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Абстрактные классы Static Keyword

Last updated: Sat, 27 Jan 2007

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# Константы в объектах

Константы также могут быть объявлены и в пределах одного класса. Отличие переменных и констант состоит в том, что при объявлении последних или при обращении к ним не используется символ `$`. Как и Разд. Static Keyword свойства и методы, значения констант, объявленных внутри класса, не могут быть получены через переменную, содержащую экземпляр этого класса.

Пример 19-17. Объявление и использование константы

`<?php
class MyClass {
const constant = 'значение константы';
function showConstant() {
echo self::constant . "\n";
}
}
echo MyClass::constant . "\n";
$class = new MyClass();
$class->showConstant();
/* echo $class::constant; - такое использование недопустимо */
?>`

add a note User Contributed Notes
Константы в объектах
Maikel
04-Mar-2007 07:44
`One way to access class constants from an instance is using the __get method, example:
<?php
class MyClass
{
const MY_CONSTANT = "Constant class";
public static function __get($name)
{
if(defined("self::$name"))
{
return constant("self::$name");
}
trigger_error ("Constant $name isn't defined");
}
}
$instance = new MyClass();
echo $instance->MY_CONSTANT; //it works!!
?>`
php at blindchaos dot net
13-Dec-2006 10:46
`I was having problems accessing my constants from an extended class in my abstract class, i.e.:
abstract class a {
public function showConst() {
echo self::foo;
}
}
class b extends a {
const foo = 'something';
}
I found a way to work around this is to switch self::foo with eval("return " . get_class($this) . "::foo;")
hope this helps someone.`
michikono at symbol gmail dot com
07-Dec-2006 08:58
`In realizing it is impossible to create dynamic constants, I opted for a "read only" constants class.
<?php
abstract class aClassConstant {
/**
* Setting is not permitted.
*
* @param string constant name
* @param mixed new value
* @return void
* @throws Exception
*/
final function __set($member, $value) {
throw new Exception('You cannot set a constant.');
}
/**
* Get the value of the constant
*
* @param string constant name
* @return void
*/
final function __get($member) {
return $this->$member;
}
}
?>
The class would be extended by another class that would compartmentalize the purpose of the constants. Thus, for example, you would extend the class with a DbConstant class for managing database related constants, that might look like this:
<?php
/**
* Constants that deal only with the database
*/
class DbConstant extends aClassConstant {
protected $host = 'localhost';
protected $user = 'user';
protected $password = 'pass';
protected $database = 'db';
protected $time;
/**
* Constructor. This is so fully dynamic values can be set. This can be skipped and the values can be directly assigned for non dynamic values as shown above.
*
* @return void
*/
function __construct() {
$this->time = time() + 1; // dynamic assignment
}
}
?>
You would use the class like thus:
<?php
$dbConstant = new DbConstant();
echo $dbConstant->host;
?>
The following would cause an exception:
<?php
$dbConstant = new DbConstant();
$dbConstant->host = '127.0.0.1'; // EXCEPTION
?>
It's not pretty, nor ideal, but at least you don't pollute the global name space with long winded global names and it is relatively elegant.
Variables must be *protected*, not public. Public variables will bypass the __get and __set methods!! This class is, by design, not meant to be extended much further than one level, as it is really meant to only contain constants. By keeping the constant definition class seperate from the rest of your classes (if you are calling this from a class), you minimize the possibility of accidental variable assignment.
Managing this instance may be a slight pain that requires either caching a copy of the instance in a class variable, or using the factory pattern. Unfortunately, static methods can't detect the correct class name when the parent name is used during the call (e.g., DbConstant::instance()). Thus there is no elegant, inheriting solution to that problem. Thus, it is easier to simply manage a single instance that is declared using conventional notation (e.g., new DbConstant...).
- Michi Kono`
gt at realvertex.com
15-Nov-2006 10:20
`If you would like to generate a constant names dynamically, then one could use the functions defined() and constant():
class CONFIG {
const MYCONST1 = 'Hello ';
const MYCONST2 = 'World';
public static function get($_name) {
if(defined("self::$_name"))
return constant("self::$_name");
throw new Exception('Constant ' . $_name . ' is not defined');
}
}
$vars = array(1,2);
try {
foreach($vars as $const_suffix) {
echo CONFIG::get('MYCONST' . $const_suffix);
}
} catch (Exception $e) {
$msg = $e->getMessage();
echo $msg;
}
Output will print:
Hello World`
kevin at metalaxe dot com
10-Nov-2006 01:32
`In response to anon on 31-May-2006 02:03:
If you can define a global constant based on the return of a function, please explain why you do not see it justifiable that a class constant should be able to be assigned in the same manner.
This affects ALL members of a class, not just constants and, for the sake of class constants, I see this as quite severe limitation to PHP. Take the following class for example:
<?php
class parser
{
private $magic_quotes = false;
public function __construct()
{
$this->magic_quotes = (bool)get_magic_quotes_gpc();
}
public my_stripslashes( $str )
{
if( $this->magic_quotes )
{
$str = stripslashes( $str );
}
return $str;
}
}
?>
Wouldn't this be much cleaner and easier to impliment like this?
<?php
class parser
{
const magic_quotes = (bool)get_magic_quotes_gpc();
public my_stripslashes( $str )
{
if( self::magic_quotes_gpc )
{
$str = stripslashes( $str );
}
return $str;
}
}
?>
In the second iteration, there isn't a need to scour through the script to find where magic_quotes is assigned a value and there is no confusion as to EXACTLY what magic_quotes is. More so, there is no worry that some silly end user will be able to change the value of magic_quotes in any of their "modifications" to your code as constants are a read only property. Personally I find this practical for script operation., but completely impossible in PHP.`
lucas dot s at ono dot com
06-Nov-2006 06:09
`In the same way "define()" can be used to create a GLOBAL constant that can be assigned as the value of a CLASS constant (like anonymous (31-May-2006 10:03) noted a few posts back), MAGIC constants (__LINE__, __FILE__, etc.) can also be assigned to a CLASS constant :-) Note that an instance of ReflectionClass can be used to obtain the exact same info that magic constants can offer you...
<?php
class MyClass
{
const FILE_I_AM_IN = __FILE__;
}
echo MyClass::FILE_I_AM_IN;
?>
This outputs the file the class definition is located in, as expected.
Notes on the other magic constants:
__LINE__ = does output the correct line but... is of course completely useless...
__FUNCTION__ = does not output anything.
__METHOD__ = outputs the class name!`
webmaster at chaosonline dot de
24-Sep-2006 04:57
`Since constants of a child class are not accessible from the parent class via self::CONST and there is no special keyword to access the constant (like this::CONST), i use private static variables and these two methods to make them read-only accessible from object's parent/child classes as well as statically from outside:
<?php
class b extends a {
private static $CONST = 'any value';
public static function getConstFromOutside($const) {
return self::$$const;
}
protected function getConst($const) {
return self::$$const;
}
}
?>
With those methods in the child class, you are now able to read the variables from the parent or child class:
<?php
class a {
private function readConst() {
return $this->getConst('CONST');
}
abstract public static function getConstFromOutside($const);
abstract protected function getConst($const);
}
?>
From outside of the object:
<?php
echo b::getConstFromOutside('CONST');
?>
You maybe want to put the methods into an interface.
However, class b's attribute $CONST is not a constant, so it is changeable by methods inside of class b, but it works for me and in my opinion, it is better than using real constants and accessing them by calling with eval:
<?php
protected function getConst($const) {
eval('$value = '.get_class($this).'::'.$const.';');
return $value;
}
?>`
sw at knip dot pol dot lublin dot pl
05-Jul-2006 12:14
`It might be obvious,
but I noticed that you can't define an array as a class constant.
Insteed you can define AND initialize an static array variable:
<?php
class AClass {
const an_array = Array (1,2,3,4);
//this WILL NOT work
// and will throw Fatal Error:
//Fatal error: Arrays are not allowed in class constants in...
public static $an_array = Array (1,2,3,4);
//this WILL work
//however, you have no guarantee that it will not be modified outside your class
}
?>`
31-May-2006 02:03
`In addition to what "tobias_demuth at web dot de" wrote:
Assigning the return value of a function to a constant does not work. Thus you may assign the return value of a function to a global constant defintion using "define()" and assign this global constant to the class constant.
The following example works as expected.
<?php
define("MYTIME", time());
class test {
const time = MYTIME;
}
print test::time;
?>
Will output the current timestamp. Whatsoever: IMHO this is "bad style" and so I suggest NOT to use this as "workaround".`
awbacker at gmail dot com
31-Mar-2006 07:15
`"Lest anyone think this is somehow an omission in PHP, there is simply no point to having a protected or private constant. Access specifiers identify who has the right to *change* members, not who has the right to read them"
I do see this as an omission. They are not only access modifiers, but they limit visibility as well. As it is, I can not make a constant that is private to my class, which I see as a problem. I would settle for multiple modifiers like private const $var = 'me'; but that is not allowed either.`
spiritus.canis at gmail dot com
14-Dec-2005 08:12
`RE: mail at erwindoornbos dot nl
Sure, that'll work, but you'll have the same constant defined for your entire application. Using class constants allows you to re-use the name of a constant while 1) holding a different value in different classes, and 2) giving you the ability to reference them from a static context.`
mail at erwindoornbos dot nl
12-Dec-2005 06:21
`<?php
define('SOMETHING', 'Foo bar');
class something
{
function getsomething()
{
echo SOMETHING;
}
}
$class = new something();
$class->getsomething();
?>
Works for me! This prints "Foo bar" without any warnings :)`
17-Jun-2005 09:29
`It's important to note that constants cannot be overridden by an extended class, if you with to use them in virtual functions. For example :
<?php
class abc
{
const avar = "abc's";
function show()
{
echo self::avar . "\r\n";
}
};
class def extends abc
{
const avar = "def's";
function showmore ()
{
echo self::avar . "\r\n";
$this->show();
}
};
$bob = new def();
$bob->showmore();
?>
Will display:
def's
abc's
However, if you use variables instead the output is different, such as:
<?php
class abc
{
protected $avar = "abc's";
function show()
{
echo $this->avar . "\r\n";
}
};
class def extends abc
{
protected $avar = "def's";
function showmore ()
{
echo $this->avar . "\r\n";
$this->show();
}
};
$bob = new def();
$bob->showmore();
?>
Will output:
def's
def's`
esad at 25novembar dot com
25-Apr-2005 01:03
`Refering to caliban at darklock dot com's article:
The whole idea of visibility is implementing the concept of data hiding and encapsulation. This means exposing as little as possible of the class variables/methods, in order to maintain loose coupling. If you reference all your variables in your class directly, you've probably missed the point of OOP.
If the variable visibility is set to private it shouldn't be readable outside the class (performing tricks to read it is pointless, if you want to read something, make it public, it's your code). This is not used to obfuscate/hide a variable from someone but to enforce good coding practice of maintaining the loose coupling between objects.
http://c2.com/cgi/wiki?CouplingAndCohesion`
tobias_demuth at web dot de
24-Apr-2005 10:39
`Please note, that it is not possible to initialize an object's constant with a not-constant value. Something like this won't work:
<?php
class Testing {
const TIME = time();
// Some useful code
}
echo Testing::TIME;
?>
It will break with: Parse error: syntax error, unexpected '(', expecting ',' or ';' in path/to/file on line anylinenumber
Hope this will help somebody...`
douglass_davis at earthlink dot net
03-Feb-2005 04:34
`Re: caliban at darklock dot com
most people are not going to do all of this:
<?php
if(isset($y["@$classname@$b"]))
echo "\"$b\" is private: {$y["@$classname@$b"]}<br/>";
?>
to read an object variable.
My point is: what you said is true, however access specifiers do have an effect on who gets to read the variables when you are not trying to bypass encapsulation:
<?php
class Foo
{
private $varname=2;
}
$obj=new Foo();
echo $obj->varname; // accessing in the usual way doesn't work
?>
So: const gives you a constant that is public in terms of reading them the usual way. A private const would mean you could not read the variable using the 2nd method above. Not to say it's an "omission in PHP," but, realize that there would be some value added in allowing consts to be made private.`
pcarmody at c-spanarchives dot org
18-Jan-2005 06:54
`It should be noted that you cannot use the return from a function to assign a value to a class constant:
<?php
class MyClass {
const good = "blah";
const bad = str("blah", 0, 3); // causes a parse error
}
?>`
caliban at darklock dot com
15-Dec-2004 10:55
`Lest anyone think this is somehow an omission in PHP, there is simply no point to having a protected or private constant. Access specifiers identify who has the right to *change* members, not who has the right to read them:
<?php
// define a test class
class Test
{
public static $open=2;
protected static $var=1;
private static $secret=3;
}
$classname="Test";
// reflect class information
$x=new ReflectionClass($classname);
$y=array();
foreach($x->GetStaticProperties() as $k=>$v)
$y[str_replace(chr(0),"@",$k)]=$v;
// define the variables to search for
$a=array("open","var","secret","nothing");
foreach($a as $b)
{
if(isset($y["$b"]))
echo "\"$b\" is public: {$y["$b"]}<br/>";
elseif(isset($y["@*@$b"]))
echo "\"$b\" is protected: {$y["@*@$b"]}<br/>";
elseif(isset($y["@$classname@$b"]))
echo "\"$b\" is private: {$y["@$classname@$b"]}<br/>";
else
echo "\"$b\" is not a static member of $classname<br/>";
}
?>
As you can see from the results of this code, the protected and private static members of Test are still visible if you know where to look. The protection and privacy are applicable only on writing, not reading -- and since nobody can write to a constant at all, assigning an access specifier to it is just redundant.`
pezskwerl at gmail dot com
08-Dec-2004 09:02
`Unlike static members, constants always have public visibility, so trying to set a constant's visibility won't work, such as in this example:
<?php
class MyClass {
protected static $nonConstant = "this will work";
protected const constant = "this won't work";
}
?>`
add a note
Абстрактные классы Static Keyword

Last updated: Sat, 27 Jan 2007
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