title: PHP

paragraphs: PHP is a general-purpose scripting language especially suited to web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1994. The PHP reference implementation is now produced by The PHP Group. PHP originally stood for "Personal Home Page", but it now stands for the recursive initialism "PHP: Hypertext Preprocessor".

paragraphs: PHP code is usually processed on a web server by a PHP interpreter implemented as a module, a daemon or as a Common Gateway Interface (CGI) executable. On a web server, the result of the interpreted and executed PHP code which may be any type of data, such as generated HTML or binary image data would form the whole or part of an HTTP response. Various web template systems, web content management systems, and web frameworks exist which can be employed to orchestrate or facilitate the generation of that response. Additionally, PHP can be used for many programming tasks outside of the web context, such as standalone graphical applications and robotic drone control. Arbitrary PHP code can also be interpreted and executed via command-line interface (CLI).

paragraphs: The standard PHP interpreter, powered by the Zend Engine, is free software released under the PHP License. PHP has been widely ported and can be deployed on most web servers on almost every operating system and platform, free of charge.

paragraphs: The PHP language evolved without a written formal specification or standard until 2014, with the original implementation acting as the "de facto" standard which other implementations aimed to follow. Since 2014, work has gone on to create a formal PHP specification.

paragraphs: , 72% of PHP websites use discontinued versions of PHP, i.e. PHP 7.2 or lower no longer supported by The PHP Development Team; and large additional fraction is on PHP 7.3 that is by now "supported for critical security issues only." Over 40% of all PHP websites use version 5.6 or older, that not even Debian supports (Debian 9 supported version 7.0 and 7.1).

paragraphs: PHP development began in 1994 when Rasmus Lerdorf wrote several Common Gateway Interface (CGI) programs in C, which he used to maintain his personal homepage. He extended them to work with web forms and to communicate with databases, and called this implementation "Personal Home Page/Forms Interpreter" or PHP/FI.

paragraphs: PHP/FI could be used to build simple, dynamic web applications. To accelerate bug reporting and improve the code, Lerdorf initially announced the release of PHP/FI as "Personal Home Page Tools (PHP Tools) version 1.0" on the Usenet discussion group "comp.infosystems.www.authoring.cgi" on June 8, 1995. This release already had the basic functionality that PHP has today. This included Perl-like variables, form handling, and the ability to embed HTML. The syntax resembled that of Perl, but was simpler, more limited and less consistent.

paragraphs: <!--sql database select * from table where user='$username'-->

paragraphs: Early PHP was not intended to be a new programming language, and grew organically, with Lerdorf noting in retrospect: "I don't know how to stop it, there was never any intent to write a programming language [...] I have absolutely no idea how to write a programming language, I just kept adding the next logical step on the way." A development team began to form and, after months of work and beta testing, officially released PHP/FI 2 in November 1997.

paragraphs: The fact that PHP was not originally designed, but instead was developed organically has led to inconsistent naming of functions and inconsistent ordering of their parameters. In some cases, the function names were chosen to match the lower-level libraries which PHP was "wrapping", while in some very early versions of PHP the length of the function names was used internally as a hash function, so names were chosen to improve the distribution of hash values.

paragraphs: Zeev Suraski and Andi Gutmans rewrote the parser in 1997 and formed the base of PHP 3, changing the language's name to the recursive acronym "PHP: Hypertext Preprocessor". Afterwards, public testing of PHP 3 began, and the official launch came in June 1998. Suraski and Gutmans then started a new rewrite of PHP's core, producing the Zend Engine in 1999. They also founded Zend Technologies in Ramat Gan, Israel.

paragraphs: On May 22, 2000, PHP 4, powered by the Zend Engine 1.0, was released. By August 2008 this branch reached version 4.4.9. PHP 4 is no longer under development nor will any security updates be released.

paragraphs: On July 14 2004, PHP 5 was released powered by the new Zend Engine II. PHP 5 included new features such as improved support for object-oriented programming, the PHP Data Objects (PDO) extension (which defines a lightweight and consistent interface for accessing databases), and numerous performance enhancements. In 2008, PHP 5 became the only stable version under development. Late static binding had been missing from PHP and was added in version 5.3.

paragraphs: Many high-profile open-source projects ceased to support PHP 4 in new code from February 5, 2008, because of the GoPHP5 initiative, provided by a consortium of PHP developers promoting the transition from PHP 4 to PHP 5.

paragraphs: Over time, PHP interpreters became available on most existing 32-bit and 64-bit operating systems, either by building them from the PHP source code, or by using pre-built binaries. For PHP versions 5.3 and 5.4, the only available Microsoft Windows binary distributions were 32-bit IA-32 builds, requiring Windows 32-bit compatibility mode while using Internet Information Services (IIS) on a 64-bit Windows platform. PHP version 5.5 made the 64-bit x86-64 builds available for Microsoft Windows.

paragraphs: Official security support for PHP 5.6 ended on 31 December 2018.

paragraphs: PHP received mixed reviews due to lacking native Unicode support at the core language level. In 2005, a project headed by Andrei Zmievski was initiated to bring native Unicode support throughout PHP, by embedding the International Components for Unicode (ICU) library, and representing text strings as UTF-16 internally. Since this would cause major changes both to the internals of the language and to user code, it was planned to release this as version 6.0 of the language, along with other major features then in development.

paragraphs: However, a shortage of developers who understood the necessary changes, and performance problems arising from conversion to and from UTF-16, which is rarely used in a web context, led to delays in the project. As a result, a PHP 5.3 release was created in 2009, with many non-Unicode features back-ported from PHP 6, notably namespaces. In March 2010, the project in its current form was officially abandoned, and a PHP 5.4 release was prepared containing most remaining non-Unicode features from PHP 6, such as traits and closure re-binding. Initial hopes were that a new plan would be formed for Unicode integration, but by 2014 none had been adopted.

paragraphs: During 2014 and 2015, a new major PHP version was developed, which was numbered PHP 7. The numbering of this version involved some debate among internal developers. While the PHP 6 Unicode experiment had never been released, several articles and book titles referenced the PHP 6 name, which might have caused confusion if a new release were to reuse the name. After a vote, the name PHP 7 was chosen.

paragraphs: The foundation of PHP 7 is a PHP branch that was originally dubbed "PHP next generation" ("phpng"). It was authored by Dmitry Stogov, Xinchen Hui and Nikita Popov, and aimed to optimize PHP performance by refactoring the Zend Engine while retaining near-complete language compatibility. By 14 July 2014, WordPress-based benchmarks, which served as the main benchmark suite for the phpng project, showed an almost 100% increase in performance. Changes from phpng are also expected to make it easier to improve performance in the future, as more compact data structures and other changes are seen as better suited for a successful migration to a just-in-time (JIT) compiler. Because of the significant changes, the reworked Zend Engine is called "Zend Engine 3", succeeding Zend Engine 2 used in PHP 5.

paragraphs: Because of major internal changes in phpng it must receive a new major version number of PHP, rather than a minor PHP 5 release, according to PHP's release process. Major versions of PHP are allowed to break backward-compatibility of code and therefore PHP 7 presented an opportunity for other improvements beyond phpng that require backward-compatibility breaks. In particular, it involved the following changes:

paragraphs: PHP 7 also included new language features. Most notably, it introduces return type declarations for functions which complement the existing parameter type declarations, and support for the scalar types (integer, float, string, and boolean) in parameter and return type declarations.

paragraphs: PHP 8 was released on November 26, 2020. PHP 8 is a major version and has breaking changes from previous versions. New features and notable changes include:

paragraphs: Just-in-time compilation is supported in PHP 8.

paragraphs: PHP 8's JIT compiler can provide substantial performance improvements for some use cases. PHP developer Nikita Popov has stated that the performance improvements for most websites will be less substantial than the upgrade from PHP 5 to PHP 7. Performance improvements due to the addition of the JIT compiler are expected to have more substantial performance improvements in mathematical-type operations than for common web-development use cases. Additionally, the JIT compiler provides future potential to move some code from C to PHP, due to the performance improvements for some use cases.

paragraphs: PHP 8 introduced the expression. The match expression is conceptually similar to a statement and is more compact for some use cases. Because is an expression, its result can be caught into a variable or returned from a function.

paragraphs: PHP 8 introduced union types, a new return type, and a new type.

paragraphs: "Attributes," often referred to as "annotations" in other programming languages, were added in PHP 8, which allow metadata to be added to classes.

paragraphs: PHP 8 includes changes to allow alternate, more concise, or more consistent syntaxes in a number of scenarios. For example, the nullsafe operator is similar to the null coalescing operator , but used when calling methods. The following code snippet will not throw an error if returns null:

paragraphs: Constructor property promotion has been added as "syntactic sugar," allowing class properties to be set automatically when parameters are passed into a class constructor. This reduces the amount of boilerplate code that must be written.

paragraphs: Other minor changes include support for use of on objects, which serves as an alternative for the use of ; non-capturing catches in try-catch blocks; variable syntax tweaks to resolve inconsistencies; support for named arguments; and support for trailing commas in parameter lists, which adds consistency with support for trailing commas in other contexts, such as in arrays.

paragraphs: Beginning on June 28, 2011, the PHP Development Team implemented a timeline for the release of new versions of PHP. Under this system, at least one release should occur every month. Once per year, a minor release should occur which may include new features. Every minor release should at least be supported for two years with security and bug fixes, followed by at least one year of only security fixes, for a total of a three-year release process for every minor release. No new features, unless small and self-contained, are to be introduced into a minor release during the three-year release process.

paragraphs: The mascot of the PHP project is the "elePHPant", a blue elephant with the PHP logo on its side, designed by Vincent Pontier in 1998. "The (PHP) letters were forming the shape of an elephant if viewed in a sideways angle." The elePHPant is sometimes differently colored when in plush toy form.

paragraphs: Many variations of this mascot have been made over the years. Only the elePHPants based on the original design by Vincent Pontier are considered official by the community. These are collectible and some of them are extremely rare.

paragraphs: The following "Hello, World!" program is written in PHP code embedded in an HTML document:

paragraphs: However, as no requirement exists for PHP code to be embedded in HTML, the simplest version of "Hello, World!" may be written like this, with the closing tag omitted as preferred in files containing pure PHP code

paragraphs: The PHP interpreter only executes PHP code within its delimiters. Anything outside its delimiters is not processed by PHP, although non-PHP text is still subject to control structures described in PHP code. The most common delimiters are <?php to open and ?> to close PHP sections. The shortened form <? also exists. This short delimiter makes script files less portable, since support for them can be disabled in the local PHP configuration and it is therefore discouraged; there is no recommendation against the echo short tag <?=. Prior to PHP 5.4.0, this short syntax for echo() only works with the codice_13 configuration setting enabled, while for PHP 5.4.0 and later it is always available. The purpose of all these delimiters is to separate PHP code from non-PHP content, such as JavaScript code or HTML markup.

paragraphs: The first form of delimiters, <?php and ?>, in XHTML and other XML documents, creates correctly formed XML processing instructions. This means that the resulting mixture of PHP code and other markup in the server-side file is itself well-formed XML.

paragraphs: Variables are prefixed with a dollar symbol, and a type does not need to be specified in advance. PHP 5 introduced "type declarations" that allows functions to force their parameters to be objects of a specific class, arrays, interfaces or callback functions. However, before PHP 7, type declarations could not be used with scalar types such as integer or string.

paragraphs: Unlike function and class names, variable names are case sensitive. Both double-quoted ("") and heredoc strings provide the ability to interpolate a variable's value into the string. PHP treats newlines as whitespace in the manner of a free-form language, and statements are terminated by a semicolon. PHP has three types of comment syntax: /* */ marks block and inline comments; // or # are used for one-line comments. The codice_14 statement is one of several facilities PHP provides to output text.

paragraphs: In terms of keywords and language syntax, PHP is similar to the C style syntax. if conditions, for and while loops, and function returns are similar in syntax to languages such as C, C++, C#, Java and Perl.

paragraphs: PHP is a loosely coupled language, since it does not depend on the data type. This is the one of the main features of the language. It stores integers in a platform-dependent range, either as a 32, 64 or 128-bit signed integer equivalent to the C-language long type. Unsigned integers are converted to signed values in certain situations, which is different behavior to many other programming languages. Integer variables can be assigned using decimal (positive and negative), octal, hexadecimal, and binary notations.

paragraphs: Floating point numbers are also stored in a platform-specific range. They can be specified using floating point notation, or two forms of scientific notation. PHP has a native Boolean type that is similar to the native Boolean types in Java and C++. Using the Boolean type conversion rules, non-zero values are interpreted as true and zero as false, as in Perl and C++.

paragraphs: The null data type represents a variable that has no value; codice_15 is the only allowed value for this data type.

paragraphs: Variables of the "resource" type represent references to resources from external sources. These are typically created by functions from a particular extension, and can only be processed by functions from the same extension; examples include file, image, and database resources.

paragraphs: Arrays can contain elements of any type that PHP can handle, including resources, objects, and even other arrays. Order is preserved in lists of values and in hashes with both keys and values, and the two can be intermingled. PHP also supports strings, which can be used with single quotes, double quotes, nowdoc or heredoc syntax.

paragraphs: The Standard PHP Library (SPL) attempts to solve standard problems and implements efficient data access interfaces and classes.

paragraphs: PHP defines a large array of functions in the core language and many are also available in various extensions; these functions are well documented in the online PHP documentation. However, the built-in library has a wide variety of naming conventions and associated inconsistencies, as described under history above.

paragraphs: Custom functions may be defined by the developer:

paragraphs: echo 'I am currently ' . myAge(1995) . ' old.';

paragraphs: In 2021, the output of the above sample program is 'I am currently 26 years old.'

paragraphs: In lieu of function pointers, functions in PHP can be referenced by a string containing their name. In this manner, normal PHP functions can be used, for example, as callbacks or within function tables. User-defined functions may be created at any time without being prototyped. Functions may be defined inside code blocks, permitting a run-time decision as to whether or not a function should be defined. There is a codice_16 function that determines whether a function with a given name has already been defined. Function calls must use parentheses, with the exception of zero-argument class constructor functions called with the PHP operator codice_17, in which case parentheses are optional.

paragraphs: Until PHP 5.3, support for anonymous functions and closures did not exist in PHP. While codice_18 has existed since PHP 4.0.1, it is merely a thin wrapper around codice_19 that allows normal PHP functions to be created during program execution. PHP 5.3 added syntax to define an anonymous function or "closure" which can capture variables from the surrounding scope:

paragraphs: In the example above, codice_20 function creates a closure using passed argument (the keyword codice_21 imports a variable from the lexical context), which takes an additional argument , and returns the created closure to the caller. Such a function is a first-class object, meaning that it can be stored in a variable, passed as a parameter to other functions, etc.

paragraphs: Unusually for a dynamically typed language, PHP supports type declarations on function parameters, which are enforced at runtime. This has been supported for classes and interfaces since PHP 5.0, for arrays since PHP 5.1, for "callables" since PHP 5.4, and scalar (integer, float, string and boolean) types since PHP 7.0. PHP 7.0 also has type declarations for function return types, expressed by placing the type name after the list of parameters, preceded by a colon. For example, the codice_22 function from the earlier example could be annotated with types like so in PHP 7:

paragraphs: echo $adder(null); // throws an exception because an incorrect type was passed

paragraphs: $adder = getAdder([]); // would also throw an exception

paragraphs: By default, scalar type declarations follow weak typing principles. So, for example, if a parameter's type is codice_23, PHP would allow not only integers, but also convertible numeric strings, floats or booleans to be passed to that function, and would convert them. However, PHP 7 has a "strict typing" mode which, when used, disallows such conversions for function calls and returns within a file.

paragraphs: Basic object-oriented programming functionality was added in PHP 3 and improved in PHP 4. This allowed for PHP to gain further abstraction, making creative tasks easier for programmers using the language. Object handling was completely rewritten for PHP 5, expanding the feature set and enhancing performance. In previous versions of PHP, objects were handled like value types. The drawback of this method was that code had to make heavy use of PHP's "reference" variables if it wanted to modify an object it was passed rather than creating a copy of it. In the new approach, objects are referenced by handle, and not by value.

paragraphs: PHP 5 introduced private and protected member variables and methods, along with abstract classes, final classes, abstract methods, and final methods. It also introduced a standard way of declaring constructors and destructors, similar to that of other object-oriented languages such as C++, and a standard exception handling model. Furthermore, PHP 5 added interfaces and allowed for multiple interfaces to be implemented. There are special interfaces that allow objects to interact with the runtime system. Objects implementing ArrayAccess can be used with array syntax and objects implementing Iterator or IteratorAggregate can be used with the codice_6 language construct. There is no virtual table feature in the engine, so static variables are bound with a name instead of a reference at compile time.

paragraphs: If the developer creates a copy of an object using the reserved word codice_25, the Zend engine will check whether a codice_26 method has been defined. If not, it will call a default codice_26 which will copy the object's properties. If a codice_26 method is defined, then it will be responsible for setting the necessary properties in the created object. For convenience, the engine will supply a function that imports the properties of the source object, so the programmer can start with a by-value replica of the source object and only override properties that need to be changed.

paragraphs: The following is a basic example of object-oriented programming in PHP:

paragraphs: // Hello, my name is Alice, I learn Computer Science

paragraphs: // Hello, my name is Bob, I learn Computer Science

paragraphs: // Hello, my name is Charlie, I learn Business Studies

paragraphs: // Hello, my name is Dan, I teach Computer Science, Information Security

paragraphs: // Hello, my name is Erin, I teach Computer Science, 3D Graphics Programming

paragraphs: // Hello, my name is Frankie, I teach Online Marketing, Business Studies, E-commerce

paragraphs: The visibility of PHP properties and methods is defined using the keywords codice_29, codice_30, and codice_31. The default is public, if only var is used; codice_32 is a synonym for codice_29. Items declared codice_29 can be accessed everywhere. codice_31 limits access to inherited classes (and to the class that defines the item). codice_30 limits visibility only to the class that defines the item. Objects of the same type have access to each other's private and protected members even though they are not the same instance.

paragraphs: The only complete PHP implementation is the original, known simply as PHP. It is the most widely used and is powered by the Zend Engine. To disambiguate it from other implementations, it is sometimes unofficially called "Zend PHP". The Zend Engine compiles PHP source code on-the-fly into an internal format that it can execute, thus it works as an interpreter. It is also the "reference implementation" of PHP, as PHP has no formal specification, and so the semantics of Zend PHP define the semantics of PHP. Due to the complex and nuanced semantics of PHP, defined by how Zend works, it is difficult for competing implementations to offer complete compatibility.

paragraphs: PHP's single-request-per-script-execution model, and the fact the Zend Engine is an interpreter, leads to inefficiency; as a result, various products have been developed to help improve PHP performance. In order to speed up execution time and not have to compile the PHP source code every time the web page is accessed, PHP scripts can also be deployed in the PHP engine's internal format by using an opcode cache, which works by caching the compiled form of a PHP script (opcodes) in shared memory to avoid the overhead of parsing and compiling the code every time the script runs. An opcode cache, Zend Opcache, is built into PHP since version 5.5. Another example of a widely used opcode cache is the Alternative PHP Cache (APC), which is available as a PECL extension.

paragraphs: While Zend PHP is still the most popular implementation, several other implementations have been developed. Some of these are compilers or support JIT compilation, and hence offer performance benefits over Zend PHP at the expense of lacking full PHP compatibility. Alternative implementations include the following:

paragraphs: PHP is free software released under the PHP License, which stipulates that:

paragraphs: This restriction on use of "PHP" makes the PHP License incompatible with the General Public License (GPL), while the Zend License is incompatible due to an advertising clause similar to that of the original BSD license.

paragraphs: PHP includes various free and open-source libraries in its source distribution, or uses them in resulting PHP binary builds. PHP is fundamentally an Internet-aware system with built-in modules for accessing File Transfer Protocol (FTP) servers and many database servers, including PostgreSQL, MySQL, Microsoft SQL Server and SQLite (which is an embedded database), LDAP servers, and others. Numerous functions familiar to C programmers, such as those in the stdio family, are available in standard PHP builds.

paragraphs: PHP allows developers to write extensions in C to add functionality to the PHP language. PHP extensions can be compiled statically into PHP or loaded dynamically at runtime. Numerous extensions have been written to add support for the Windows API, process management on Unix-like operating systems, multibyte strings (Unicode), cURL, and several popular compression formats. Other PHP features made available through extensions include integration with IRC, dynamic generation of images and Adobe Flash content, "PHP Data Objects" (PDO) as an abstraction layer used for accessing databases, and even speech synthesis. Some of the language's core functions, such as those dealing with strings and arrays, are also implemented as extensions. The PHP Extension Community Library (PECL) project is a repository for extensions to the PHP language.

paragraphs: Some other projects, such as "Zephir", provide the ability for PHP extensions to be created in a high-level language and compiled into native PHP extensions. Such an approach, instead of writing PHP extensions directly in C, simplifies the development of extensions and reduces the time required for programming and testing.

paragraphs: By December 2018 the PHP Group consisted of ten people: Thies C. Arntzen, Stig Bakken, Shane Caraveo, Andi Gutmans, Rasmus Lerdorf, Sam Ruby, Sascha Schumann, Zeev Suraski, Jim Winstead, and Andrei Zmievski.

paragraphs: Zend Technologies provides a PHP Certification based on PHP 7 exam (and previously based on PHP 5.5) for programmers to become certified PHP developers.

paragraphs: There are two primary ways for adding support for PHP to a web server – as a native web server module, or as a CGI executable. PHP has a direct module interface called Server Application Programming Interface (SAPI), which is supported by many web servers including Apache HTTP Server, Microsoft IIS, Netscape (now defunct) and iPlanet. Some other web servers, such as OmniHTTPd, support the Internet Server Application Programming Interface (ISAPI), which is a Microsoft's web server module interface. If PHP has no module support for a web server, it can always be used as a Common Gateway Interface (CGI) or FastCGI processor; in that case, the web server is configured to use PHP's CGI executable to process all requests to PHP files.

paragraphs: PHP-FPM (FastCGI Process Manager) is an alternative FastCGI implementation for PHP, bundled with the official PHP distribution since version 5.3.3. When compared to the older FastCGI implementation, it contains some additional features, mostly useful for heavily loaded web servers.

paragraphs: When using PHP for command-line scripting, a PHP command-line interface (CLI) executable is needed. PHP supports a CLI server application programming interface (SAPI) since PHP 4.3.0. The main focus of this SAPI is developing shell applications using PHP. There are quite a few differences between the CLI SAPI and other SAPIs, although they do share many of the same behaviors.

paragraphs: PHP has a direct module interface called SAPI for different web servers; in case of PHP 5 and Apache 2.0 on Windows, it is provided in form of a DLL file called , which is a module that, among other functions, provides an interface between PHP and the web server, implemented in a form that the server understands. This form is what is known as a SAPI.

paragraphs: There are different kinds of SAPIs for various web server extensions. For example, in addition to those listed above, other SAPIs for the PHP language include the Common Gateway Interface (CGI) and command-line interface (CLI).

paragraphs: PHP can also be used for writing desktop graphical user interface (GUI) applications, by using the PHP-GTK extension. PHP-GTK is not included in the official PHP distribution, and as an extension it can be used only with PHP versions 5.1.0 and newer. The most common way of installing PHP-GTK is compiling it from the source code.

paragraphs: When PHP is installed and used in cloud environments, software development kits (SDKs) are provided for using cloud-specific features. For example:

paragraphs: Numerous configuration options are supported, affecting both core PHP features and extensions. Configuration file codice_39 is searched for in different locations, depending on the way PHP is used. The configuration file is split into various sections, while some of the configuration options can be also set within the web server configuration.

paragraphs: PHP is a general-purpose scripting language that is especially suited to server-side web development, in which case PHP generally runs on a web server. Any PHP code in a requested file is executed by the PHP runtime, usually to create dynamic web page content or dynamic images used on websites or elsewhere. It can also be used for command-line scripting and client-side graphical user interface (GUI) applications. PHP can be deployed on most web servers, many operating systems and platforms, and can be used with many relational database management systems (RDBMS). Most web hosting providers support PHP for use by their clients. It is available free of charge, and the PHP Group provides the complete source code for users to build, customize and extend for their own use.

paragraphs: Originally designed to create dynamic web pages, PHP now focuses mainly on server-side scripting, and it is similar to other server-side scripting languages that provide dynamic content from a web server to a client, such as Microsoft's ASP.NET, Sun Microsystems' JavaServer Pages, and codice_40. PHP has also attracted the development of many software frameworks that provide building blocks and a design structure to promote rapid application development (RAD). Some of these include PRADO, CakePHP, Symfony, CodeIgniter, Laravel, Yii Framework, Phalcon and Zend Framework, offering features similar to other web frameworks.

paragraphs: The LAMP architecture has become popular in the web industry as a way of deploying web applications. PHP is commonly used as the "P" in this bundle alongside Linux, Apache and MySQL, although the "P" may also refer to Python, Perl, or some mix of the three. Similar packages, WAMP and MAMP, are also available for Windows and macOS, with the first letter standing for the respective operating system. Although both PHP and Apache are provided as part of the macOS base install, users of these packages seek a simpler installation mechanism that can be more easily kept up to date.

paragraphs: For specific and more advanced usage scenarios, PHP offers a well defined and documented way for writing custom extensions in C or C++. Besides extending the language itself in form of additional libraries, extensions are providing a way for improving execution speed where it is critical and there is room for improvements by using a true compiled language. PHP also offers well defined ways for embedding itself into other software projects. That way PHP can be easily used as an internal scripting language for another project, also providing tight interfacing with the project's specific internal data structures.

paragraphs: PHP received mixed reviews due to lacking support for multithreading at the core language level, though using threads is made possible by the "pthreads" PECL extension.

paragraphs: A command line interface, php-cli, and two ActiveX Windows Script Host scripting engines for PHP have been produced.

paragraphs: PHP is used for Web content management systems including MediaWiki, WordPress, Joomla, Drupal, Moodle, eZ Publish, eZ Platform, and SilverStripe.

paragraphs: Websites using PHP include Facebook, Digg, Tumblr, Dailymotion, and Slack.

paragraphs: , PHP was used in more than 240 million websites (39% of those sampled) and was installed on 2.1 million web servers.

paragraphs: , PHP was used as the server-side programming language on 78.8% of websites, down from 83.5% previously, where the language could be determined.

paragraphs: , PHP 7 is the most used version of the language with 50.6% of sites using PHP and 39.6% of all websites on the web.

paragraphs: In 2019, 11% of all vulnerabilities listed by the National Vulnerability Database were linked to PHP; historically, about 30% of all vulnerabilities listed since 1996 in this database are linked to PHP. Technical security flaws of the language itself or of its core libraries are not frequent (22 in 2009, about 1% of the total although PHP applies to about 20% of programs listed). Recognizing that programmers make mistakes, some languages include taint checking to automatically detect the lack of input validation which induces many issues. Such a feature is being developed for PHP, but its inclusion into a release has been rejected several times in the past.

paragraphs: There are advanced protection patches such as Suhosin and Hardening-Patch, especially designed for web hosting environments.

paragraphs: Historically, old versions of PHP had some configuration parameters and default values for such runtime settings that made some PHP applications prone to security issues. Among these, codice_41 and codice_42 configuration directives were the best known; the latter made any URL parameters become PHP variables, opening a path for serious security vulnerabilities by allowing an attacker to set the value of any uninitialized global variable and interfere with the execution of a PHP script. Support for "magic quotes" and "register globals" settings has been deprecated since PHP 5.3.0, and removed from PHP 5.4.0.

paragraphs: Another example for the potential runtime-settings vulnerability comes from failing to disable PHP execution (for example by using the codice_43 configuration directive) for the directory where uploaded files are stored; enabling it can result in execution of malicious code embedded within the uploaded files. The best practice is to either locate the image directory outside of the document root available to the web server and serve it via intermediary script, or disable PHP execution for the directory which stores the uploaded files.

paragraphs: Also, enabling the dynamic loading of PHP extensions (via codice_44 configuration directive) in a shared web hosting environment can lead to security issues.

paragraphs: Implied type conversions that result in different values being treated as equal, sometimes against the programmer's intent, can lead to security issues. For example, the result of the comparison is codice_45, because strings that are parsable as numbers are converted to numbers; in this case, the first compared value is treated as scientific notation having the value (), which is zero. Errors like this resulted in authentication vulnerabilities in Simple Machines Forum, Typo3 and phpBB when MD5 password hashes were compared. The recommended way is to use hash_equals() (for timing attack safety), codice_46 or the identity operator (codice_47), as results in codice_48.

paragraphs: In a 2013 analysis of over 170,000 website defacements, published by Zone-H, the most frequently (53%) used technique was exploitation of file inclusion vulnerability, mostly related to insecure usage of the PHP language constructs codice_49, codice_50, and codice_51.


title: C++

paragraphs: C++ () is a general-purpose programming language created by Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significantly over time, and modern C++ now has object-oriented, generic, and functional features in addition to facilities for low-level memory manipulation. It is almost always implemented as a compiled language, and many vendors provide C++ compilers, including the Free Software Foundation, LLVM, Microsoft, Intel, Oracle, and IBM, so it is available on many platforms.

paragraphs: C++ was designed with a bias toward system programming and embedded, resource-constrained software and large systems, with performance, efficiency, and flexibility of use as its design highlights. C++ has also been found useful in many other contexts, with key strengths being software infrastructure and resource-constrained applications, including desktop applications, video games, servers (e.g. e-commerce, Web search, or SQL servers), and performance-critical applications (e.g. telephone switches or space probes).

paragraphs: C++ is standardized by the International Organization for Standardization (ISO), with the latest standard version ratified and published by ISO in December 2020 as "ISO/IEC 14882:2020" (informally known as C++20). The C++ programming language was initially standardized in 1998 as "ISO/IEC 14882:1998", which was then amended by the C++03, C++11, C++14, and C++17 standards. The current C++20 standard supersedes these with new features and 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; he wanted an efficient and flexible language similar to C that also provided high-level features for program organization. Since 2012, C++ is on a three-year release schedule, with C++23 the next planned standard.

paragraphs: In 1979, Bjarne Stroustrup, a Danish computer scientist, began work on "", the predecessor to C++. The motivation for creating a new language originated from Stroustrup's experience in programming for his PhD thesis. Stroustrup found that Simula had features that were very 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, portable and widely used. As well as C and Simula's influences, other languages also influenced this new language, including ALGOL 68, Ada, CLU and ML.

paragraphs: Initially, Stroustrup's "C with Classes" added features to the C compiler, Cpre, including classes, derived classes, strong typing, inlining and default arguments.

paragraphs: In 1982, Stroustrup started to develop a successor to C with Classes, which he named "C++" (++ being the increment operator in C) after going through several other names. New features were added, including virtual functions, function name and operator overloading, references, constants, type-safe free-store memory allocation (new/delete), improved type checking, and BCPL style single-line comments with two forward slashes (//). Furthermore, Stroustrup developed a new, standalone compiler for C++, Cfront.

paragraphs: In 1984, Stroustrup implemented the first stream input/output library. The idea of providing an output operator rather than a named output function was suggested by Doug McIlroy (who had previously suggested Unix pipes).

paragraphs: 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.

paragraphs: 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, and protected members. In 1990, "The Annotated C++ Reference Manual" was published. This work became the basis for the future standard. Later feature additions included templates, exceptions, namespaces, new casts, and a Boolean type.

paragraphs: In 1998, C++98 was released, standardizing the language, and a minor update (C++03) was released in 2003.

paragraphs: After C++98, C++ evolved relatively slowly until, in 2011, the C++11 standard was released, adding numerous new features, enlarging the standard library further, and providing more facilities to C++ programmers. After a minor C++14 update released in December 2014, various new additions were introduced in C++17. After becoming finalized in February 2020, a draft of the C++20 standard was approved on 4 September 2020 and officially published on 15 December 2020.

paragraphs: 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".

paragraphs: According to Stroustrup, "the name signifies the evolutionary nature of the changes from C". This name is credited to Rick Mascitti (mid-1983) 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 (which increments the value of a variable) and a common naming convention of using "+" to indicate an enhanced computer program.

paragraphs: During C++'s development period, the language had been referred to as "new C" and "C with Classes" before acquiring its final name.

paragraphs: Throughout C++'s life, its development and evolution has been guided by a set of principles:

paragraphs: C++ is standardized by an ISO working group known as JTC1/SC22/WG21. So far, it has published six revisions of the C++ standard and is currently working on the next revision, C++23.

paragraphs: In 1998, the ISO working group standardized C++ for the first time as "ISO/IEC 14882:1998", which is informally known as "C++98". In 2003, it published a new version of the C++ standard called "ISO/IEC 14882:2003", which fixed problems identified in C++98.

paragraphs: The next major revision of the standard was informally referred to as "C++0x", but it was not released until 2011. C++11 (14882:2011) included many additions to both the core language and the standard library.

paragraphs: In 2014, C++14 (also known as C++1y) was released as a small extension to C++11, featuring mainly bug fixes and small improvements. The Draft International Standard ballot procedures completed in mid-August 2014.

paragraphs: After C++14, a major revision C++17, informally known as C++1z, was completed by the ISO C++ Committee in mid July 2017 and was approved and published in December 2017.

paragraphs: As part of the standardization process, ISO also publishes technical reports and specifications:

paragraphs: More technical specifications are in development and pending approval, including static reflection.

paragraphs: The C++ language has two main components: a direct mapping of hardware features provided primarily by the C subset, and zero-overhead abstractions based on those mappings. Stroustrup describes C++ as "a light-weight abstraction programming language [designed] for building and using efficient and elegant abstractions"; and "offering both hardware access and abstraction is the basis of C++. Doing it efficiently is what distinguishes it from other languages."

paragraphs: C++ inherits most of C's syntax. The following is Bjarne Stroustrup's version of the Hello world program that uses the C++ Standard Library stream facility to write a message to standard output:

paragraphs: As in C, C++ supports four types of memory management: static storage duration objects, thread storage duration objects, automatic storage duration objects, and dynamic storage duration objects.

paragraphs: Static storage duration objects are created before codice_1 is entered (see exceptions below) and destroyed in reverse order of creation after codice_1 exits. The exact order of creation is not specified by the standard (though there are some rules defined below) to allow implementations some freedom in how to organize their implementation. More formally, objects of this type have a lifespan that "shall last for the duration of the program".

paragraphs: Static storage duration objects are initialized in two phases. First, "static initialization" is performed, and only "after" all static initialization is performed, "dynamic initialization" is performed. In static initialization, all objects are first initialized with zeros; after that, all objects that have a constant initialization phase are initialized with the constant expression (i.e. variables initialized with a literal or codice_3). Though it is not specified in the standard, the static initialization phase can be completed at compile time and saved in the data partition of the executable. Dynamic initialization involves all object initialization done via a constructor or function call (unless the function is marked with codice_3, in C++11). The dynamic initialization order is defined as the order of declaration within the compilation unit (i.e. the same file). No guarantees are provided about the order of initialization between compilation units.

paragraphs: Variables of this type are very similar to static storage duration objects. The main difference is the creation time is just prior to thread creation and destruction is done after the thread has been joined.

paragraphs: The most common variable types in C++ are local variables inside a function or block, and temporary variables. The common feature about automatic variables is that they have a lifetime that is limited to the scope of the variable. They are created and potentially initialized at the point of declaration (see below for details) and destroyed in the "reverse" order of creation when the scope is left. This is implemented by allocation on the stack.

paragraphs: Local variables are created as the point of execution passes the declaration point. If the variable has a constructor or initializer this is used to define the initial state of the object. Local variables are destroyed when the local block or function that they are declared in is closed. C++ destructors for local variables are called at the end of the object lifetime, allowing a discipline for automatic resource management termed RAII, which is widely used in C++.

paragraphs: Member variables are created when the parent object is created. Array members are initialized from 0 to the last member of the array in order. Member variables are destroyed when the parent object is destroyed in the reverse order of creation. i.e. If the parent is an "automatic object" then it will be destroyed when it goes out of scope which triggers the destruction of all its members.

paragraphs: Temporary variables are created as the result of expression evaluation and are destroyed when the statement containing the expression has been fully evaluated (usually at the codice_5 at the end of a statement).

paragraphs: These objects have a dynamic lifespan and can be created directly with a call to and destroyed explicitly with a call to . C++ also supports codice_6 and codice_7, from C, but these are not compatible with and . Use of returns an address to the allocated memory. The C++ Core Guidelines advise against using directly for creating dynamic objects in favor of smart pointers through for single ownership and for reference-counted multiple ownership, which were introduced in C++11.

paragraphs: C++ templates enable generic programming. C++ supports function, class, alias, and variable templates. Templates may be parameterized by types, compile-time constants, and other templates. Templates are implemented by "instantiation" at compile-time. To instantiate a template, compilers substitute specific arguments for a template's parameters to generate a concrete function or class instance. Some substitutions are not possible; these are eliminated by an overload resolution policy described by the phrase "Substitution failure is not an error" (SFINAE). Templates are a powerful tool that can be used for generic programming, template metaprogramming, and code optimization, but this power implies a cost. Template use may increase code size, because each template instantiation produces a copy of the template code: one for each set of template arguments, however, this is the same or smaller amount of code that would be generated if the code was written by hand. This is in contrast to run-time generics seen in other languages (e.g., Java) where at compile-time the type is erased and a single template body is preserved.

paragraphs: Templates are different from macros: while both of these compile-time language features enable conditional compilation, templates are not restricted to lexical substitution. Templates are aware of the semantics and type system of their companion language, as well as all compile-time type definitions, and can perform high-level operations including programmatic flow control based on evaluation of strictly type-checked parameters. Macros are capable of conditional control over compilation based on predetermined criteria, but cannot instantiate new types, recurse, or perform type evaluation and in effect are limited to pre-compilation text-substitution and text-inclusion/exclusion. In other words, macros can control compilation flow based on pre-defined symbols but cannot, unlike templates, independently instantiate new symbols. Templates are a tool for static polymorphism (see below) and generic programming.

paragraphs: In addition, templates are a compile-time mechanism in C++ that is Turing-complete, meaning that any computation expressible by a computer program can be computed, in some form, by a template metaprogram prior to runtime.

paragraphs: In summary, a template is a compile-time parameterized function or class written without knowledge of the specific arguments used to instantiate it. After instantiation, the resulting code is equivalent to code written specifically for the passed arguments. In this manner, templates provide a way to decouple generic, broadly applicable aspects of functions and classes (encoded in templates) from specific aspects (encoded in template parameters) without sacrificing performance due to abstraction.

paragraphs: C++ introduces object-oriented programming (OOP) features to C. It offers classes, which provide the four features commonly present in OOP (and some non-OOP) languages: abstraction, encapsulation, inheritance, and polymorphism. One distinguishing feature of C++ classes compared to classes in other programming languages is support for deterministic destructors, which in turn provide support for the Resource Acquisition is Initialization (RAII) concept.

paragraphs: Encapsulation is the hiding of information to ensure that data structures and operators are used as intended and to make the usage model more obvious to the developer. C++ provides the ability to define classes and functions as its primary encapsulation mechanisms. Within a class, members can be declared as either public, protected, or private to explicitly enforce encapsulation. A public member of the class is accessible to any function. A private member is accessible only to functions that are members of that class and to functions and classes explicitly granted access permission by the class ("friends"). A protected member is accessible to members of classes that inherit from the class in addition to the class itself and any friends.

paragraphs: The object-oriented principle ensures the encapsulation of all and only the functions that access the internal representation of a type. C++ supports this principle via member functions and friend functions, but it does not enforce it. Programmers can declare parts or all of the representation of a type to be public, and they are allowed to make public entities not part of the representation of a type. Therefore, C++ supports not just object-oriented programming, but other decomposition paradigms such as modular programming.

paragraphs: It is generally considered good practice to make all data private or protected, and to make public only those functions that are part of a minimal interface for users of the class. This can hide the details of data implementation, allowing the designer to later fundamentally change the implementation without changing the interface in any way.

paragraphs: Inheritance allows one data type to acquire properties of other data types. Inheritance from a base class may be declared as public, protected, or private. This access specifier determines whether unrelated and derived classes can access the inherited public and protected members of the base class. Only public inheritance corresponds to what is usually meant by "inheritance". The other two forms are much less frequently used. If the access specifier is omitted, a "class" inherits privately, while a "struct" inherits publicly. Base classes may be declared as virtual; this is called virtual inheritance. Virtual inheritance ensures that only one instance of a base class exists in the inheritance graph, avoiding some of the ambiguity problems of multiple inheritance.

paragraphs: Multiple inheritance is a C++ feature allowing a class to be derived from more than one base class; this allows for more elaborate inheritance relationships. For example, a "Flying Cat" class can inherit from both "Cat" and "Flying Mammal". Some other languages, such as C# or Java, accomplish something similar (although more limited) by allowing inheritance of multiple interfaces while restricting the number of base classes to one (interfaces, unlike classes, provide only declarations of member functions, no implementation or member data). An interface as in C# and Java can be defined in C++ as a class containing only pure virtual functions, often known as an abstract base class or "ABC". The member functions of such an abstract base class are normally explicitly defined in the derived class, not inherited implicitly. C++ virtual inheritance exhibits an ambiguity resolution feature called dominance.

paragraphs: C++ provides more than 35 operators, covering basic arithmetic, bit manipulation, indirection, comparisons, logical operations and others. Almost all operators can be overloaded for user-defined types, with a few notable exceptions such as member access (codice_8 and codice_9) as well as the conditional operator. The rich set of overloadable operators is central to making user-defined types in C++ seem like built-in types.

paragraphs: Overloadable operators are also an essential part of many advanced C++ programming techniques, such as smart pointers. Overloading an operator does not change the precedence of calculations involving the operator, nor does it change the number of operands that the operator uses (any operand may however be ignored by the operator, though it will be evaluated prior to execution). Overloaded "codice_10" and "codice_11" operators lose their short-circuit evaluation property.

paragraphs: Polymorphism enables one common interface for many implementations, and for objects to act differently under different circumstances.

paragraphs: C++ supports several kinds of "static" (resolved at compile-time) and "dynamic" (resolved at run-time) polymorphisms, supported by the language features described above. Compile-time polymorphism does not allow for certain run-time decisions, while runtime polymorphism typically incurs a performance penalty.

paragraphs: Function overloading allows programs to declare multiple functions having the same name but with different arguments (i.e. "ad hoc" polymorphism). The functions are distinguished by the number or types of their formal parameters. Thus, the same function name can refer to different functions depending on the context in which it is used. The type returned by the function is not used to distinguish overloaded functions and would result in a compile-time error message.

paragraphs: When declaring a function, a programmer can specify for one or more parameters a default value. Doing so allows the parameters with defaults to optionally be omitted when the function is called, in which case the default arguments will be used. When a function is called with fewer arguments than there are declared parameters, explicit arguments are matched to parameters in left-to-right order, with any unmatched parameters at the end of the parameter list being assigned their default arguments. In many cases, specifying default arguments in a single function declaration is preferable to providing overloaded function definitions with different numbers of parameters.

paragraphs: Templates in C++ provide a sophisticated mechanism for writing generic, polymorphic code (i.e. parametric polymorphism). In particular, through the curiously recurring template pattern, it's possible to implement a form of static polymorphism that closely mimics the syntax for overriding virtual functions. Because C++ templates are type-aware and Turing-complete, they can also be used to let the compiler resolve recursive conditionals and generate substantial programs through template metaprogramming. Contrary to some opinion, template code will not generate a bulk code after compilation with the proper compiler settings.

paragraphs: Variable pointers and references to a base class type in C++ can also refer to objects of any derived classes of that type. This allows arrays and other kinds of containers to hold pointers to objects of differing types (references cannot be directly held in containers). This enables dynamic (run-time) polymorphism, where the referred objects can behave differently, depending on their (actual, derived) types.

paragraphs: C++ also provides the dynamic_cast operator, which allows code to safely attempt conversion of an object, via a base reference/pointer, to a more derived type: "downcasting". The "attempt" is necessary as often one does not know which derived type is referenced. ("Upcasting", conversion to a more general type, can always be checked/performed at compile-time via static_cast, as ancestral classes are specified in the derived class's interface, visible to all callers.) dynamic_cast relies on run-time type information (RTTI), metadata in the program that enables differentiating types and their relationships. If a dynamic_cast to a pointer fails, the result is the nullptr constant, whereas if the destination is a reference (which cannot be null), the cast throws an exception. Objects "known" to be of a certain derived type can be cast to that with static_cast, bypassing RTTI and the safe runtime type-checking of dynamic_cast, so this should be used only if the programmer is very confident the cast is, and will always be, valid.

paragraphs: Ordinarily, when a function in a derived class overrides a function in a base class, the function to call is determined by the type of the object. A given function is overridden when there exists no difference in the number or type of parameters between two or more definitions of that function. Hence, at compile time, it may not be possible to determine the type of the object and therefore the correct function to call, given only a base class pointer; the decision is therefore put off until runtime. This is called dynamic dispatch. Virtual member functions or "methods" allow the most specific implementation of the function to be called, according to the actual run-time type of the object. In C++ implementations, this is commonly done using virtual function tables. If the object type is known, this may be bypassed by prepending a fully qualified class name before the function call, but in general calls to virtual functions are resolved at run time.

paragraphs: In addition to standard member functions, operator overloads and destructors can be virtual. As a rule of thumb, if any function in the class is virtual, the destructor should be as well. As the type of an object at its creation is known at compile time, constructors, and by extension copy constructors, cannot be virtual. Nonetheless a situation may arise where a copy of an object needs to be created when a pointer to a derived object is passed as a pointer to a base object. In such a case, a common solution is to create a clone() (or similar) virtual function that creates and returns a copy of the derived class when called.

paragraphs: A member function can also be made "pure virtual" by appending it with = 0 after the closing parenthesis and before the semicolon. A class containing a pure virtual function is called an "abstract class". Objects cannot be created from an abstract class; they can only be derived from. Any derived class inherits the virtual function as pure and must provide a non-pure definition of it (and all other pure virtual functions) before objects of the derived class can be created. A program that attempts to create an object of a class with a pure virtual member function or inherited pure virtual member function is ill-formed.

paragraphs: C++ provides support for anonymous functions, also known as lambda expressions, with the following form:

paragraphs: If the lambda takes no parameters, the () can be omitted, that is,

paragraphs: Also, the return type of a lambda expression can be automatically inferred, if possible, e.g.:

paragraphs: [](int x, int y) { return x + y; } // inferred

paragraphs: [](int x, int y) -> int { return x + y; } // explicit

paragraphs: The [capture] list supports the definition of closures. Such lambda expressions are defined in the standard as syntactic sugar for an unnamed function object.

paragraphs: Exception handling is used to communicate the existence of a runtime problem or error from where it was detected to where the issue can be handled. It permits this to be done in a uniform manner and separately from the main code, while detecting all errors. Should an error occur, an exception is thrown (raised), which is then caught by the nearest suitable exception handler. The exception causes the current scope to be exited, and also each outer scope (propagation) until a suitable handler is found, calling in turn the destructors of any objects in these exited scopes. At the same time, an exception is presented as an object carrying the data about the detected problem.

paragraphs: Some C++ style guides, such as Google's, LLVM's, and Qt's forbid the usage of exceptions.

paragraphs: The exception-causing code is placed inside a try block. The exceptions are handled in separate catch blocks (the handlers); each try block can have multiple exception handlers, as it is visible in the example below.

paragraphs: It is also possible to raise exceptions purposefully, using the throw keyword; these exceptions are handled in the usual way. In some cases, exceptions cannot be used due to technical reasons. One such example is a critical component of an embedded system, where every operation must be guaranteed to complete within a specified amount of time. This cannot be determined with exceptions as no tools exist to determine the maximum time required for an exception to be handled.

paragraphs: Unlike signal handling, in which the handling function is called from the point of failure, exception handling exits the current scope before the catch block is entered, which may be located in the current function or any of the previous function calls currently on the stack.

paragraphs: The C++ standard consists of two parts: the core language and the standard library. C++ programmers expect the latter on every major implementation of C++; it includes aggregate types (vectors, lists, maps, sets, queues, stacks, arrays, tuples), algorithms (find, for_each, binary_search, random_shuffle, etc.), input/output facilities (iostream, for reading from and writing to the console and files), filesystem library, localisation support, smart pointers for automatic memory management, regular expression support, multi-threading library, atomics support (allowing a variable to be read or written to by at most one thread at a time without any external synchronisation), time utilities (measurement, getting current time, etc.), a system for converting error reporting that doesn't use C++ exceptions into C++ exceptions, a random number generator and a slightly modified version of the C standard library (to make it comply with the C++ type system).

paragraphs: A large part of the C++ library is based on the Standard Template Library (STL). Useful tools provided by the STL include containers as the collections of objects (such as vectors and lists), iterators that provide array-like access to containers, and algorithms that perform operations such as searching and sorting.

paragraphs: Furthermore, (multi)maps (associative arrays) and (multi)sets are provided, all of which export compatible interfaces. Therefore, using templates it is possible to write generic algorithms that work with any container or on any sequence defined by iterators. As in C, the features of the library are accessed by using the #include directive to include a standard header. The C++ Standard Library provides 105 standard headers, of which 27 are deprecated.

paragraphs: The standard incorporates the STL that was originally designed by Alexander Stepanov, who experimented with generic algorithms and containers for many years. When he started with C++, he finally found a language where it was possible to create generic algorithms (e.g., STL sort) that perform even better than, for example, the C standard library qsort, thanks to C++ features like using inlining and compile-time binding instead of function pointers. The standard does not refer to it as "STL", as it is merely a part of the standard library, but the term is still widely used to distinguish it from the rest of the standard library (input/output streams, internationalization, diagnostics, the C library subset, etc.).

paragraphs: Most C++ compilers, and all major ones, provide a standards-conforming implementation of the C++ standard library.

paragraphs: The C++ Core Guidelines are an initiative led by Bjarne Stroustrup, the inventor of C++, and Herb Sutter, the convener and chair of the C++ ISO Working Group, to help programmers write 'Modern C++' by using best practices for the language standards C++14 and newer, and to help developers of compilers and static checking tools to create rules for catching bad programming practices.

paragraphs: The main aim is to efficiently and consistently write type and resource safe C++.

paragraphs: The Core Guidelines were announced in the opening keynote at CPPCon 2015.

paragraphs: The Guidelines are accompanied by the Guideline Support Library (GSL), a header only library of types and functions to implement the Core Guidelines and static checker tools for enforcing Guideline rules.

paragraphs: To give compiler vendors greater freedom, the C++ standards committee decided not to dictate the implementation of name mangling, exception handling, and other implementation-specific features. The downside of this decision is that object code produced by different compilers is expected to be incompatible. There were, however, attempts to standardize compilers for particular machines or operating systems (for example C++ ABI), though they seem to be largely abandoned now.

paragraphs: C++ is often considered to be a superset of C but this is not strictly true. Most C code can easily be made to compile correctly in C++ but there are a few differences that cause some valid C code to be invalid or behave differently in C++. For example, C allows implicit conversion from void* to other pointer types but C++ does not (for type safety reasons). Also, C++ defines many new keywords, such as new and class, which may be used as identifiers (for example, variable names) in a C program.

paragraphs: Some incompatibilities have been removed by the 1999 revision of the C standard (C99), which now supports C++ features such as line comments (//) and declarations mixed with code. On the other hand, C99 introduced a number of new features that C++ did not support that were incompatible or redundant in C++, such as variable-length arrays, native complex-number types (however, the std::complex class in the C++ standard library provides similar functionality, although not code-compatible), designated initializers, compound literals, and the restrict keyword. Some of the C99-introduced features were included in the subsequent version of the C++ standard, C++11 (out of those which were not redundant). However, the C++11 standard introduces new incompatibilities, such as disallowing assignment of a string literal to a character pointer, which remains valid C.

paragraphs: To intermix C and C++ code, any function declaration or definition that is to be called from/used both in C and C++ must be declared with C linkage by placing it within an extern "C" {/*...*/} block. Such a function may not rely on features depending on name mangling (i.e., function overloading).

paragraphs: Despite its widespread adoption, some notable programmers have criticized the C++ language, including Linus Torvalds, Richard Stallman, Joshua Bloch, Ken Thompson, and Donald Knuth.

paragraphs: One of the most often criticised points of C++ is its perceived complexity as a language, with the criticism that a large number of non-orthogonal features in practice necessitates restricting code to subset of C++, thus eschewing the readability benefits of common style and idioms. As expressed by Joshua Bloch: I think C++ was pushed well beyond its complexity threshold, and yet there are a lot of people programming it. But what you do is you force people to subset it. So almost every shop that I know of that uses C++ says, “Yes, we’re using C++ but we’re not doing multiple-implementation inheritance and we’re not using operator overloading.” There are just a bunch of features that you’re not going to use because the complexity of the resulting code is too high. And I don’t think it’s good when you have to start doing that. You lose this programmer portability where everyone can read everyone else’s code, which I think is such a good thing.

paragraphs: Donald Knuth (1993, commenting on pre-standardized C++), who said of Edsger Dijkstra that "to think of programming in C++" "would make him physically ill": The problem that I have with them today is that... C++ is too complicated. At the moment, it's impossible for me to write portable code that I believe would work on lots of different systems, unless I avoid all exotic features. Whenever the C++ language designers had two competing ideas as to how they should solve some problem, they said "OK, we'll do them both". So the language is too baroque for my taste.

paragraphs: Ken Thompson, who was a colleague of Stroustrup at Bell Labs, gives his assessment: It certainly has its good points. But by and large I think it’s a bad language. It does a lot of things half well and it’s just a garbage heap of ideas that are mutually exclusive. Everybody I know, whether it’s personal or corporate, selects a subset and these subsets are different. So it’s not a good language to transport an algorithm—to say, “I wrote it; here, take it.” It’s way too big, way too complex. And it’s obviously built by a committee.

paragraphs: Stroustrup campaigned for years and years and years, way beyond any sort of technical contributions he made to the language, to get it adopted and used. And he sort of ran all the standards committees with a whip and a chair. And he said “no” to no one. He put every feature in that language that ever existed. It wasn’t cleanly designed—it was just the union of everything that came along. And I think it suffered drastically from that.

paragraphs: However Brian Kernighan, also a colleague at Bell Labs, disputes this assessment: C++ has been enormously influential. ... Lots of people say C++ is too big and too complicated etc. etc. but in fact it is a very powerful language and pretty much everything that is in there is there for a really sound reason: it is not somebody doing random invention, it is actually people trying to solve real world problems. Now a lot of the programs that we take for granted today, that we just use, are C++ programs.

paragraphs: Stroustrup himself comments that C++ semantics are much cleaner than its syntax: "within C++, there is a much smaller and cleaner language struggling to get out".

paragraphs: Other complaints may include a lack of reflection or garbage collection, long compilation times, perceived feature creep, and verbose error messages, particularly from template metaprogramming.