Within a class, you can have dynamic or object variables and methods, where the code and the values associated with it are applied to individual objects. They're far and away the majority of your variables / methods - in yesterday's public transport service exercise, each train or bus had a potentially different start time, for example.
But just occasionally, you need a static or class variable or method, which applies to the class as a whole.
In PHP, you can define a static variable within your class using the
static keyword (and you should initialise it as you do so):
static $id_service = 0;
You can then reference that variable within both static and dynamic methodsusing the notation
self::...:
self::$id_service++;
You can define static or class functions too, but they can only access static variables - they do NOT have any
$this-> access:
static function next_id() {
return self::$id_service + 1;
}
Within the
calling code, you access the static method using the notation
class::method:
print ("The next service will have id ". service::next_id() ."\n");
There is a complete example
[here] from which the above lines of code have been "Cherry Picked"
When should you use static members?
Very rarely indeed!! There's a great temptation to use them to hold a count of the number of objects you have of a type, but that approach can fall over on you if you want to create a number of different groups - you should hold them in arrays and count the arrays if that's what you need to do.
My example for this post uses a static variable to assign a unique ascending ID number to each object of a type; that's subtley different, and a good use of statics. |
(written 2010-03-16)
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
H108 - Objects in PHP [67] Object Oriented Programming in PHP - (2004-09-29)
[124] PHP v Java - (2004-11-20)
[205] PHP5 lets you say no - (2005-02-07)
[343] Should I use structured or object oriented? - (2005-06-10)
[421] Don't repeat code - use loops or functions - (2005-08-21)
[485] North, Norther and Northest - PHP 5 Objects - (2005-11-04)
[656] Think about your design even if you don't use full UML - (2006-03-24)
[720] Planning a hotel refurb - an example of a Gant chart in PHP - (2006-05-14)
[836] Build on what you already have with OO - (2006-08-17)
[1027] Cue the music, I'm happy. - (2007-01-09)
[1153] Object Oriented Model - a summary of changes from PHP4 to PHP5 - (2007-04-18)
[1217] What are factory and singleton classes? - (2007-06-04)
[1535] OO PHP demonstration - comparing objects and more - (2008-02-08)
[1682] Accounts in PHP - an OO demo - (2008-06-19)
[1819] Calling base class constructors - (2008-10-03)
[1820] Sorting objects in PHP - (2008-10-04)
[1925] Introduction to Object Oriented Programming - (2008-12-06)
[2160] PHP - getclass v instanceof - (2009-05-07)
[2169] When should I use OO techniques? - (2009-05-11)
[2171] Cleaning up redundant objects - (2009-05-11)
[2172] PHP4 v PHP5 - Object Model Difference - (2009-05-11)
[2380] Object Oriented programming - a practical design example - (2009-08-27)
[2434] Abstract classes, Interfaces, PHP and Java - (2009-10-03)
[2435] Serialization - storing and reloading objects - (2009-10-04)
[2632] Shipping a test harness with your class in PHP - (2010-02-12)
[2641] Object Oriented Programming in PHP - (2010-02-19)
[2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
[2741] What is a factory? - (2010-04-26)
[2774] PHP - Object Oriented Design in use - (2010-05-21)
[2921] Does copying a variable duplicate the contents? - (2010-08-14)
[2922] Getting the OO design write - with PHP a example - (2010-08-14)
[3142] Private and Public - and things between - (2011-01-22)
[3210] Catchable fatal error in PHP ... How to catch, and alternative solutions such as JSON - (2011-03-22)
[3211] Computer Graphics in PHP - World (incoming data) to Pixel (screen) conversion - (2011-03-24)
[3607] Designing your application - using UML techniques - (2012-02-11)
[3608] Design Patterns - what are they? Why use them? - (2012-02-12)
[3609] How do classes relate to each other? Associated Classes - (2012-02-12)
[3840] Autoload in PHP - (2012-08-17)
[3841] Copying, duplicating, cloning an object in PHP - (2012-08-18)
[3843] Caching Design Patterns - (2012-08-20)
[3953] Objects in PHP - Revision - (2012-12-16)
[4057] stdClass in PHP - using an object rather than an associative array - (2013-04-02)
[4073] Learning about Object Orientation in PHP - a new set of examples - (2013-04-28)
[4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
[4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
[4626] Singleton design pattern - examples and uses - (2016-01-20)
[4627] Caching results in an object for efficiency - avoiding re-calculation - (2016-01-20)
[4628] Associative objects - one object within another. - (2016-01-20)
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Exception handling in PHPCar Parking in MelkshamAdding extensions to PHP Open Source applications - callbacksTcl - a great engineering languageStatic class members in PHP - a documented exampleHow to build a test harness into your PHPReaching out to the community - a good case for supportBlowing out the winter cobwebsChanging TimesRedirecting to your main domain for correct security keys