmaster Verification Page:

Bing Webmaster Tools  is very similar to Google Webmaster Tools. You can follow the same procedure here and verify your ownership. In Bing webmaster tools, you need to apply even your website sitemap page. I will tell you how to add that below.

Sitemap.xml Page: 

Your site should have a Sitemap.xml page, so that search engines can index your site links and pages quickly. To create a sitemap, just install the Google XML Sitemaps (WordPress) and create a sitemap.

Robots.txt

You need to create a robots.txt file in your site. No matter if it’s a WordPress site or any other CMS based, you need to have a robots.txt file. This file will tell the search engines, what to index and what not to. You can check SEOSiren robots.txt file for clear idea. To create a sitemap, just open your hosting cpanel, click on File Manager, and select your site folder to open it. Once opened, you can create a file and name it as robots.txt. You can enter the basic commands to restrict search engines to over crawl you site.

Alexa Rank:

A lot of people disagree with this statement. But still your site should have a decent Alexa ranking to get adsense approved. Even if your site doesn’t have a decent Alexa rank, you can still apply. But make sure your site rankings are atleast under 400k. Else the chances are quite less to be taken into consideration.

Additional Pages:

Your site should have a Privacy Policy page, Disclaimer page, and Terms of use page. You need to clearly mention what is your site about, and how it is useful for people.

Own Email ID:

If you have your own domain and hosting, its better to apply Adsense via domain email ID. Which means, instead of example@gmail.com, you can apply with example@domain.com. Just like satish@seosiren.com or admin@seosiren.com.

Decent Traffic: 

Your site should have a minimum of 50+ visitors per day. Please don’t be in a hurry to apply adsense without  having consistent traffic. And even if you follow all the above mentioned requirements without consistent traffic, your account will not be approved.

These are the most effective ways to get Google Adsense approval within 3 days. I hope your Adsense account will be approved. Drop us a line if you have any queries related to Adsense approval.  Stay tuned to SEOSiren for more!

VTU Recruitment for 73 Professors, Associate Professors and Assistant Professor

Visvesvaraya Tecnological University (VTU) , Belgaum has invited applications for the recruitmentof a total of 73 Professors, Associate Professors and Assistant Professors for various fields of Master of Technology (M Tech), Master of Computer Application (MCA) and Master of Business Administration (MBA). Candidates are required for the posts in VTU's Constituent College, UBDTCE, Davanagere. Selected candidates will be on an initial probation period of two years. Last date for application is April 5, 2014.

Professor Vacancies
1. M. Tech. in Production Engineering and System Technology: 1 post
2. M. Tech. in Thermal Engineering and System Technology: 1 post
3. M. Tech. in Machine Design: 1 post
4. M. Tech. in Computer Science and Engineering: 1 post
5. M. Tech. in Environmental Engineering: 1 post
6. M. Tech. in Computer Aided Design of Structure: 1 post
7. M. Tech. in Power Systems and Power Electronics: 1 post
8. M. Tech. in Digital Communication and Network: 1 post
9. MCA: 2 posts
10. MBA: 1 post
11. Department of Industrial and Production Engineering (UG): 1 post

Total: 12 posts

Salary: Rs 37,400 - 67,000 + AGP of Rs 10,000

Eligibility: Age of the candidate should be above 18 years and below 35 years, candidate should also have completed B.E.,/B.Tech., and M.E., / M.Tech., in relevant subject with First Class or equivalent either in B.E.,/B.Tech., or M.E., / M.Tech and PhD.

Experience: Minimum of 10 years teaching and / or research and / or industrial experience of which at least 5 years should be at the level of Associate Professor.

Associate Professor Vacancies1. M. Tech. in Production Engineering and System Technology: 1 post
2. M. Tech. in Thermal Engineering and System Technology: 1 post
3. M. Tech. in Machine Design: 1 post
4. M. Tech. in Computer Science and Engineering: 1 post
5. M. Tech. in Environmental Engineering: 1 post
6. M. Tech. in Computer Aided Design of Structure: 1 post
7. M. Tech. in Power Systems and Power Electronics: 1 post
8. M. Tech. in Digital Communication and Network: 1 post
9. MCA: 6 posts
10. MBA: 4 posts
11. Department of Industrial and Production Engineering (UG): 3 posts

Total: 21 posts

Salary: Rs 37,400 - 67,000 + AGP of Rs 9,000

Eligibility: Age of the candidate should be above 18 years and below 35 years, candidate should also have completed B.E.,/B.Tech., and M.E., / M.Tech., in relevant subject with First Class or equivalent either in B.E.,/B.Tech., or M.E., / M.Tech and PhD.

Experience: Minimum of 5 years teaching and / or research and / or industrial experience

Assistant Professor Vacancies1. M. Tech. in Production Engineering and System Technology: 1 post
2. M. Tech. in Thermal Engineering and System Technology: 1 post
3. M. Tech. in Machine Design: 1 post
4. M. Tech. in Computer Science and Engineering: 1 post
5. M. Tech. in Environmental Engineering: 1 post
6. M. Tech. in Computer Aided Design of Structure: 1 post
7. M. Tech. in Power Systems and Power Electronics: 1 post
8. M. Tech. in Digital Communication and Network: 1 post
9. MCA: 14 posts
10. MBA: 11 posts
11. Department of Industrial and Production Engineering (UG): 7 posts

Total: 40 posts

Salary: Rs 15,600 - 39,100 + AGP of Rs 6,000 

Eligibility: Age of the candidate should be above 18 years and below 35 years, candidate should also have completed B.E.,/B.Tech., and M.E., / M.Tech., in relevant subject with First Class or equivalent either in B.E.,/B.Tech., or M.E., / M.Tech

Selection
Eligible candidates will be selected on the basis of an interview.

How to apply
Application form is to be filled online and a print out copy is to be sent to the given address with required documents to complete the application.

Application fees
General: Rs 1, 000
SC/ST: Rs 500

The Visvesvaraya Technological University (VTU), Belgaum has revised the scheme and syllabus of Postgraduate Master of Computer Application (MCA) course from the year 2013-14. As per the University policy, the revision of scheme and syllabus of the MCA course has to be taken up for every three years. Accordingly the revision of scheme & syllabus of MCA course falls due from 2013-14. In the last academic year 2010-11 VTU has revised the MCA course scheme and syllabus. On this regard, the chairman of Board of Syllabus in MCA has prepared the 'Draft Scheme and syllabus of the MCA' course to be revised with respect to the 2013-14 and submitted to the university. The new scheme and syllabus details has been uploaded in the official website of VTU. The notice has been sent to all Engineering colleges affiliated to the University. Click here to the changed MCA course scheme and syllabus. It has been bring to notice that, If any suggestions and comments needed for changed scheme and syllabus. The students and concerned teaching faculty may sent same to the Chairman,

BOS in MCA either in soft/ hard copy to the below address not later than March 30, 2013.

Dr. C.V. Srikrishna Chairman,

 BOS in MCA of VTU,

Belgaum & Professor,

 P.E.S Institute of technology,

100fe ring Road, 2nd Main,

Banashankri, Hosakerahalli, Bangalore - 560085.

 Tel: 080 - 26721983 E-Mail: cvsrikrishna@yahoo.co.in .

VTU released MCA 2nd, 3rd, 4th and 5th Semester Results released for all the Regions of Examination December 2014/ January 2015 at www.results.vtu.ac.in : Apply for Revaluation

Belgaum: University had conducted MCA 2nd, 3rd, 4th and 5th Semester Exam for all the Regions during December 2014/ January 2015. Several candidates are waiting for the results. As all the candidates are in rush to check the results server may be slow but keep patience to open the home page of VTU university website and check the results. All the best for the candidates who had written the examinations.

Click here to check the results

Procedure to check the results by Web:

Log on to vtu.ac.in
After appearing the home page candidates can see the option Results by Web under Examinations Click on that option
After appearing the results option Candidates can see the option MCA 2nd, 3rd, 4th and 5th Semester Results released for all the Regions of Examination December 2014/ January 2015. Under in the appropriate column Enter the University seat number
Click on the submit button to get the result
After appearing the result at the screen take the print out of the same
Procedure to check the results by SMS:

Log on to vtu.ac.in
After appearing the home page candidates can see the option Results by SMS under Examinations Click on that option
To get the results in the form of SMS candidates need to type the message as RESULT<space>USN<space>yourEmailID and send the message to 5424204
After checking the results Candidates apply for the revaluation. For more details candidates need to check at the official website of VTU. Visvesvaraya Technological University is a public state University in Karnataka. With the intention to improve the quality of technical education in the state this university was established. In India, it is one of the largest universities. Now it is the time to check the results of MCA examinations.

C++ Interview Questions

for the Data Structures in C++ course

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

(c) 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

(c) when you construct an object based on another

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.

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?

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'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];

...

}

Q: Can I overload the destructor for my class?

A: No.

You can have only one destructor for a class Fred. It's always called Fred::~Fred(). It never takes

any parameters, and it never returns anything.

You can't pass parameters to the destructor anyway, since you never explicitly call a destructor

(well, almost never).

Q: Should I explicitly call a destructor on a local variable?

A: No!

The destructor will get called again at the close } of the block in which the local was created.

This is a guarantee of the language; it happens automagically; there's no way to stop it from

happening. But you can get really bad results from calling a destructor on the same object a

second time! Bang! You're dead!

Q: What if I want a local to "die" before the close } of the scope in which it was created? Can I

call a destructor on a local if I really want to?

A: No! [For context, please read the previous FAQ].

Suppose the (desirable) side effect of destructing a local File object is to close the File. Now

suppose you have an object f of a class File and you want File f to be closed before the end of the

scope (i.e., the }) of the scope of object f:

void someCode()

{

File f;

...insert code that should execute when f is still open...

We want the side-effect of f's destructor here!

...insert code that should execute after f is closed...

}

There is a simple solution to this problem. But in the mean time, remember: Do not explicitly

call the destructor!

Q: OK, OK already; I won't explicitly call the destructor of a local; but how do I handle the

above situation?

A: Simply wrap the extent of the lifetime of the local in an artificial block {...}:

void someCode()

{

{

File f;

...insert code that should execute when f is still open...

} f's destructor will automagically be called here!

...insert code here that should execute after f is closed...}

Q: What if I can't wrap the local in an artificial block?

A: Most of the time, you can limit the lifetime of a local by wrapping the local in an artificial

block ({...}). But if for some reason you can't do that, add a member function that has a similar

effect as the destructor. But do not call the destructor itself!

For example, in the case of class File, you might add a close() method. Typically the destructor

will simply call this close() method. Note that the close() method will need to mark the File

object so a subsequent call won't re-close an already-closed File. E.g., it might set the

fileHandle_ data member to some nonsensical value such as -1, and it might check at the

beginning to see if the fileHandle_ is already equal to -1:

class File {

public:

void close();

~File();

...

private:

int fileHandle_; // fileHandle_ >= 0 if/only-if it's open

};

File::~File()

{

close();

}

void File::close()

{

if (fileHandle_ >= 0) {

...insert code to call the OS to close the file...

fileHandle_ = -1;

}

}

Note that the other File methods may also need to check if the fileHandle_ is -1 (i.e., check if the

File is closed).

Note also that any constructors that don't actually open a file should set fileHandle_ to -1.

Q: But can I explicitly call a destructor if I've allocated my object with new?

A: Probably not.

Unless you used placement new, you should simply delete the object rather than explicitly

calling the destructor. For example, suppose you allocated the object via a typical new

expression:

Fred* p = new Fred();

Then the destructor Fred::~Fred() will automagically get called when you delete it via:

delete p; // Automagically calls p->~Fred()

You should not explicitly call the destructor, since doing so won't release the memory that was

allocated for the Fred object itself. Remember: delete p does two things: it calls the destructor

and it deallocates the memory.

Q: What is "placement new" and why would I use it?

A: There are many uses of placement new. The simplest use is to place an object at a particular

location in memory. This is done by supplying the place as a pointer parameter to the new part of

a new expression:

#include // Must #include this to use "placement new"

#include "Fred.h" // Declaration of class Fred

void someCode()

{

char memory[sizeof(Fred)]; // Line #1

void* place = memory; // Line #2

Fred* f = new(place) Fred(); // Line #3 (see "DANGER" below)

// The pointers f and place will be equal

...

}

Line #1 creates an array of sizeof(Fred) bytes of memory, which is big enough to hold a Fred

object. Line #2 creates a pointer place that points to the first byte of this memory (experienced C

programmers will note that this step was unnecessary; it's there only to make the code more

obvious). Line #3 essentially just calls the constructor Fred::Fred(). The this pointer in the Fred

constructor will be equal to place. The returned pointer f will therefore be equal to place.

ADVICE: Don't use this "placement new" syntax unless you have to. Use it only when you really

care that an object is placed at a particular location in memory. For example, when your

hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that

memory location.

DANGER: You are taking sole responsibility that the pointer you pass to the "placement new"

operator points to a region of memory that is big enough and is properly aligned for the object

type that you're creating. Neither the compiler nor the run-time system make any attempt to

check whether you did this right. If your Fred class needs to be aligned on a 4 byte boundary but

you supplied a location that isn't properly aligned, you can have a serious disaster on your hands

(if you don't know what "alignment" means, please don't use the placement new syntax). You

have been warned.

You are also solely responsible for destructing the placed object. This is done by explicitly

calling the destructor:

void someCode()

{

char memory[sizeof(Fred)];

void* p = memory;

Fred* f = new(p) Fred();

...

f->~Fred(); // Explicitly call the destructor for the placed object

}

This is about the only time you ever explicitly call a destructor.

Note: there is a much cleaner but more sophisticated way of handling the destruction / deletion

situation.

Q: When I write a destructor, do I need to explicitly call the destructors for my member objects?

A: No. You never need to explicitly call a destructor (except with placement new).

A class's destructor (whether or not you explicitly define one) automagically invokes the

destructors for member objects. They are destroyed in the reverse order they appear within the

declaration for the class.

class Member {

public:

~Member();

...

};

class Fred {

public:

~Fred();

...

private:

Member x_;

Member y_;

Member z_;

};

Fred::~Fred()

{

// Compiler automagically calls z_.~Member()

// Compiler automagically calls y_.~Member()

// Compiler automagically calls x_.~Member()

}

Q: When I write a derived class's destructor, do I need to explicitly call the destructor for my

base class?

A: No. You never need to explicitly call a destructor (except with placement new).

A derived class's destructor (whether or not you explicitly define one) automagically invokes the

destructors for base class subobjects. Base classes are destructed after member objects. In the

event of multiple inheritance, direct base classes are destructed in the reverse order of their

appearance in the inheritance list.

class Member {

public:

~Member();

...

};

class Base {

public:

virtual ~Base(); // A virtual destructor

...

};

class Derived : public Base {

public:

~Derived();

...

private:

Member x_;

};

Derived::~Derived()

{

// Compiler automagically calls x_.~Member()

// Compiler automagically calls Base::~Base()

}

Note: Order dependencies with virtual inheritance are trickier. If you are relying on order

dependencies in a virtual inheritance hierarchy, you'll need a lot more information than is in this

FAQ.

Q: Is there any difference between List x; and List x();?

A: A big difference!

Suppose that List is the name of some class. Then function f() declares a local List object called

x:

void f()

{

List x; // Local object named x (of class List)

...

}

But function g() declares a function called x() that returns a List:

void g()

{

List x(); // Function named x (that returns a List)

...

}

Q: Can one constructor of a class call another constructor of the same class to initialize the this

object?

A: Nope.

Let's work an example. Suppose you want your constructor Foo::Foo(char) to call another

constructor of the same class, say Foo::Foo(char,int), in order that Foo::Foo(char,int) would help

initialize the this object. Unfortunately there's no way to do this in C++.

Some people do it anyway. Unfortunately it doesn't do what they want. For example, the line

Foo(x, 0); does not call Foo::Foo(char,int) on the this object. Instead it calls Foo::Foo(char,int) to

initialize a temporary, local object (not this), then it immediately destructs that temporary when

control flows over the ;.

class Foo {

public:

Foo(char x);

Foo(char x, int y);

...

};

Foo::Foo(char x)

{

...

Foo(x, 0); // this line does NOT help initialize the this object!!

...

}

You can sometimes combine two constructors via a default parameter:

class Foo {

public:

Foo(char x, int y=0); // this line combines the two constructors

...

};

If that doesn't work, e.g., if there isn't an appropriate default parameter that combines the two

constructors, sometimes you can share their common code in a private init() member function:

class Foo {

public:

Foo(char x);

Foo(char x, int y);

...

private:

void init(char x, int y);

};

Foo::Foo(char x)

{

init(x, int(x) + 7);

...

}

Foo::Foo(char x, int y)

{

init(x, y);

...

}

void Foo::init(char x, int y)

{

...

}

BTW do NOT try to achieve this via placement new. Some people think they can say new(this)

Foo(x, int(x)+7) within the body of Foo::Foo(char). However that is bad, bad, bad. Please don't

write me and tell me that it seems to work on your particular version of your particular compiler;

it's bad. Constructors do a bunch of little magical things behind the scenes, but that bad

technique steps on those partially constructed bits. Just say no.

Q: Is the default constructor for Fred always Fred::Fred()?

A: No. A "default constructor" is a constructor that can be called with no arguments. One

example of this is a constructor that takes no parameters:

class Fred {

public:

Fred(); // Default constructor: can be called with no args

...

};

Another example of a "default constructor" is one that can take arguments, provided they are

given default values:

class Fred {

public:

Fred(int i=3, int j=5); // Default constructor: can be called with no args

...

};

Q: Which constructor gets called when I create an array of Fred objects?

A: Fred's default constructor (except as discussed below).

class Fred {

public:

Fred();

...

};

int main()

{

Fred a[10]; calls the default constructor 10 times

Fred* p = new Fred[10]; calls the default constructor 10 times

...

}

If your class doesn't have a default constructor, you'll get a compile-time error when you attempt

to create an array using the above simple syntax:

class Fred {

public:

Fred(int i, int j); assume there is no default constructor

...

};

int main()

{

Fred a[10]; ERROR: Fred doesn't have a default constructor

Fred* p = new Fred[10]; ERROR: Fred doesn't have a default constructor

...

}

However, even if your class already has a default constructor, you should try to use std::vector

rather than an array (arrays are evil). std::vector lets you decide to use any constructor, not just

the default constructor:

#include

int main()

{

std::vector a(10, Fred(5,7)); the 10 Fred objects in std::vector a will be initialized with Fred(5,7)

...

}

Even though you ought to use a std::vector rather than an array, there are times when an array

might be the right thing to do, and for those, you might need the "explicit initialization of arrays"

syntax. Here's how:

class Fred {

public:

Fred(int i, int j); assume there is no default constructor

...

};

int main()

{

Fred a[10] = {

Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7), // The 10 Fred objects are

Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7), Fred(5,7) // initialized using Fred(5,7)

};

...

}

Of course you don't have to do Fred(5,7) for every entry you can put in any numbers you want,

even parameters or other variables.

Finally, you can use placement-new to manually initialize the elements of the array. Warning: it's

ugly: the raw array can't be of type Fred, so you'll need a bunch of pointer-casts to do things like

compute array index operations. Warning: it's compiler- and hardware-dependent: you'll need to

make sure the storage is aligned with an alignment that is at least as strict as is required for

objects of class Fred. Warning: it's tedious to make it exception-safe: you'll need to manually

destruct the elements, including in the case when an exception is thrown part-way through the

loop that calls the constructors. But if you really want to do it anyway, read up on placementnew.

(BTW placement-new is the magic that is used inside of std::vector. The complexity of

getting everything right is yet another reason to use std::vector.)

By the way, did I ever mention that arrays are evil? Or did I mention that you ought to use a

std::vector unless there is a compelling reason to use an array?

Q: Should my constructors use "initialization lists" or "assignment"?

A: Initialization lists. In fact, constructors should initialize as a rule all member objects in the

initialization list. One exception is discussed further down.

Consider the following constructor that initializes member object x_ using an initialization list:

Fred::Fred() : x_(whatever) { }. The most common benefit of doing this is improved

performance. For example, if the expression whatever is the same type as member variable x_,

the result of the whatever expression is constructed directly inside x_ the compiler does not

make a separate copy of the object. Even if the types are not the same, the compiler is usually

able to do a better job with initialization lists than with assignments.

The other (inefficient) way to build constructors is via assignment, such as: Fred::Fred() { x_ =

whatever; }. In this case the expression whatever causes a separate, temporary object to be

created, and this temporary object is passed into the x_ object's assignment operator. Then that

temporary object is destructed at the ;. That's inefficient.

As if that wasn't bad enough, there's another source of inefficiency when using assignment in a

constructor: the member object will get fully constructed by its default constructor, and this

might, for example, allocate some default amount of memory or open some default file. All this

work could be for naught if the whatever expression and/or assignment operator causes the

object to close that file and/or release that memory (e.g., if the default constructor didn't allocate

a large enough pool of memory or if it opened the wrong file).

Conclusion: All other things being equal, your code will run faster if you use initialization lists

rather than assignment.

Note: There is no performance difference if the type of x_ is some built-in/intrinsic type, such as

int or char* or float. But even in these cases, my personal preference is to set those data members

in the initialization list rather than via assignment for consistency. Another symmetry argument

in favor of using initialization lists even for built-in/intrinsic types: non-static const and nonstatic

reference data members can't be assigned a value in the constructor, so for symmetry it

makes sense to initialize everything in the initialization list.

Now for the exceptions. Every rule has exceptions (hmmm; does "every rule has exceptions"

have exceptions? reminds me of Gdel's Incompleteness Theorems), and there are a couple of

exceptions to the "use initialization lists" rule. Bottom line is to use common sense: if it's

cheaper, better, faster, etc. to not use them, then by all means, don't use them. This might happen

when your class has two constructors that need to initialize the this object's data members in

different orders. Or it might happen when two data members are self-referential. Or when a datamember

needs a reference to the this object, and you want to avoid a compiler warning about

using the this keyword prior to the { that begins the constructor's body (when your particular

compiler happens to issue that particular warning). Or when you need to do an if/throw test on a

variable (parameter, global, etc.) prior to using that variable to initialize one of your this

members. This list is not exhaustive; please don't write me asking me to add another "Or

when...". The point is simply this: use common sense.

Q: Should you use the this pointer in the constructor?

A: Some people feel you should not use the this pointer in a constructor because the object is not

fully formed yet. However you can use this in the constructor (in the {body} and even in the

initialization list) if you are careful.

Here is something that always works: the {body} of a constructor (or a function called from the

constructor) can reliably access the data members declared in a base class and/or the data

members declared in the constructor's own class. This is because all those data members are

guaranteed to have been fully constructed by the time the constructor's {body} starts executing.

Here is something that never works: the {body} of a constructor (or a function called from the

constructor) cannot get down to a derived class by calling a virtual member function that is

overridden in the derived class. If your goal was to get to the overridden function in the derived

class, you won't get what you want. Note that you won't get to the override in the derived class

independent of how you call the virtual member function: explicitly using the this pointer (e.g.,

this->method()), implicitly using the this pointer (e.g., method()), or even calling some other

function that calls the virtual member function on your this object. The bottom line is this: even

if the caller is constructing an object of a derived class, during the constructor of the base class,

your object is not yet of that derived class. You have been warned.

Here is something that sometimes works: if you pass any of the data members in this object to

another data member's initializer, you must make sure that the other data member has already

been initialized. The good news is that you can determine whether the other data member has (or

has not) been initialized using some straightforward language rules that are independent of the

particular compiler you're using. The bad news it that you have to know those language rules

(e.g., base class sub-objects are initialized first (look up the order if you have multiple and/or

virtual inheritance!), then data members defined in the class are initialized in the order in which

they appear in the class declaration). If you don't know these rules, then don't pass any data

member from the this object (regardless of whether or not you explicitly use the this keyword) to

any other data member's initializer! And if you do know the rules, please be careful.

Q: What is the "Named Constructor Idiom"?

A: A technique that provides more intuitive and/or safer construction operations for users of your

class.

The problem is that constructors always have the same name as the class. Therefore the only way

to differentiate between the various constructors of a class is by the parameter list. But if there

are lots of constructors, the differences between them become somewhat subtle and error prone.

With the Named Constructor Idiom, you declare all the class's constructors in the private or

protected sections, and you provide public static methods that return an object. These static

methods are the so-called "Named Constructors." In general there is one such static method for

each different way to construct an object.

For example, suppose we are building a Point class that represents a position on the X-Y plane.

Turns out there are two common ways to specify a 2-space coordinate: rectangular coordinates

(X+Y), polar coordinates (Radius+Angle). (Don't worry if you can't remember these; the point

isn't the particulars of coordinate systems; the point is that there are several ways to create a

Point object.) Unfortunately the parameters for these two coordinate systems are the same: two

floats. This would create an ambiguity error in the overloaded constructors:

class Point {

public:

Point(float x, float y); // Rectangular coordinates

Point(float r, float a); // Polar coordinates (radius and angle)

// ERROR: Overload is Ambiguous: Point::Point(float,float)

};

int main()

{

Point p = Point(5.7, 1.2); // Ambiguous: Which coordinate system?

...

}

One way to solve this ambiguity is to use the Named Constructor Idiom:

#include // To get sin() and cos()

class Point {

public:

static Point rectangular(float x, float y); // Rectangular coord's

static Point polar(float radius, float angle); // Polar coordinates

// These static methods are the so-called "named constructors"

...

private:

Point(float x, float y); // Rectangular coordinates

float x_, y_;

};

inline Point::Point(float x, float y)

: x_(x), y_(y) { }

inline Point Point::rectangular(float x, float y)

{ return Point(x, y); }

inline Point Point::polar(float radius, float angle)

{ return Point(radius*cos(angle), radius*sin(angle)); }

Now the users of Point have a clear and unambiguous syntax for creating Points in either

coordinate system:

int main()

{

Point p1 = Point::rectangular(5.7, 1.2); // Obviously rectangular

Point p2 = Point::polar(5.7, 1.2); // Obviously polar

...

}

Make sure your constructors are in the protected section if you expect Point to have derived

classes.

The Named Constructor Idiom can also be used to make sure your objects are always created via

new.

Note that the Named Constructor Idiom, at least as implemented above, is just as fast as directly

calling a constructor modern compilers will not make any extra copies of your object.

 _ _ _       _ _

Q: What is virtual function?

A: When derived class overrides the base class method by redefining the same function, then if

client wants to access redefined the method from derived class through a pointer from base class

object, then you must define this function in base class as virtual function.

class parent

{

void Show()

{

cout << "i'm parent" << endl;

}

};

class child: public parent

{

void Show()

{

cout << "i'm child" << endl;

}

};

parent * parent_object_ptr = new child;

parent_object_ptr->show() // calls parent->show()

now we goto virtual world...

class parent

{

virtual void Show()

{

cout << "i'm parent" << endl;

}

};

class child: public parent

{

void Show()

{

cout << "i'm child" << endl;

}

};

parent * parent_object_ptr = new child;

parent_object_ptr->show() // calls child->show()

Q: What is a "pure virtual" member function?

A: The abstract class whose pure virtual method has to be implemented by all the classes which

derive on these. Otherwise it would result in a compilation error. This construct should be used

when one wants to ensure that all the derived classes implement the method defined as pure

virtual in base class.

Q: How virtual functions are implemented C++?

A: Virtual functions are implemented using a table of function pointers, called the vtable. There

is one entry in the table per virtual function in the class. This table is created by the constructor

of the class. When a derived class is constructed, its base class is constructed _rst which creates

the vtable. If the derived class overrides any of the base classes virtual functions, those entries in

the vtable are overwritten by the derived class constructor. This is why you should never call

virtual functions from a constructor: because the vtable entries for the object may not have

been set up by the derived class constructor yet, so you might end up calling base class

implementations of those virtual functions

Q: What is pure virtual function? or what is abstract class?

A: When you de_ne only function prototype in a base class without implementation and do the

complete implementation in derived class. This base class is called abstract class and client won't

able to instantiate an object using this base class. You can make a pure virtual function or

abstract class this way..

class Boo

{

void foo() = 0;

}

Boo MyBoo; // compilation error

Q: What is Pure Virtual Function? Why and when it is used?

A: The abstract class whose pure virtual method has to be implemented by all the classes which

derive on these. Otherwise it would result in a compilation error. This construct should be used

when one wants to ensure that all the derived classes implement the method defined as pure

virtual in base class.

Q: How Virtual functions call up is maintained?

A: Through Look up tables added by the compile to every class image. This also leads to

performance penalty.

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 caller

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: What is inheritance?

A: Inheritance allows one class to reuse the state and behavior of another class. The derived class

inherits the properties and method implementations of the base class and extends it by overriding

methods and adding additional properties and methods.

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." 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: 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. This relationship is best implemented by embedding an object

of the Salary class in the Employee class.

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 generality) to the

designer of the container or manager class.

Q: What is multiple inheritance(virtual inheritance)? What are its advantages and disadvantages?

A: Multiple Inheritance is the process whereby a child can be derived from more than one parent

class. The advantage of multiple inheritance is that it allows a class to inherit the functionality of

more than one base class thus allowing for modeling of complex relationships.

The disadvantage of multiple inheritance is that it can lead to a lot of confusion(ambiguity)

when two base classes implement a method with the same name.

Q: What a derived class inherits or doesn't inherit?

A: Inherits:

Every data member defined in the parent class (although such members may not always be

accessible in the derived class!)

Every ordinary member function of the parent class (although such members may not always be

accessible in the derived class!)

The same initial data layout as the base class.

Doesn't Inherit :

The base class's constructors and destructor.

The base class's assignment operator.

The base class's friends

_ __ _ _ __ _ _ _ __ _

Q: What is Polymorphism??

A: Polymorphism allows a client to treat di_erent objects in the same way even if they were

created from di_erent classes and exhibit di_erent behaviors. You can use implementation

inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's

pointer can invoke methods in derived class objects. You can also achieve polymorphism in C++

by function overloading and operator overloading.

Q: What is problem with Runtime type identification?

A: The run time type identification comes at a cost of performance penalty. Compiler maintains

the class.

Q: What is Polymorphism?

A: Polymorphism allows a client to treat different objects in the same way even if they were

created from different classes and exhibit different behaviors. You can use implementation

inheritance to achieve polymorphism in languages such as C++ and Java. Base class object's

pointer can invoke methods in derived class objects. You can also achieve polymorphism in

C++ by function overloading and operator overloading.

__ _ _____

Q: What is a class?

A: A class is an expanded concept of a data structure: instead of holding only data, it can hold

both data and functions.

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

A: The default member and base class access specifies 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 specifes, member

functions, overloaded operators, and so on. 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 specified and private base-class

inheritance.

Q: How do you know that your class needs a virtual destructor?

A: If your class has at least one virtual function, you should make a destructor for this class

virtual. This will allow you to delete a dynamic object through a caller to a base class object. If

the destructor is non-virtual, then wrong destructor will be invoked during deletion of the

dynamic object.

Q: What is encapsulation?

A: Containing and hiding Information about an object, such as internal data structures and code.

Encapsulation isolates the internal complexity of an object's operation from the rest of the

application. For example, a client component asking for net revenue from a business object need

not know the data's origin.

Q: What is "this" pointer?

A: The this pointer is a pointer accessible only within the member functions of a class, struct, or

union type. It points to the object for which the member function is called. Static member

functions do not have a this pointer. When a nonstatic member function is called for an

object, the address of the object is passed as a hidden argument to the function. For example, the

following function call

myDate.setMonth( 3 );

can be interpreted this way:

setMonth( &myDate, 3 );

The object's address is available from within the member function as the this pointer. It is legal,

though unnecessary, to use the this pointer when referring to members of the class.

Q: What happens when you make call "delete this;"?

A: 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.

You should never do this. Since compiler does not know whether the object was allocated on the

stack or on the heap, "delete this" could cause a disaster.

Q: What is assignment operator?

A: Default assignment operator handles assigning one object to another of the same class.

Member to member copy (shallow copy)

Q: What are all the implicit member functions of the class? Or what are all the functions which

compiler implements for us if we don't define one?

A:

(a) default ctor

(b) copy ctor

(c) assignment operator

(d) default destructor

(e) address operator

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

A: 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 wellknown

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 Overriding?

A: To override a method, a subclass of the class that originally declared the method must declare

a method with the same name, return type (or a subclass of that return type), and same parameter

list.

The definition of the method overriding is:

· Must have same method name.

· Must have same data type.

· Must have same argument list.

Overriding a method means that replacing a method functionality in child class. To imply

overriding functionality we need parent and child classes. In the child class you define the same

method signature as one defined in the parent class.

Q: How do you access the static member of a class?

A: ::

Q: What is a nested class? Why can it be useful?

A: A nested class is a class enclosed within the scope of another class. For example:

// Example 1: Nested class

//

class OuterClass

{

class NestedClass

{

// ...

};

// ...

};

Nested classes are useful for organizing code and controlling access and dependencies. Nested

classes obey access rules just like other parts of a class do; so, in Example 1, if NestedClass is

public then any code can name it as OuterClass::NestedClass. Often nested classes contain

private implementation details, and are therefore made private; in Example 1, if NestedClass

is private, then only OuterClass's members and friends can use NestedClass. When you

instantiate as outer class, it won't instantiate inside class.

Q: What is a local class? Why can it be useful?

A: Local class is a class defined within the scope of a function _ any function, whether a

member function or a free function. For example:

// Example 2: Local class

//

int f()

{

class LocalClass

{

// ...

};

// ...

};

Like nested classes, local classes can be a useful tool for managing code dependencies.

Q: What a derived class can add?

A: New data members

New member functions

New constructors and destructor

New friends

Q: What happens when a derived-class object is created and destroyed?

A: Space is allocated (on the stack or the heap) for the full object (that is, enough space to store

the data members inherited from the base class plus the data members defined in the derived

class itself)

The base class's constructor is called to initialize the data members inherited from the base class

The derived class's constructor is then called to initialize the data members added in the derived

class

The derived-class object is then usable

When the object is destroyed (goes out of scope or is deleted) the derived class's destructor is

called on the object first

Then the base class's destructor is called on the object

Finally the allocated space for the full object is reclaimed

Q: How do I create a subscript operator for a Matrix class?

A: Use operator() rather than operator[].

When you have multiple subscripts, the cleanest way to do it is with operator() rather than with

operator[]. The reason is that operator[] always takes exactly one parameter, but operator() can

take any number of parameters (in the case of a rectangular matrix, two parameters are needed).

For example:

class Matrix {

public:

Matrix(unsigned rows, unsigned cols);

double& operator() (unsigned row, unsigned col); subscript operators often come in pairs

double operator() (unsigned row, unsigned col) const; subscript operators often come in pairs

...

~Matrix(); // Destructor

Matrix(const Matrix& m); // Copy constructor

Matrix& operator= (const Matrix& m); // Assignment operator

...

private:

unsigned rows_, cols_;

double* data_;

};

inline

Matrix::Matrix(unsigned rows, unsigned cols)

: rows_ (rows)

, cols_ (cols)

//data_ <--initialized below (after the 'if/throw' statement)

{

if (rows == 0 || cols == 0)

throw BadIndex("Matrix constructor has 0 size");

data_ = new double[rows * cols];

}

inline

Matrix::~Matrix()

{

delete[] data_;

}

inline

double& Matrix::operator() (unsigned row, unsigned col)

{

if (row >= rows_ || col >= cols_)

throw BadIndex("Matrix subscript out of bounds");

return data_[cols_*row + col];

}

inline

double Matrix::operator() (unsigned row, unsigned col) const

{

if (row >= rows_ || col >= cols_)

throw BadIndex("const Matrix subscript out of bounds");

return data_[cols_*row + col];

}

Then you can access an element of Matrix m using m(i,j) rather than m[i][j]:

int main()

{

Matrix m(10,10);

m(5,8) = 106.15;

std::cout << m(5,8);

...

}

Q: Why shouldn't my Matrix class's interface look like an array-of-array?

A: Here's what this FAQ is really all about: Some people build a Matrix class that has an

operator[] that returns a reference to an Array object (or perhaps to a raw array, shudder), and

that Array object has an operator[] that returns an element of the Matrix (e.g., a reference to a

double). Thus they access elements of the matrix using syntax like m[i][j] rather than syntax like

m(i,j).

The array-of-array solution obviously works, but it is less flexible than the operator() approach.

Specifically, there are easy performance tuning tricks that can be done with the operator()

approach that are more difficult in the [][] approach, and therefore the [][] approach is more

likely to lead to bad performance, at least in some cases.

For example, the easiest way to implement the [][] approach is to use a physical layout of the

matrix as a dense matrix that is stored in row-major form (or is it column-major; I can't ever

remember). In contrast, the operator() approach totally hides the physical layout of the matrix,

and that can lead to better performance in some cases.

Put it this way: the operator() approach is never worse than, and sometimes better than, the [][]

approach.

The operator() approach is never worse because it is easy to implement the dense, row-major

physical layout using the operator() approach, so when that configuration happens to be the

optimal layout from a performance standpoint, the operator() approach is just as easy as the [][]

approach (perhaps the operator() approach is a tiny bit easier, but I won't quibble over minor

nits).

The operator() approach is sometimes better because whenever the optimal layout for a given

application happens to be something other than dense, row-major, the implementation is often

significantly easier using the operator() approach compared to the [][] approach.

As an example of when a physical layout makes a significant difference, a recent project

happened to access the matrix elements in columns (that is, the algorithm accesses all the

elements in one column, then the elements in another, etc.), and if the physical layout is rowmajor,

the accesses can "stride the cache". For example, if the rows happen to be almost as big as

the processor's cache size, the machine can end up with a "cache miss" for almost every element

access. In this particular project, we got a 20% improvement in performance by changing the

mapping from the logical layout (row,column) to the physical layout (column,row).

Of course there are many examples of this sort of thing from numerical methods, and sparse

matrices are a whole other dimension on this issue. Since it is, in general, easier to implement a

sparse matrix or swap row/column ordering using the operator() approach, the operator()

approach loses nothing and may gain something it has no down-side and a potential up-side.

Use the operator() approach.

Q: Should I design my classes from the outside (interfaces first) or from the inside (data first)?

A: From the outside!

A good interface provides a simplified view that is expressed in the vocabulary of a user. In the

case of OO software, the interface is normally the set of public methods of either a single class or

a tight group of classes.

First think about what the object logically represents, not how you intend to physically build it.

For example, suppose you have a Stack class that will be built by containing a LinkedList:

class Stack {

public:

...

private:

LinkedList list_;

};

Should the Stack have a get() method that returns the LinkedList? Or a set() method that takes a

LinkedList? Or a constructor that takes a LinkedList? Obviously the answer is No, since you

should design your interfaces from the outside-in. I.e., users of Stack objects don't care about

LinkedLists; they care about pushing and popping.

Now for another example that is a bit more subtle. Suppose class LinkedList is built using a

linked list of Node objects, where each Node object has a pointer to the next Node:

class Node { /*...*/ };

class LinkedList {

public:

...

private:

Node* first_;

};

Should the LinkedList class have a get() method that will let users access the first Node? Should

the Node object have a get() method that will let users follow that Node to the next Node in the

chain? In other words, what should a LinkedList look like from the outside? Is a LinkedList

really a chain of Node objects? Or is that just an implementation detail? And if it is just an

implementation detail, how will the LinkedList let users access each of the elements in the

LinkedList one at a time?

The key insight is the realization that a LinkedList is not a chain of Nodes. That may be how it is

built, but that is not what it is. What it is is a sequence of elements. Therefore the LinkedList

abstraction should provide a LinkedListIterator class as well, and that LinkedListIterator might

have an operator++ to go to the next element, and it might have a get()/set() pair to access its

value stored in the Node (the value in the Node element is solely the responsibility of the

LinkedList user, which is why there is a get()/set() pair that allows the user to freely manipulate

that value).