I have recently started learning design patterns. I have understood the basics of few patterns. Now I want to get familiar with some real code examples where these patterns are being utilized. Can somebody here please point out to some open source projects where design patterns are being used ? I would like to see how design patterns are being used in real world code.
Following are some of the links. JUnit Cookbook provides brilliant example of quite a few patterns - really worth checking.
JUnit Cook's Tour
Vince Huton's DP
Design PAtterns
Hope that helps.
Well, this is tooting my own horn a bit, but an Open Source project I'm working on makes extensive use of the factory method pattern. I implement the guts of the pattern itself here as a set of templates, define a factory here, register a subclass for that factory here, and use the factory to instantiate objects here.
IMHO, ACE (Adaptive Communication Environment) can be a good example. It also uses c++ templates with design patterns, which is also pragmatical. This framework takes basis of Pattern-Oriented Software Architecture: Patterns for Concurrent and Networked Objects book, which is also a good reference for design patterns, besides GoF
In the book by GOF which you are probably reading, for each Design Pattern they provide a REAL code example, I mean not the sample code, but they actually say in which projects each DP has been used. Those projects are mostly OpenSource. HTH
Related
What is the most common C++ Design Pattern libraries?
I've read about Loki library in Alexandrescu's book, but looks like it somewhat dead now. Is there something similar out there?
“Design patterns are bug reports against your programming language” --
Peter Norvig
To answer the question why there are not many C++ design pattern libraries, it is useful to know what design patterns were meant to solve in the first place. The classic GoF book states in the preface
Design patterns describe simple and elegant solutions to specific
problems in object-oriented software design.
The 90s style of object-oriented programming relied heavily on using abstract classes as interfaces, with concrete implementation classes deriving from these interfaces. The GoF patterns describe creational, structural and behavioral relationships between objects of different class types. They key element was: encapsulate and parameterize whatever will frequently change. Many of the GoF patterns can also be reformulated using templates, but then the flexibility is constrained to compile-time rather than run-time.
Object-oriented programming makes it very easy to add different concrete implementations of an interface. What OOP has a hard time with is adding new functionality to existing interfaces. The Visitor pattern is the prime example: it is essentially a work-around that relies on an extra level of indirection to allow new algorithms to work on existing data structures.
This is the exact opposite of functional programming: with functional programming it is very easy to add new functions for existing data, but it is much harder to add new data types to which such functions apply. The difficulty in getting extensibility in both functions and types is called the expression problem.
OOP style polymorphism is heavily based on internal polymorphism: the dynamic function dispatch is based on the object's type. Modern C++ also uses external polymorphism where techniques such as type erasure allow run-time flexibility with a static interface. The new std::shared_ptr and boost::any or adobe::poly classes are prime example of these techniques.
A recent ACCU presentation by Tobias Darm showed many examples of transforming the old internally polymporhic GoF patterns to this new style externally polymorphic patterns. The rough idea is to replace abstract classes with a function argument that can take std::function as a parameter. The std::function then controls the polymorphic flexibility from the outside. Many of the GoF patterns can be greatly enhanced in terms of boilerplate this way.
TL;DR: The classic GoF patterns were tailored to solve OOP shortcomings. But OOP is no longer the dominant C++ style. A combination of generic programming (the Standard Library, Boost) and OOP can solve many problems more elegantly, making classic design patterns no longer the go-to solution.
The original definition of a design pattern was a reusable approach to a reoccurring problem that could not be conveniently encapsulated in a library. Thus, the moment you can encapsulate a pattern in a library, it ceases to be a pattern, in my opinion. This has, for instance, largely happened with iterators in C++, as the standard C++ library has a comprehensive framework for implementing iterators now.
I’ve never tried to use Loki, but reading Alexandrescu’s book, I was not persuaded that a library based approach really had much to offer for many patterns.
May seems tautology, but the most common is ... the standard library itself!
It is not -strictly speaking- a "pattern library", but a folder for a number of tools addressing common pattern implementation.
Note that your question is not answerable, being a pattern just a conceptual definition commonly used in a variety of problems. Libraries don't provide patterns, they (can) use patterns (like anybody else can) to provide implementation of specific problem solutions.
Patterns are at an higher abstraction layer than coding.
In an effort to improve code maintainability, re-usability and readability, some researchers (such as GoF, Booch) started examining best practices. They have noticed that there are some patterns adopted by experienced developers to address specific design problems.
As you can see, experience created design patterns. So using design patterns is coding like a specialist. And there is no silver bullet for this.
It is true that some straightforward design patterns such as decorators find support from specific languages. But that's the limit. Domain specific frameworks also guide you to use their interfaces to complete the design pattern decided by the authors of those.
Libraries will only help you understand how design patterns used in that library will facilitate your implementation. It won't even give you a choice to change the design.
I want to get my hands dirty with some projects (in C++) which have used design patterns extensively.
I have already read design pattern documentation (as well as code) from net and other books (gang of four and Head first), but i am looking for a place where i can get already implemented projects (using design patterns), get my hands dirty with them, understand them, enhance them etc.
Could anybody point me to a place(s) from where I can get design experience in the best possible way? (Please note: Language C++, Complexity of the project can be intermediate to difficult)
ACE is a good example - uses many concurrency and communications patterns. There's a list of related tutorials on their website here.
If you are feeling ambitious take a look at Loki.
I am learning C++
I don't know much about this stuff except the fact that programming design pattern is neccesary when actually working in large projects.I hope its correct to some extent.
Is this common to all object oriented languages or do I need to look specifically into
C++ design patterns.
Also How does it helps you.Is this realy important to learn as a C++ programmer.
Please suggest
Design patterns are often misunderstood. They are really a way for experienced users to have shorthand for describing common situations. You don't need to worry about them too much while you are learning C++.
You will hear discording opinions about design patterns, in the programming community at large.
In my opinion, it is sure that there are abstractions that patterns encapsulate that are really useful (factory, singleton, delegate, etc.). I use patterns a lot, but I myself am sometime puzzled by the apparent lack of depth or level of insight that you get by reading a pattern description. This is also in tune with the proliferation of design patterns that specialize for any kind of things.
When the design hey are useful, they are a very good means of communication and certainly they guide you through the process of designing or defining the architecture of your app. They are useful both for small project and for large ones, and they can be applied at different granularity levels.
Patters are a generic concept and all programming languages support them. Anyway, if you work in C++, a book focusing on it is best, because you will get the pattern adapted to the characteristics of the language.
In my opinion, the really fundamental book about design patterns are:
GoF, Design Patterns: Elements of Reusable Object-Oriented Software
VV.AA., Pattern-Oriented Software Architecture Volume 1: A System of Patterns
VV.AA., Pattern-Oriented Software Architecture Volume 2: Patterns for Concurrent and Networked Objects
Most of the design pattern are common to all object oriented languages.
For me design patterns are nothing but abstractions that shorten communication time between programmers. Instead of expressing a complex idea of how your program is designed, you can probably find a name of a pattern that describes the design of your program.
E.g. TPM says "We should use singleton here", and you interpret "We should use one instance of the class here".
Patterns are useful to learn, but one can program without knowing any patterns, yet using them a lot.
I see design patterns as collections of wisdom gained by experience, particularly addressing issues of flexibility and maintainability of code. For example: by using a facade we can change implementations without changing the code that uses the facade.
I think it's fair to say that all programming languages have wisdom to be captured, and design patterns of some sort will be useful. The C/C++/Java/C# heritage languages seem to offer a particularly rich seem to be mined for wisdom. C++ being quite gnarly really does benefit from some key design patterns.
Some folk see Design Patterns as a way of patching over language deficiencies, that would imply that "better" langauges might need fewer patterns. My feeling is that in all cases we have something to learn from experienced folks and Patterns help us codify their wisdom.
Edited to add: Interesting point made by Munish, it is best to do a little reinvention first? By writing some real code without explicit use of Patterns you may well start to feel dissatisfied with what you write ... when I change this, I need to change all that, is there a better way to write this? ... this may drive you to really need patterns and hence motivate your study.
If you have the luxury of time to learn this way then I think you'll probably benefit.
Design patterns are solutions to commonly occuring problems in Design phase of a project.These patterns provide the solutions which we can use independent of programming language.For e.g. there is Singleton design pattern which ensures that there is only one instance of a class.Now there are numerous occaions on which this may be required.You can use the solution of these pattern and use in your code.
They provide the re usability in Software development .Put simply, design patterns
help a designer get a design right faster.
For more better understanding you could refer Design Patterns: Elements of Reusable Object-Oriented Software
Lately I am losing my trust in OOP. I have already seen many
complaints about common OOP misuses or just simple overuse. I do not
mean the common confusion between is-a and has-a relationship. I mean
stuff like the problems of ORM when dealing with relational databases,
the excessive use of inheritance from C# and also several years of looking
at code with the same false encapsulation belief that Scott Meyers
mentions in the item 23 of Effective C++
I am interested in learning more about this and non OOP software
patterns that can solve certain problems better than their OOP
counterparts. I am convinced that out there there are many people
giving good advice on how to use this as an advantage with non pure OOP
languages such as C++.
Does anyone knows any good reference (author, book, article) to get
started?
Please, notice that I am looking for two related but different things:
Common misuses of OOP concepts (like item 23)
Patterns where OOP is not the best solution (with alternatives)
Well I can recommend you a book Agile Principles, Patterns, and Practices in C#.
Examples are in C# of course, but the idea of the book is universal. Not only it covers Agile but also focuses on bad practices and shows in examples how to convert bad code to a good code. It also contains descriptions of many design pattern and shows how to implement them in semi-real example of Payroll application.
This has to be done but if you truly want to get away from OOP or at least take a look at concepts which are not OOP but are used with great effectiveness: Learn you a Haskell. Try a new programming paradigm and then start seeing where you can apply much of the concepts back to OOP languages. This addresses your second bullet, not in a direct way but trust me, it'll help more than you can think.
It's a bit odd that you mention C#. It has very powerful keywords to keep the usual inheritance misery in check. The first one ought to be the internal keyword. The notion of restricting the visibility to a module. That concept is completely absent in C++, the build model just doesn't support it. Otherwise a great concept, "I only trust the members of my team to get it right". Of course you do.
Then there's the slammer one, the sealed keyword. Extraordinary powerful, "the buck stops here, don't mess with me". Used with surgical precision in the .NET framework, I've never yet found a case where sealed was used inappropriately. Also missing in C++, but with obscure ways to get that working.
But yes, the WPF object model sucks fairly heavy. Inheriting 6 levels deep and using backdoors like a dependency property is offensive. Inheritance is hard, let's go shopping.
I would say to look at game engines. For the most part, OOP has a tendency to cause slight performance decreases, and the gaming industry is seemingly obsessed with eliminating minor slowdowns (and sometimes ignoring large ones). As such, their code, though usually written in a language that supports OOP, will end up using only those elements of OOP that are necessary for clean code / ease of maintenance that also balances performance.
EDIT:
Having said that, I don't know if I would really go look at Unreal. They do some strange things for the sake of making their content pipeline easier for developers... it makes their code... well, look if you really want to know.
One common overuse is forcing OOP in programs/scripts that take some input, turn it to output, then exit (and not receiving input from anywhere else during the process). Procedural way is much cleaner in these cases.
Typical example of this is forcing OOP in PHP scripts.
I am working on a project written in C++ which involves modification of existing code. The code uses object oriented principles(design patterns) heavily and also complicated stuff like smart pointers.
While trying to understand the code using gdb,I had to be very careful about the various polymorphic functions being called by the various subclasses.
Everyone knows that the intent of using design patterns and other complicated stuff in your code is to make it more reusable i.e maintainable but I personally feel that, it is much easier to understand and debug a procedure oriented code as you definitely know which function will actually be called.
Any insights or tips to handle such situations is greatly appreciated.
P.S: I am relatively less experienced with OOP and large projects.
gdb is not a tool for understanding code, it is a low-level debugging tool. Especially when using C++ as a higher level language on a larger project, it's not going to be easy to get the big picture from stepping through code in a debugger.
If you consider smart pointers and design patterns to be 'complicated stuff' then I respectfully suggest that you study their use until they don't seem complicated. They should be used to make things simpler, not more complex.
While procedural code may be simple to understand in the small, using object oriented design principals can provide the abstractions required to build a very large project without it turning into unmaintainable spaghetti.
For large projects, reading code is a much more important skill than operating a debugger. If a function is operating on a polymorphic base class then you need to read the code and understand what abstract operations it is performing. If there is an issue with a derived class' behaviour, then you need to look at the overrides to see if these are consistent with the base class contract.
If and only if you have a specific question about a specific circumstance that the debugger can answer should you step through code in a debugger. Questions might be something like 'Is this overriden function being called?'. This can be answered by putting a breakpoint in the overriden function and stepping over the call which you believe should be calling the overriden function to see if the breakpoint is hit.
Port it into Doxygen as a first step.
Modifying comments should have no effect on the code.
Doxygen will allow you to get an overview of the structure of the program.
Over time, as you figure out more about the program, you add comments that get picked up by Doxygen. The quality of the document grows over time, and will be helpful to the next poor SOB that gets stuck with the program
There is an excellent book called Object-Oriented Reengineering Patterns that, in a first part, provides patterns on how to understand legacy code (e.g. "refactor to understand").
A pdf version of the book is available for free at http://scg.unibe.ch/download/oorp/
Diagrams. Does your IDE have a tool that can reverse-engineer class diagrams from the code? That may help you understand the relationships between classes. Also, have the other developers actually written documentation on what they are doing and why? Is there a decisions document explaining why they designed and built in the way they did (Ok, sometimes this is not necessary - but if it exists, it would also help).
Also, do you know WHAT design patterns were used? Do they have names? Can you look them up and find other simpler examples of them? Maybe try writing a small app that also implements the design pattern, just to try it for yourself. That can also improve understanding.
I generally do the following:
Draw a simplified class diagram
Write some pseudocode
Ask a developer who is likely to be familiar with the code layout