Traits class

In computer programming, a traits class is a class template used to associate information or behaviour to a compile-time entity, typically a data type or a constant, without modifying the existing entity. In the C++ programming language and PHP programming language, this is normally achieved by defining a primary class template, and creating explicit or partial specializations for the relevant types.

Usage

It is used in Standard Template Library and the C++ standard library to support generic container classes. The technique is used extensively in the Boost TypeTraits library.

PHP uses this in some other way. Since version 5.4.0 it allows you to specify templates that are not as dynamic as C++ in using different data types but they gives you the ability to "inherit" from more than one (trait-)class.

Examples

PHP

Since version 5.4.0 PHP gives you traits. This example uses one template class (trait) to enhance another class:

// the template
trait TSingleton {
  private static $_instance = null;
 
  public static getInstance() {
    if (null === static::$_instance)
    {
      static::$_instance = new static();
    }
 
    return static::$_instance;
  }
}
 
class FrontController {
  use TSingleton;
}
 
// can even be used in already extended classes
class WebSite extends SomeClass {
  use TSingleton;
}

This gives you the power to "inherit" multiple functionalities:

trait TBounding {
  public $x, $y, $width, $height;
}
 
trait TMoveable {
  public function moveTo($x, $y) {
    // ...
  }
}
 
trait TResizeable {
  public function resize($newWidth, $newHeight) {
    // ...
  }
}
 
class Rectangle {
  use TBounding, TMoveable, TResizeable;
 
  function fillColor($color) {
    // ...
  }
}

External links

• Generic Programming Techniques from Boost
• iterator_traits<Iterator> from SGI
• Traits: a new and useful template technique, Nathan C. Myers, C++ Report, June 1995 issue
• Traits - PHP Documentation