Related
See the following example (PHP)
class Parent
{
protected $_property;
protected $_anotherP;
public function __construct($var)
{
$this->_property = $var;
$this->someMethod(); #Sets $_anotherP
}
protected function someMethod()
...
}
class Child extends Parent
{
protected $parent;
public function __construct($parent)
{
$this->parent = $parent;
}
private function myMethod()
{
return $this->parent->_anotherP; #Note this line
}
}
I am new to OOP and am a bit ignorant.
Here to access the parents property I am using an instance of that class, which seems wrong :S (no need of being i child then). Is there an easy way, so that i can sync the parent properties with the child properties and can directly access $this->anotherP without having to use $this->parent->anotherP ?
As your Child class is extending your Parent class, every properties and methods that are either public or protected in the Parent class will be seen by the Child class as if they were defined in the Child class -- and the other way arround.
When the Child class extends the Parent class, it can be seen as "Child is a Parent" -- which means the Child has the properties of the Parent, unless it redefines those another way.
(BTW, note that "parent" is a reserved keyword, in PHP -- which means you can't name a class with that name)
Here's a quick example of a "parent" class :
class MyParent {
protected $data;
public function __construct() {
$this->someMethodInTheParentClass();
}
protected function someMethodInTheParentClass() {
$this->data = 123456;
}
}
And it's "child" class :
class Child extends MyParent {
public function __construct() {
parent::__construct();
}
public function getData() {
return $this->data; // will return the $data property
// that's defined in the MyParent class
}
}
That can be used this way :
$a = new Child();
var_dump($a->getData());
And you'll get as output :
int 123456
Which means the $data property, defined in the MyParent class, and initialized in a method of that same MyParent class, is accessible by the Child class as if it were its own.
To make things simple : as the Child "is a" MyParent, it doesn't need to keep a pointer to... itself ;-)
This may save you a few hours of searching around.
Remember: Your Child class only inherits the properties DEFINED in the Parent class... So if you instantiate an object using Parent class and then populate it with data, then this data will NOT be available in your child class...
It's super obvious of course, but I'm guessing others may run into the same issue.
A super simple solution is not to extend anything, simply pass the $object of your parent class into your child class through a constructor. This way you have access to all the properties and methods of the object generated by parent class
Example
class child {
public parentObject;
public function __construct($parentObject) {
$this->parentObject = $parentObject;
}
}
If your $parentObject has a public property $name, then you can access it inside the child class with a function like:
public function print_name() {
echo $this->parentObject->name;
}
Is there any way to define abstract class properties in PHP?
abstract class Foo_Abstract {
abstract public $tablename;
}
class Foo extends Foo_Abstract {
//Foo must 'implement' $property
public $tablename = 'users';
}
There is no such thing as defining a property.
You can only declare properties because they are containers of data reserved in memory on initialization.
A function on the other hand can be declared (types, name, parameters) without being defined (function body missing) and thus, can be made abstract.
"Abstract" only indicates that something was declared but not defined and therefore before using it, you need to define it or it becomes useless.
No, there is no way to enforce that with the compiler, you'd have to use run-time checks (say, in the constructor) for the $tablename variable, e.g.:
class Foo_Abstract {
public final function __construct(/*whatever*/) {
if(!isset($this->tablename))
throw new LogicException(get_class($this) . ' must have a $tablename');
}
}
To enforce this for all derived classes of Foo_Abstract you would have to make Foo_Abstract's constructor final, preventing overriding.
You could declare an abstract getter instead:
abstract class Foo_Abstract {
abstract public function get_tablename();
}
class Foo extends Foo_Abstract {
protected $tablename = 'tablename';
public function get_tablename() {
return $this->tablename;
}
}
Depending on the context of the property, if I want to force declaration of an abstract class property in an extended class, I like to use a constant with the static keyword for the property in the abstract object constructor or setter/getter methods. You can optionally use final to prevent the method from being overridden in extended classes.
Example: https://3v4l.org/WH5Xl
abstract class AbstractFoo
{
public $bar;
final public function __construct()
{
$this->bar = static::BAR;
}
}
class Foo extends AbstractFoo
{
//const BAR = 'foobar'; //uncomment to prevent exception
}
$foo = new Foo();
//Fatal Error: Undefined class constant 'BAR'
However, the extended class overrides the parent class properties and methods if redefined.
For example; if a property is declared as protected in the parent and redefined as public in the extended class, the resulting property is public. Otherwise, if the property is declared private in the parent it will remain private and not available to the extended class.
http://www.php.net//manual/en/language.oop5.static.php
As stated above, there is no such exact definition.
I, however, use this simple workaround to force the child class to define the "abstract" property:
abstract class Father
{
public $name;
abstract protected function setName(); // now every child class must declare this
// function and thus declare the property
public function __construct()
{
$this->setName();
}
}
class Son extends Father
{
protected function setName()
{
$this->name = "son";
}
function __construct(){
parent::__construct();
}
}
The need for abstract properties can indicate design problems. While many of answers implement kind of Template method pattern and it works, it always looks kind of strange.
Let's take a look at the original example:
abstract class Foo_Abstract {
abstract public $tablename;
}
class Foo extends Foo_Abstract {
//Foo must 'implement' $property
public $tablename = 'users';
}
To mark something abstract is to indicate it a must-have thing. Well, a must-have value (in this case) is a required dependency, so it should be passed to the constructor during instantiation:
class Table
{
private $name;
public function __construct(string $name)
{
$this->name = $name;
}
public function name(): string
{
return $this->name;
}
}
Then if you actually want a more concrete named class you can inherit like so:
final class UsersTable extends Table
{
public function __construct()
{
parent::__construct('users');
}
}
This can be useful if you use DI container and have to pass different tables for different objects.
I've asked myself the same question today, and I'd like to add my two cents.
The reason we would like abstract properties is to make sure that subclasses define them and throw exceptions when they don't. In my specific case, I needed something that could work with statically.
Ideally I would like something like this:
abstract class A {
abstract protected static $prop;
}
class B extends A {
protected static $prop = 'B prop'; // $prop defined, B loads successfully
}
class C extends A {
// throws an exception when loading C for the first time because $prop
// is not defined.
}
I ended up with this implementation
abstract class A
{
// no $prop definition in A!
public static final function getProp()
{
return static::$prop;
}
}
class B extends A
{
protected static $prop = 'B prop';
}
class C extends A
{
}
As you can see, in A I don't define $prop, but I use it in a static getter. Therefore, the following code works
B::getProp();
// => 'B prop'
$b = new B();
$b->getProp();
// => 'B prop'
In C, on the other hand, I don't define $prop, so I get exceptions:
C::getProp();
// => Exception!
$c = new C();
$c->getProp();
// => Exception!
I must call the getProp() method to get the exception and I can't get it on class loading, but it is quite close to the desired behavior, at least in my case.
I define getProp() as final to avoid that some smart guy (aka myself in 6 months) is tempted to do
class D extends A {
public static function getProp() {
// really smart
}
}
D::getProp();
// => no exception...
As you could have found out by just testing your code:
Fatal error: Properties cannot be declared abstract in ... on line 3
No, there is not. Properties cannot be declared abstract in PHP.
However you can implement a getter/setter function abstract, this might be what you're looking for.
Properties aren't implemented (especially public properties), they just exist (or not):
$foo = new Foo;
$foo->publicProperty = 'Bar';
PHP 7 makes it quite a bit easier for making abstract "properties". Just as above, you will make them by creating abstract functions, but with PHP 7 you can define the return type for that function, which makes things a lot easier when you're building a base class that anyone can extend.
<?php
abstract class FooBase {
abstract public function FooProp(): string;
abstract public function BarProp(): BarClass;
public function foo() {
return $this->FooProp();
}
public function bar() {
return $this->BarProp()->name();
}
}
class BarClass {
public function name() {
return 'Bar!';
}
}
class FooClass extends FooBase {
public function FooProp(): string {
return 'Foo!';
}
public function BarProp(): BarClass {
// This would not work:
// return 'not working';
// But this will!
return new BarClass();
}
}
$test = new FooClass();
echo $test->foo() . PHP_EOL;
echo $test->bar() . PHP_EOL;
if tablename value will never change during the object's lifetime, following will be a simple yet safe implementation.
abstract class Foo_Abstract {
abstract protected function getTablename();
public function showTableName()
{
echo 'my table name is '.$this->getTablename();
}
}
class Foo extends Foo_Abstract {
//Foo must 'implement' getTablename()
protected function getTablename()
{
return 'users';
}
}
the key here is that the string value 'users' is specified and returned directly in getTablename() in child class implementation. The function mimics a "readonly" property.
This is fairly similar to a solution posted earlier on which uses an additional variable. I also like Marco's solution though it can be a bit more complicated.
Just define the property in the base class without assigning it a (default) value.
Getting the property value without redefining it with a default value or assigning it a value will throw an Error.
<?php
class Base {
protected string $name;
public function i_am() : string {
return $this->name;
}
}
class Wrong extends Base {
...
}
class Good extends Base {
protected string $name = 'Somebody';
}
$test = new Good();
echo $test->i_am(), '<br>'; // Will show "Nobody"
$test = new Wrong();
echo $test->i_am(), '<br>'; // Will throw an Error:
// Error: Typed property Base::$name must not be accessed before initialization in ....
?>
You can define a static property in an abstract class.
<?php
abstract class Foo {
private static $bar = "1234";
public static function func() {
echo self::$bar;
}
}
Foo::func(); // It will be printed 1234
Too late to answer the question, but you may use the difference between self and static as follows
<?php
class A { // Base Class
protected static $name = 'ClassA';
public static function getSelfName() {
return self::$name;
}
public static function getStaticName() {
return static::$name;
}
}
class B extends A {
protected static $name = 'ClassB';
}
echo A::getSelfName(); // ClassA
echo A::getStaticName(); // ClassA
echo B::getSelfName(); // ClassA
echo B::getStaticName(); // ClassB
I have a class called Foo with a constructor and I want to have one variable of it to be the name of the child class.
Example:
class Foo {
protected static $name;
protected static $something;
public function __construct( $something ) {
self::$something = $something;
self::$name = strtolower( __CLASS__ );
}
static public function hello() {
echo self::$name;
}
}
class Bar extends Foo {}
new Foo( 'lazy' );
Bar::hello();
The result will be always foo(), because the constructor is called only by new Foo().
So, what is the way? Is there a magic method called when a static method is invoked? Sorry for the bad explanation.
get_called_class() should be used instead of __CLASS__ magic constant.
http://php.net/manual/en/function.get-called-class.php
More info in the manual : late static binding
As #JesusTheHun said, first of all you'll want to change __CLASS__ to either get_called_class() or static::class (as of PHP 5.5). The string __CLASS__ gets read at compile-time, and will always refer to the class in which it's specified. Either of the other options will take effect at run-time.
However, by setting a static class variable in a constructor, you're changing that variable for all instances of the class (past and present) as well as changing it for any static calls. So whenever a instance of any class in the tree is instantiated, all future calls to hello() will display that class name, i.e.
<?php
class Foo {
protected static $name;
protected static $something;
public function __construct() {
self::$name = strtolower( static::class );
}
static public function hello() {
echo self::$name;
}
}
class Bar extends Foo {}
class Baz extends Foo {}
new Foo;
Bar::hello(); // "foo"
new Baz;
Bar::hello(); // "baz"
This seems like it probably isn't the behaviour you're looking for. If all you want is a static method that will echo the class name that called it, then you can use one of the methods above, e.g.
static public function hello() {
echo static::class;
}
Is there any way to define abstract class properties in PHP?
abstract class Foo_Abstract {
abstract public $tablename;
}
class Foo extends Foo_Abstract {
//Foo must 'implement' $property
public $tablename = 'users';
}
There is no such thing as defining a property.
You can only declare properties because they are containers of data reserved in memory on initialization.
A function on the other hand can be declared (types, name, parameters) without being defined (function body missing) and thus, can be made abstract.
"Abstract" only indicates that something was declared but not defined and therefore before using it, you need to define it or it becomes useless.
No, there is no way to enforce that with the compiler, you'd have to use run-time checks (say, in the constructor) for the $tablename variable, e.g.:
class Foo_Abstract {
public final function __construct(/*whatever*/) {
if(!isset($this->tablename))
throw new LogicException(get_class($this) . ' must have a $tablename');
}
}
To enforce this for all derived classes of Foo_Abstract you would have to make Foo_Abstract's constructor final, preventing overriding.
You could declare an abstract getter instead:
abstract class Foo_Abstract {
abstract public function get_tablename();
}
class Foo extends Foo_Abstract {
protected $tablename = 'tablename';
public function get_tablename() {
return $this->tablename;
}
}
Depending on the context of the property, if I want to force declaration of an abstract class property in an extended class, I like to use a constant with the static keyword for the property in the abstract object constructor or setter/getter methods. You can optionally use final to prevent the method from being overridden in extended classes.
Example: https://3v4l.org/WH5Xl
abstract class AbstractFoo
{
public $bar;
final public function __construct()
{
$this->bar = static::BAR;
}
}
class Foo extends AbstractFoo
{
//const BAR = 'foobar'; //uncomment to prevent exception
}
$foo = new Foo();
//Fatal Error: Undefined class constant 'BAR'
However, the extended class overrides the parent class properties and methods if redefined.
For example; if a property is declared as protected in the parent and redefined as public in the extended class, the resulting property is public. Otherwise, if the property is declared private in the parent it will remain private and not available to the extended class.
http://www.php.net//manual/en/language.oop5.static.php
As stated above, there is no such exact definition.
I, however, use this simple workaround to force the child class to define the "abstract" property:
abstract class Father
{
public $name;
abstract protected function setName(); // now every child class must declare this
// function and thus declare the property
public function __construct()
{
$this->setName();
}
}
class Son extends Father
{
protected function setName()
{
$this->name = "son";
}
function __construct(){
parent::__construct();
}
}
The need for abstract properties can indicate design problems. While many of answers implement kind of Template method pattern and it works, it always looks kind of strange.
Let's take a look at the original example:
abstract class Foo_Abstract {
abstract public $tablename;
}
class Foo extends Foo_Abstract {
//Foo must 'implement' $property
public $tablename = 'users';
}
To mark something abstract is to indicate it a must-have thing. Well, a must-have value (in this case) is a required dependency, so it should be passed to the constructor during instantiation:
class Table
{
private $name;
public function __construct(string $name)
{
$this->name = $name;
}
public function name(): string
{
return $this->name;
}
}
Then if you actually want a more concrete named class you can inherit like so:
final class UsersTable extends Table
{
public function __construct()
{
parent::__construct('users');
}
}
This can be useful if you use DI container and have to pass different tables for different objects.
I've asked myself the same question today, and I'd like to add my two cents.
The reason we would like abstract properties is to make sure that subclasses define them and throw exceptions when they don't. In my specific case, I needed something that could work with statically.
Ideally I would like something like this:
abstract class A {
abstract protected static $prop;
}
class B extends A {
protected static $prop = 'B prop'; // $prop defined, B loads successfully
}
class C extends A {
// throws an exception when loading C for the first time because $prop
// is not defined.
}
I ended up with this implementation
abstract class A
{
// no $prop definition in A!
public static final function getProp()
{
return static::$prop;
}
}
class B extends A
{
protected static $prop = 'B prop';
}
class C extends A
{
}
As you can see, in A I don't define $prop, but I use it in a static getter. Therefore, the following code works
B::getProp();
// => 'B prop'
$b = new B();
$b->getProp();
// => 'B prop'
In C, on the other hand, I don't define $prop, so I get exceptions:
C::getProp();
// => Exception!
$c = new C();
$c->getProp();
// => Exception!
I must call the getProp() method to get the exception and I can't get it on class loading, but it is quite close to the desired behavior, at least in my case.
I define getProp() as final to avoid that some smart guy (aka myself in 6 months) is tempted to do
class D extends A {
public static function getProp() {
// really smart
}
}
D::getProp();
// => no exception...
As you could have found out by just testing your code:
Fatal error: Properties cannot be declared abstract in ... on line 3
No, there is not. Properties cannot be declared abstract in PHP.
However you can implement a getter/setter function abstract, this might be what you're looking for.
Properties aren't implemented (especially public properties), they just exist (or not):
$foo = new Foo;
$foo->publicProperty = 'Bar';
PHP 7 makes it quite a bit easier for making abstract "properties". Just as above, you will make them by creating abstract functions, but with PHP 7 you can define the return type for that function, which makes things a lot easier when you're building a base class that anyone can extend.
<?php
abstract class FooBase {
abstract public function FooProp(): string;
abstract public function BarProp(): BarClass;
public function foo() {
return $this->FooProp();
}
public function bar() {
return $this->BarProp()->name();
}
}
class BarClass {
public function name() {
return 'Bar!';
}
}
class FooClass extends FooBase {
public function FooProp(): string {
return 'Foo!';
}
public function BarProp(): BarClass {
// This would not work:
// return 'not working';
// But this will!
return new BarClass();
}
}
$test = new FooClass();
echo $test->foo() . PHP_EOL;
echo $test->bar() . PHP_EOL;
if tablename value will never change during the object's lifetime, following will be a simple yet safe implementation.
abstract class Foo_Abstract {
abstract protected function getTablename();
public function showTableName()
{
echo 'my table name is '.$this->getTablename();
}
}
class Foo extends Foo_Abstract {
//Foo must 'implement' getTablename()
protected function getTablename()
{
return 'users';
}
}
the key here is that the string value 'users' is specified and returned directly in getTablename() in child class implementation. The function mimics a "readonly" property.
This is fairly similar to a solution posted earlier on which uses an additional variable. I also like Marco's solution though it can be a bit more complicated.
Just define the property in the base class without assigning it a (default) value.
Getting the property value without redefining it with a default value or assigning it a value will throw an Error.
<?php
class Base {
protected string $name;
public function i_am() : string {
return $this->name;
}
}
class Wrong extends Base {
...
}
class Good extends Base {
protected string $name = 'Somebody';
}
$test = new Good();
echo $test->i_am(), '<br>'; // Will show "Nobody"
$test = new Wrong();
echo $test->i_am(), '<br>'; // Will throw an Error:
// Error: Typed property Base::$name must not be accessed before initialization in ....
?>
You can define a static property in an abstract class.
<?php
abstract class Foo {
private static $bar = "1234";
public static function func() {
echo self::$bar;
}
}
Foo::func(); // It will be printed 1234
Too late to answer the question, but you may use the difference between self and static as follows
<?php
class A { // Base Class
protected static $name = 'ClassA';
public static function getSelfName() {
return self::$name;
}
public static function getStaticName() {
return static::$name;
}
}
class B extends A {
protected static $name = 'ClassB';
}
echo A::getSelfName(); // ClassA
echo A::getStaticName(); // ClassA
echo B::getSelfName(); // ClassA
echo B::getStaticName(); // ClassB
See the following example (PHP)
class Parent
{
protected $_property;
protected $_anotherP;
public function __construct($var)
{
$this->_property = $var;
$this->someMethod(); #Sets $_anotherP
}
protected function someMethod()
...
}
class Child extends Parent
{
protected $parent;
public function __construct($parent)
{
$this->parent = $parent;
}
private function myMethod()
{
return $this->parent->_anotherP; #Note this line
}
}
I am new to OOP and am a bit ignorant.
Here to access the parents property I am using an instance of that class, which seems wrong :S (no need of being i child then). Is there an easy way, so that i can sync the parent properties with the child properties and can directly access $this->anotherP without having to use $this->parent->anotherP ?
As your Child class is extending your Parent class, every properties and methods that are either public or protected in the Parent class will be seen by the Child class as if they were defined in the Child class -- and the other way arround.
When the Child class extends the Parent class, it can be seen as "Child is a Parent" -- which means the Child has the properties of the Parent, unless it redefines those another way.
(BTW, note that "parent" is a reserved keyword, in PHP -- which means you can't name a class with that name)
Here's a quick example of a "parent" class :
class MyParent {
protected $data;
public function __construct() {
$this->someMethodInTheParentClass();
}
protected function someMethodInTheParentClass() {
$this->data = 123456;
}
}
And it's "child" class :
class Child extends MyParent {
public function __construct() {
parent::__construct();
}
public function getData() {
return $this->data; // will return the $data property
// that's defined in the MyParent class
}
}
That can be used this way :
$a = new Child();
var_dump($a->getData());
And you'll get as output :
int 123456
Which means the $data property, defined in the MyParent class, and initialized in a method of that same MyParent class, is accessible by the Child class as if it were its own.
To make things simple : as the Child "is a" MyParent, it doesn't need to keep a pointer to... itself ;-)
This may save you a few hours of searching around.
Remember: Your Child class only inherits the properties DEFINED in the Parent class... So if you instantiate an object using Parent class and then populate it with data, then this data will NOT be available in your child class...
It's super obvious of course, but I'm guessing others may run into the same issue.
A super simple solution is not to extend anything, simply pass the $object of your parent class into your child class through a constructor. This way you have access to all the properties and methods of the object generated by parent class
Example
class child {
public parentObject;
public function __construct($parentObject) {
$this->parentObject = $parentObject;
}
}
If your $parentObject has a public property $name, then you can access it inside the child class with a function like:
public function print_name() {
echo $this->parentObject->name;
}