Objected-oriented programming conventions

PSR-4, namespaces, class naming, and other conventions.

A lot of the PHP code in Drupal is object oriented (OO), making use of PHP classes, interfaces, and traits (which are loosely referred to as "classes" in the rest of this topic). The following conventions and standards apply to this version of Drupal:

• Each class must be in its own file.
• Classes must be namespaced. If a module defines a class, the namespace must start with \Drupal\module_name. If it is defined by Drupal Core for use across many modules, the namespace should be \Drupal\Core or \Drupal\Component, with the exception of the global class \Drupal. See https://www.drupal.org/node/1353118 for more about namespaces.
• In order for the PSR-4-based class auto-loader to find the class, it must be located in a directory corresponding to the namespace. For module-defined classes, if the namespace is \Drupal\module_name\foo\bar, then the class goes under the main module directory in directory src/foo/bar. For Drupal-wide classes, if the namespace is \Drupal\Core\foo\bar, then it goes in directory core/lib/Drupal/Core/foo/bar. See https://www.drupal.org/node/2156625 for more information about PSR-4.
• Some classes have annotations added to their documentation headers. See the Annotation topic for more information.
• Standard plugin discovery requires particular namespaces and annotation for most plugin classes. See the Plugin API topic for more information.
• There are project-wide coding standards for OO code, including naming: https://www.drupal.org/node/608152
• Documentation standards for classes are covered on: https://www.drupal.org/coding-standards/docs#classes

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core/core.api.php, line 1483

Documentation landing page and topics, plus core library hooks.