The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, notifies them automatically of any state changes by calling one of their methods. It is used to implement distributed event handling systems, in "event driven" software. Most modern languages such as C# have built-in "event" constructs which implement the observer pattern components; the observer pattern is a key part in the familiar model–view–controller architectural pattern. The observer pattern is implemented in numerous programming libraries and systems, including all GUI toolkits; the Observer design pattern is one of the twenty-three well-known "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. The Observer pattern addresses the following problems: A one-to-many dependency between objects should be defined without making the objects coupled.
It should be ensured that when one object changes state an open-ended number of dependent objects are updated automatically. It should be possible. Defining a one-to-many dependency between objects by defining one object that updates the state of dependent objects directly is inflexible because it couples the subject to particular dependent objects. Coupled objects are hard to implement, change and reuse because they refer to and know about many different objects with different interfaces. Define Subject and Observer objects. So that when a subject changes state, all registered observers are notified and updated automatically; the sole responsibility of a subject is to maintain a list of observers and to notify them of state changes by calling their update operation. The responsibility of observers is to register themselves on a subject and to update their state when they are notified; this makes subject and observers loosely coupled. Subject and observers have no explicit knowledge of each other.
Observers can be removed independently at run-time. This notification-registration interaction is known as publish-subscribe. See the UML class and sequence diagram below; the observer pattern can cause memory leaks, known as the lapsed listener problem, because in basic implementation it requires both explicit registration and explicit deregistration, as in the dispose pattern, because the subject holds strong references to the observers, keeping them alive. This can be prevented by the subject holding weak references to the observers; the observer pattern is implemented with the "subject" being part of the object, whose state change is being observed, to be communicated to the observers upon occurrence. This type of implementation is considered "tightly coupled", forcing both the observers and the subject to be aware of each other and have access to their internal parts, creating possible issues of scalability, message recovery and maintenance, the lack of flexibility in conditional dispersion and possible hindrance to desired security measures.
In some implementations of the publish-subscribe pattern, this is solved by creating a dedicated "message queue" server and at times an extra "message handler" object, as added stages between the observer and the observed object whose state is being checked, thus "decoupling" the software components. In these cases, the message queue server is accessed by the observers with the observer pattern, "subscribing to certain messages" knowing only about the expected message, but knowing nothing about the message sender itself, the sender may know nothing about the receivers. Other implementations of the publish-subscribe pattern, which achieve a similar effect of notification and communication to interested parties, do not use the observer pattern altogether. Still, in early implementations of multi-window operating systems like OS2 and Windows, the terms "publish-subscribe pattern" and "event driven software development" were used as a synonym for the observer pattern; the observer pattern, as described in the GOF book, is a basic concept and does not deal with observance removal or with any conditional or complex logic handling to be done by the observed "subject" before or after notifying the observers.
The pattern does not deal with recording the "events", the asynchronous passing of the notifications or guaranteeing they are being received. These concerns are dealt with in message queueing systems of which the observer pattern is only a small part. Related patterns: Publish–subscribe pattern, singleton. In the above UML class diagram, the Subject class doesn't update the state of dependent objects directly. Instead, Subject refers to the Observer interface for updating state, which makes the Subject independent of how the state of dependent objects is updated; the Observer1 and Observer2 classes implement the Observer interface by synchronizing their state with subject's state. The UML sequence diagram shows the run-time interactions: The Observer1 and Observer2 objects call attach on Subject1 to register themselves. Assuming that the state of Subject1 changes, Subject1 calls notify on itself. Notify calls update on the registered Observer1 and Observer2 objects, which request the changed data from Subject1 to update their state.
While the library classes java.util. Observer and java.util. Observabl
An operating system is system software that manages computer hardware and software resources and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may include accounting software for cost allocation of processor time, mass storage and other resources. For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is executed directly by the hardware and makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers; the dominant desktop operating system is Microsoft Windows with a market share of around 82.74%. MacOS by Apple Inc. is in second place, the varieties of Linux are collectively in third place. In the mobile sector, use in 2017 is up to 70% of Google's Android and according to third quarter 2016 data, Android on smartphones is dominant with 87.5 percent and a growth rate 10.3 percent per year, followed by Apple's iOS with 12.1 percent and a per year decrease in market share of 5.2 percent, while other operating systems amount to just 0.3 percent.
Linux distributions are dominant in supercomputing sectors. Other specialized classes of operating systems, such as embedded and real-time systems, exist for many applications. A single-tasking system can only run one program at a time, while a multi-tasking operating system allows more than one program to be running in concurrency; this is achieved by time-sharing, where the available processor time is divided between multiple processes. These processes are each interrupted in time slices by a task-scheduling subsystem of the operating system. Multi-tasking may be characterized in co-operative types. In preemptive multitasking, the operating system slices the CPU time and dedicates a slot to each of the programs. Unix-like operating systems, such as Solaris and Linux—as well as non-Unix-like, such as AmigaOS—support preemptive multitasking. Cooperative multitasking is achieved by relying on each process to provide time to the other processes in a defined manner. 16-bit versions of Microsoft Windows used cooperative multi-tasking.
32-bit versions of both Windows NT and Win9x, used preemptive multi-tasking. Single-user operating systems have no facilities to distinguish users, but may allow multiple programs to run in tandem. A multi-user operating system extends the basic concept of multi-tasking with facilities that identify processes and resources, such as disk space, belonging to multiple users, the system permits multiple users to interact with the system at the same time. Time-sharing operating systems schedule tasks for efficient use of the system and may include accounting software for cost allocation of processor time, mass storage and other resources to multiple users. A distributed operating system manages a group of distinct computers and makes them appear to be a single computer; the development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine; when computers in a group work in cooperation, they form a distributed system.
In an OS, distributed and cloud computing context, templating refers to creating a single virtual machine image as a guest operating system saving it as a tool for multiple running virtual machines. The technique is used both in virtualization and cloud computing management, is common in large server warehouses. Embedded operating systems are designed to be used in embedded computer systems, they are designed to operate on small machines like PDAs with less autonomy. They are able to operate with a limited number of resources, they are compact and efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems. A real-time operating system is an operating system that guarantees to process events or data by a specific moment in time. A real-time operating system may be single- or multi-tasking, but when multitasking, it uses specialized scheduling algorithms so that a deterministic nature of behavior is achieved. An event-driven system switches between tasks based on their priorities or external events while time-sharing operating systems switch tasks based on clock interrupts.
A library operating system is one in which the services that a typical operating system provides, such as networking, are provided in the form of libraries and composed with the application and configuration code to construct a unikernel: a specialized, single address space, machine image that can be deployed to cloud or embedded environments. Early computers were built to perform a series of single tasks, like a calculator. Basic operating system features were developed in the 1950s, such as resident monitor functions that could automatically run different programs in succession to speed up processing. Operating systems did not exist in their more complex forms until the early 1960s. Hardware features were added, that enabled use of runtime libraries and parallel processing; when personal computers became popular in the 1980s, operating systems were made for them similar in concept to those used on larger computers. In the 1940s, the earliest electronic digital systems had no operating systems.
Electronic systems of this time were programmed on rows of mechanical switches or by jumper wires on plug boards. These were special-purpose systems that, for example, generated ballistics tables for the military or controlled the pri
Graphical user interface
The graphical user interface is a form of user interface that allows users to interact with electronic devices through graphical icons and visual indicators such as secondary notation, instead of text-based user interfaces, typed command labels or text navigation. GUIs were introduced in reaction to the perceived steep learning curve of command-line interfaces, which require commands to be typed on a computer keyboard; the actions in a GUI are performed through direct manipulation of the graphical elements. Beyond computers, GUIs are used in many handheld mobile devices such as MP3 players, portable media players, gaming devices and smaller household and industrial controls; the term GUI tends not to be applied to other lower-display resolution types of interfaces, such as video games, or not including flat screens, like volumetric displays because the term is restricted to the scope of two-dimensional display screens able to describe generic information, in the tradition of the computer science research at the Xerox Palo Alto Research Center.
Designing the visual composition and temporal behavior of a GUI is an important part of software application programming in the area of human–computer interaction. Its goal is to enhance the efficiency and ease of use for the underlying logical design of a stored program, a design discipline named usability. Methods of user-centered design are used to ensure that the visual language introduced in the design is well-tailored to the tasks; the visible graphical interface features of an application are sometimes referred to as chrome or GUI. Users interact with information by manipulating visual widgets that allow for interactions appropriate to the kind of data they hold; the widgets of a well-designed interface are selected to support the actions necessary to achieve the goals of users. A model–view–controller allows flexible structures in which the interface is independent from and indirectly linked to application functions, so the GUI can be customized easily; this allows users to select or design a different skin at will, eases the designer's work to change the interface as user needs evolve.
Good user interface design relates to users more, to system architecture less. Large widgets, such as windows provide a frame or container for the main presentation content such as a web page, email message or drawing. Smaller ones act as a user-input tool. A GUI may be designed for the requirements of a vertical market as application-specific graphical user interfaces. Examples include automated teller machines, point of sale touchscreens at restaurants, self-service checkouts used in a retail store, airline self-ticketing and check-in, information kiosks in a public space, like a train station or a museum, monitors or control screens in an embedded industrial application which employ a real-time operating system. By the 1980s, cell phones and handheld game systems employed application specific touchscreen GUIs. Newer automobiles use GUIs in their navigation systems and multimedia centers, or navigation multimedia center combinations. Sample graphical desktop environments A GUI uses a combination of technologies and devices to provide a platform that users can interact with, for the tasks of gathering and producing information.
A series of elements conforming a visual language have evolved to represent information stored in computers. This makes it easier for people with few computer skills to use computer software; the most common combination of such elements in GUIs is the windows, menus, pointer paradigm in personal computers. The WIMP style of interaction uses a virtual input device to represent the position of a pointing device, most a mouse, presents information organized in windows and represented with icons. Available commands are compiled together in menus, actions are performed making gestures with the pointing device. A window manager facilitates the interactions between windows and the windowing system; the windowing system handles hardware devices such as pointing devices, graphics hardware, positioning of the pointer. In personal computers, all these elements are modeled through a desktop metaphor to produce a simulation called a desktop environment in which the display represents a desktop, on which documents and folders of documents can be placed.
Window managers and other software combine to simulate the desktop environment with varying degrees of realism. Smaller mobile devices such as personal digital assistants and smartphones use the WIMP elements with different unifying metaphors, due to constraints in space and available input devices. Applications for which WIMP is not well suited may use newer interaction techniques, collectively termed post-WIMP user interfaces; as of 2011, some touchscreen-based operating systems such as Apple's iOS and Android use the class of GUIs named post-WIMP. These support styles of interaction using more than one finger in contact with a display, which allows actions such as pinching and rotating, which are unsupported by one pointer and mouse. Human interface devices, for the efficient interaction with a GUI include a computer keyboard used together with keyboard shortcuts, pointing devices for the cursor control: mouse, pointing stick, trackball, virtual keyboards, head-up displays. There are actions performed by programs that affect the GUI.
For example, there are components like inotify or D-Bus to facilitate communication between computer programs. Ivan Sutherland developed Sketchpad in 1963 held as the first graphical co
"Hello, World!" program
A "Hello, World!" program is a computer program that outputs or displays the message "Hello, World!". Because it is simple in most programming languages, it is used to illustrate the basic syntax of a programming language and is the first program that those learning to code write. A "Hello, World!" program is traditionally used to introduce novice programmers to a programming language. "Hello, world!" is traditionally used in a sanity test to make sure that a computer language is installed, that the operator understands how to use it. While small test programs have existed since the development of programmable computers, the tradition of using the phrase "Hello, world!" as a test message was influenced by an example program in the seminal book The C Programming Language. The example program from that book prints "hello, world", was inherited from a 1974 Bell Laboratories internal memorandum by Brian Kernighan, Programming in C: A Tutorial: The C version was preceded by Kernighan's own 1972 A Tutorial Introduction to the Language B, where the first known version of the program is found in an example used to illustrate external variables: main a'hell'.
The phrase is divided into multiple variables because in B, a character constant is limited to four ASCII characters. The previous example in the tutorial printed hi! on the terminal, the phrase hello, world! was introduced as a longer greeting that required several character constants for its expression. The Jargon File claims that hello, world originated instead with BCPL; this claim is supported by the archived notes of the inventors of BCPL, Prof. Brian Kernighan at Princeton and Martin Richards at Cambridge. For modern languages, world programs vary in sophistication. For example, the Go programming language introduced a multilingual program, Sun demonstrated a Java hello, world based on scalable vector graphics, the XL programming language features a spinning Earth hello, world using 3D graphics. While some languages such as Perl, Python or Ruby may need only a single statement to print "hello, world", a low-level assembly language may require dozens of commands. Mark Guzdial and Elliot Soloway have suggested that the "hello, world" test message may be outdated now that graphics and sound can be manipulated as as text.
There are many variations on the punctuation and casing of the phrase. Variations include the presence or absence of the comma and exclamation mark, the capitalization of the'H', both the'H' and the'W', or neither; some languages are forced to implement different forms, such as "HELLO WORLD", on systems that support only capital letters, while many "hello, world" programs in esoteric languages print out a modified string. For example, the first non-trivial Malbolge program printed "HEllO WORld", this having been determined to be good enough. There are variations in spirit, as well. Functional programming languages, like Lisp, ML and Haskell, tend to substitute a factorial program for Hello, World, as functional programming emphasizes recursive techniques, whereas the original examples emphasize I/O, which violates the spirit of pure functional programming by producing side effects. Languages otherwise capable of Hello, World may be used in embedded systems, where text output is either difficult or nonexistent.
For devices such as microcontrollers, field-programmable gate arrays, CPLD's, "Hello, World" may thus be substituted with a blinking LED, which demonstrates timing and interaction between components. The Debian and Ubuntu Linux distributions provide the "hello, world" program through the apt packaging system. While of itself useless, it serves as a sanity check and a simple example to newcomers of how to install a package, it is more useful for developers, however, as it provides an example of how to create a.deb package, either traditionally or using debhelper, the version of hello used, GNU Hello, serves as an example of how to write a GNU program. Time to "Hello World" is a metric for how long it takes to get a "Hello World" program running from scratch in a given programming language. "99 Bottles of Beer" as used in computer science Foobar Java Pet Store Just another Perl hacker List of basic computer science topics Trabb Pardo-Knuth algorithm List of hello world programs at Wikibooks Rösler, Wolfram.
"Hello World Collection". Helloworldcollection.de. "Hello world/Text". Rosetta Code. "Unsung Heroes of IT / Part One: Brian Kernighan". TheUnsungHeroesOfIT.com. Archived from the original on 2016-03-26. Retrieved 2014-08-23
GNOME is a free and open-source desktop environment for Unix-like operating systems. GNOME was an acronym for GNU Network Object Model Environment, but the acronym was dropped because it no longer reflected the vision of the GNOME project. GNOME is part of the GNU Project and developed by The GNOME Project, composed of both volunteers and paid contributors, the largest corporate contributor being Red Hat, it is an international project that aims to develop software frameworks for the development of software, to program end-user applications based on these frameworks, to coordinate efforts for internationalization and localization and accessibility of that software. GNOME 3 is the default desktop environment on many major Linux distributions including Fedora, Ubuntu, SUSE Linux Enterprise, Red Hat Enterprise Linux, CentOS, Oracle Linux, Scientific Linux, SteamOS, Kali Linux and Endless OS; the continued fork of the last GNOME 2 release that goes under the name MATE is default on many distributions that targets low usage of system resources.
GNOME was started on August 15 1997 by Miguel de Icaza and Federico Mena as a free software project to develop a desktop environment and applications for it. It was founded in part because K Desktop Environment, growing in popularity, relied on the Qt widget toolkit which used a proprietary software license until version 2.0. In place of Qt, the GTK toolkit was chosen as the base of GNOME. GTK uses the GNU Lesser General Public License, a free software license that allows software linking to it to use a much wider set of licenses, including proprietary software licenses. GNOME itself is licensed under the LGPL for its libraries, the GNU General Public License for its applications; the name "GNOME" was an acronym of GNU Network Object Model Environment, referring to the original intention of creating a distributed object framework similar to Microsoft's OLE, but the acronym was dropped because it no longer reflected the vision of the GNOME project. The California startup Eazel developed the Nautilus file manager from 1999 to 2001.
De Icaza and Nat Friedman founded Helix Code in 1999 in Massachusetts. During the transition to GNOME 2 around the year 2001 and shortly thereafter there were brief talks about creating a GNOME Office suite. On September 15, 2003 GNOME-Office 1.0, consisting of AbiWord 2.0, GNOME-DB 1.0 and Gnumeric 1.2.0 was released. Although some release planning for GNOME Office 1.2 was happening on gnome-office mailing list, Gnumeric 1.4 was announced as a part of it, the 1.2 release of the suite itself never materialized. As of May 4, 2014 GNOME wiki only mentions "GNOME/Gtk applications that are useful in an office environment". GNOME 2 was similar to a conventional desktop interface, featuring a simple desktop in which users could interact with virtual objects, such as windows and files. GNOME 2 started out with Sawfish, but switched to Metacity as its default window manager; the handling of windows and files in GNOME 2 is similar to that of contemporary desktop operating systems. In the default configuration of GNOME 2, the desktop has a launcher menu for quick access to installed programs and file locations.
However, these features can be moved to any position or orientation the user desires, replaced with other functions or removed altogether. As of 2009, GNOME 2 was the default desktop for OpenSolaris. GNOME 1 and 2 followed the traditional desktop metaphor. GNOME 3, released in 2011, changed this with GNOME Shell, a more abstract metaphor where switching between different tasks and virtual desktops takes place in a separate area called "Overview". Since Mutter replaced Metacity as the default window manager, the minimize and maximize buttons no longer appear by default, the title bar, menu bar and tool bar combinated in one horizontal bar called "header bar" via Client-Side Decoration mechanism. Adwaita replaced Clearlooks as the default theme. Many GNOME Core Applications went through redesigns to provide a more consistent user experience; the release of GNOME 3, notable for its move away from the traditional menu bar and taskbar, has caused considerable controversy in the GNU and Linux community.
Many users and developers have expressed concerns about usability. A few projects have been initiated to continue development of GNOME 2.x or to modify GNOME 3.x to be more like the 2.x releases. GNOME 3 aims to provide a single interface for desktop computers and tablet computers; this means using only input techniques that work on all those devices, requiring abandonment of certain concepts to which desktop users were accustomed, such as right-clicking, or saving files on the desktop. These major changes evoked widespread criticism; the MATE desktop environment was forked from the GNOME 2 code-base with the intent of retaining the traditional GNOME 2 interface, whilst keeping compatibility with modern Linux technology, such as GTK 3. The Linux Mint team addressed the issue in another way by developing the "Mint GNOME Shell Extensions" that ran on top of GNOME Shell and allowed it to be used via the traditional desktop metaphor; this led to the creation of the Cinnamon user interface, forked from the GNOME 3 codebase.
Among those critical of the early releases of GNOME 3 is Linus Torvalds, the creator of the Linux kernel. Torvalds abandoned GNOME for a wh
C++ is a general-purpose programming language, developed by Bjarne Stroustrup as an extension of the C language, or "C with Classes". It has imperative, object-oriented and generic programming features, while providing facilities for low-level memory manipulation, it is always implemented as a compiled language, many vendors provide C++ compilers, including the Free Software Foundation, Intel, IBM, so it is available on many platforms. C++ was designed with a bias toward system programming and embedded, resource-constrained software and large systems, with performance and flexibility of use as its design highlights. C++ has been found useful in many other contexts, with key strengths being software infrastructure and resource-constrained applications, including desktop applications and performance-critical applications. C++ is standardized by the International Organization for Standardization, with the latest standard version ratified and published by ISO in December 2017 as ISO/IEC 14882:2017.
The C++ programming language was standardized in 1998 as ISO/IEC 14882:1998, amended by the C++03, C++11 and C++14 standards. The current C++ 17 standard supersedes these with an enlarged standard library. Before the initial standardization in 1998, C++ was developed by Danish computer scientist Bjarne Stroustrup at Bell Labs since 1979 as an extension of the C language. C++20 is the next planned standard, keeping with the current trend of a new version every three years. In 1979, Bjarne Stroustrup, a Danish computer scientist, began work on "C with Classes", the predecessor to C++; the motivation for creating a new language originated from Stroustrup's experience in programming for his Ph. D. thesis. Stroustrup found that Simula had features that were helpful for large software development, but the language was too slow for practical use, while BCPL was fast but too low-level to be suitable for large software development; when Stroustrup started working in AT&T Bell Labs, he had the problem of analyzing the UNIX kernel with respect to distributed computing.
Remembering his Ph. D. experience, Stroustrup set out to enhance the C language with Simula-like features. C was chosen because it was general-purpose, fast and used; as well as C and Simula's influences, other languages influenced C++, including ALGOL 68, Ada, CLU and ML. Stroustrup's "C with Classes" added features to the C compiler, including classes, derived classes, strong typing and default arguments. In 1983, "C with Classes" was renamed to "C++", adding new features that included virtual functions, function name and operator overloading, constants, type-safe free-store memory allocation, improved type checking, BCPL style single-line comments with two forward slashes. Furthermore, it included the development of a standalone compiler for Cfront. In 1985, the first edition of The C++ Programming Language was released, which became the definitive reference for the language, as there was not yet an official standard; the first commercial implementation of C++ was released in October of the same year.
In 1989, C++ 2.0 was released, followed by the updated second edition of The C++ Programming Language in 1991. New features in 2.0 included multiple inheritance, abstract classes, static member functions, const member functions, protected members. In 1990, The Annotated C++ Reference Manual was published; this work became the basis for the future standard. Feature additions included templates, namespaces, new casts, a boolean type. After the 2.0 update, C++ evolved slowly until, in 2011, the C++11 standard was released, adding numerous new features, enlarging the standard library further, providing more facilities to C++ programmers. After a minor C++14 update released in December 2014, various new additions were introduced in C++17, further changes planned for 2020; as of 2017, C++ remains the third most popular programming language, behind Java and C. On January 3, 2018, Stroustrup was announced as the 2018 winner of the Charles Stark Draper Prize for Engineering, "for conceptualizing and developing the C++ programming language".
According to Stroustrup: "the name signifies the evolutionary nature of the changes from C". This name is credited to Rick Mascitti and was first used in December 1983; when Mascitti was questioned informally in 1992 about the naming, he indicated that it was given in a tongue-in-cheek spirit. The name comes from C's ++ operator and a common naming convention of using "+" to indicate an enhanced computer program. During C++'s development period, the language had been referred to as "new C" and "C with Classes" before acquiring its final name. Throughout C++'s life, its development and evolution has been guided by a set of principles: It must be driven by actual problems and its features should be useful in real world programs; every feature should be implementable. Programmers should be free to pick their own programming style, that style should be supported by C++. Allowing a useful feature is more important than preventing every possible misuse of C++, it should provide facilities for organising programs into separate, well-defined parts, provide facilities for combining separately developed parts.
No implicit violations of the type system (but allow explicit violations.