i have become a bit confused about abstract class ! i have read more of the post written in stackoverflow and another website but i didn't understand ! so i took a look at my book again but i didn't understand it either . so please analyze the code below step by step :
thanks in advance
<?php
abstract class AbstractClass
{
abstract protected function getValue();
public function printOut() {
print $this->getValue();
}
}
class ConcreteClass1 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass1";
}
}
class ConcreteClass2 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass2";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
$class2 = new ConcreteClass2;
$class2->printOut();
?>
By definition
'An abstract class is a class that is declared abstract —it may or may
not include abstract methods. Abstract classes cannot be instantiated,
but they can be subclassed. An abstract method is a method that is
declared without an implementation'.
If defined an abstract class, you should extend that class with another.
In case of having abstract methods within the abstract class, you should write them in the child class in order to instantiate the child.
Related to the code, that is why when you instantiate the ConcreteClass, the getValue function is 'overwritten' to the pattern, while calling to the printOut method is from the father itself, because It is already written and not overwritten by the child. (See also that method was not abstract, that is why you can also use it from the father class)
Your code is right. Abstact class mean, when you can not make a instance of it. You can not do this:
$abstract = new AbstractClass();
Related
I have a code like following ---
class CartItem{
var $v;
function __construct(){
$this->f();
}
function f(){
echo 'In parent';
}
}
class m extends CartItem{
function f(){
echo 'In child';
}
}
new m();
Now when creating instance of m()... it doesn't have any constructor, so it is calling parent classes constructor. Now inside that a function f is called.
What I want is -
if class m() have defined function f()... is should call it instead of parent class's function f().
But anyway it is calling parent classes function, as it was called from parent's constructor, irrespective of child class/ context :(
You want to call in __construct() a method that is not defined in the class. This is a sign that the CartItem class is an abstract concept and you don't intend to instantiate it (because an instance of CartItem probably doesn't contain enough information or behaviour for your project).
An abstract concept is implemented using an abstract class that defines as much as it can and defines abstract methods to be implemented in the concrete classes that extend it. The method f() is such a method that cannot be defined in the abstract concept and has to be defined in each class that extend it:
abstract class CartItem
{
public function __construct()
{
$this->f();
}
abstract protected function f();
}
class m extends CartItem
{
protected function f()
{
// Implement behaviour specific to this class
}
}
This is actually a really interesting question.
so, as I understand it, you're asking (if this isnt right please say):
can you call a function of a class that's extending a parent?
yes, you can... sort of, if the method in the child is static.
Take this example (Ive not used it in the constructor for simplicity of example, but it will work there too):
class ClassA {
public function testMeAsWell() {
return ClassB::testMe();
}
}
class ClassB extends ClassA {
static function testMe() {
return 'do something';
}
}
$child = new ClassB();
echo $child->testMe();
// outputs 'do something'
$parent = new ClassA();
echo $parent->testMeAsWell();
// also outputs 'do something'
the reason this works needs more research, but as a guess I would say that because PHP is compiled, it will know about both classes at run-time and therefore will be able to figure out what we wanted it to do.
So, further on, you want to use variables. Yes you can, but they would have to be static as well.
working example
If I have an abstract class like this:
abstract class MyApp
{
public function init()
{
$this->stuff = $this->getStuff();
}
public function getStuff()
{
return new BlueStuff();
}
}
And then I have a class that extends from this abstract class like this:
class MyExtendedClass extends MyApp
{
public function init()
{
parent::init();
}
public function getStuff()
{
return new RedStuff();
}
}
If I do:
$myobj = new MyExtendedClass();
$myobj->init();
Why does the method getStuff from the child class get called? Isn't $this in the context of the abstract class? If so, shouldn't the method of the abstract class get called?
Thanks!
New answer
In PHP you can use subclasses as if all methods in the parent class that don't exist in the subclass have been copied to the subclass. So your example would be the same as:
class MyExtendedClass extends MyApp {
public function init() {
$this->stuff = $this->getStuff();
}
public function getStuff() {
return new RedStuff();
}
}
Just think of the subclass as having all code of the parent class and you're normally all right. There is one exception to this rule: properties. A private property of a class can only be accessed by that class, subclasses can't access the private properties of parent classes. In order to do that you need to change the private property into a protected property.
Original answer
Abstract classes in PHP are just like regular classes with one big difference: they can't get initiated. This means you can't do new AbstractClass();.
Since they work exactly like regular classes for everything else, this also is the case for extending classes. This means that PHP first tries to find the method in the initiated class, and only looks in the abstract classes if it doesn't exist.
So in your example this would mean that the getStuff() method from MyExtendedClass is called. Furthermore, this means you can leave out the init() method in MyExtendedClass.
Having the following class hierarchy:
class TheParent{
public function parse(){
$this->validate();
}
}
class TheChild extends TheParent{
private function validate(){
echo 'Valid!!';
}
}
$child= new TheChild();
$child->parse();
What is the sequence of steps in which this is going to work?
The problem is when I ran that code it gave the following error:
Fatal error: Call to private method TheChild::validate() from context 'TheParent' on line 4
Since TheChild inherits from TheParent shouldn't $this called in parse() be referring to the instance of $child, so validate() will be visible to parse()?
Note:
After doing some research I found that the solution to this problem would either make the validate() function protected according to this comment in the PHP manual, although I don't fully understand why it is working in this case.
The second solution is to create an abstract protected method validate() in the parent and override it in the child (which will be redundant) to the first solution as protected methods of a child can be accessed from the parent?!!
Can someone please explain how the inheritance works in this case?
Other posters already pointed out that the mehods need to be protected in order to access them.
I think you should change one more thing in your code. Your base class parent relies on a method that is defined in a child class. That is bad programming. Change your code like this:
abstract class TheParent{
public function parse(){
$this->validate();
}
abstract function validate();
}
class TheChild extends TheParent{
protected function validate(){
echo 'Valid!!';
}
}
$child= new TheChild();
$child->parse();
creating an abstract function ensures that the child class will definitely have the function validate because all abstract functions of an abstract class must be implemented for inheriting from such a class
Your idea of inheritence is correct, just not the visibility.
Protected can be used by the class and inherited and parent classes, private can only be used in the actual class it was defined.
Private can only be accessed by the class which defines, neither parent nor children classes.
Use protected instead:
class TheParent{
public function parse(){
$this->validate();
}
}
class TheChild extends TheParent{
protected function validate(){
echo 'Valid!!';
}
}
$child= new TheChild();
$child->parse();
FROM PHP DOC
Visibility from other objects
Objects of the same type will have access to each others private and protected members even though they are not the same instances. This is because the implementation specific details are already known when inside those objects.
Private can only be accessed by the class which defines or Same object type Example
class TheChild {
public function parse(TheChild $new) {
$this->validate();
$new->validate(); // <------------ Calling Private Method of $new
}
private function validate() {
echo 'Valid!!';
}
}
$child = new TheChild();
$child->parse(new TheChild());
Output
Valid!!Valid!!
What is an abstract class in PHP?
How can it be used?
An abstract class is a class that contains at least one abstract method, which is a method without any actual code in it, just the name and the parameters, and that has been marked as "abstract".
The purpose of this is to provide a kind of template to inherit from and to force the inheriting class to implement the abstract methods.
An abstract class thus is something between a regular class and a pure interface. Also interfaces are a special case of abstract classes where ALL methods are abstract.
See this section of the PHP manual for further reference.
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation. Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
1. Can not instantiate abstract class: Classes defined as abstract may not be instantiated, and any class that contains at least one abstract method must also be abstract.
Example below :
abstract class AbstractClass
{
abstract protected function getValue();
abstract protected function prefixValue($prefix);
public function printOut() {
echo "Hello how are you?";
}
}
$obj=new AbstractClass();
$obj->printOut();
//Fatal error: Cannot instantiate abstract class AbstractClass
2. Any class that contains at least one abstract method must also be abstract: Abstract class can have abstract and non-abstract methods, but it must contain at least one abstract method. If a class has at least one abstract method, then the class must be declared abstract.
Note: Traits support the use of abstract methods in order to impose requirements upon the exhibiting class.
Example below :
class Non_Abstract_Class
{
abstract protected function getValue();
public function printOut() {
echo "Hello how are you?";
}
}
$obj=new Non_Abstract_Class();
$obj->printOut();
//Fatal error: Class Non_Abstract_Class contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (Non_Abstract_Class::getValue)
3. An abstract method can not contain body: Methods defined as abstract simply declare the method's signature - they cannot define the implementation. But a non-abstract method can define the implementation.
abstract class AbstractClass
{
abstract protected function getValue(){
return "Hello how are you?";
}
public function printOut() {
echo $this->getValue() . "\n";
}
}
class ConcreteClass1 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass1";
}
public function prefixValue($prefix) {
return "{$prefix}ConcreteClass1";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
echo $class1->prefixValue('FOO_') ."\n";
//Fatal error: Abstract function AbstractClass::getValue() cannot contain body
4. When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child :If you inherit an abstract class you have to provide implementations to all the abstract methods in it.
abstract class AbstractClass
{
// Force Extending class to define this method
abstract protected function getValue();
// Common method
public function printOut() {
print $this->getValue() . "<br/>";
}
}
class ConcreteClass1 extends AbstractClass
{
public function printOut() {
echo "dhairya";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
//Fatal error: Class ConcreteClass1 contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (AbstractClass::getValue)
5. Same (or a less restricted) visibility:When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child; additionally, these methods must be defined with the same (or a less restricted) visibility. For example, if the abstract method is defined as protected, the function implementation must be defined as either protected or public, but not private.
Note that abstract method should not be private.
abstract class AbstractClass
{
abstract public function getValue();
abstract protected function prefixValue($prefix);
public function printOut() {
print $this->getValue();
}
}
class ConcreteClass1 extends AbstractClass
{
protected function getValue() {
return "ConcreteClass1";
}
public function prefixValue($prefix) {
return "{$prefix}ConcreteClass1";
}
}
$class1 = new ConcreteClass1;
$class1->printOut();
echo $class1->prefixValue('FOO_') ."<br/>";
//Fatal error: Access level to ConcreteClass1::getValue() must be public (as in class AbstractClass)
6. Signatures of the abstract methods must match:When inheriting from an abstract class, all methods marked abstract in the parent's class declaration must be defined by the child;the signatures of the methods must match, i.e. the type hints and the number of required arguments must be the same. For example, if the child class defines an optional argument, where the abstract method's signature does not, there is no conflict in the signature.
abstract class AbstractClass
{
abstract protected function prefixName($name);
}
class ConcreteClass extends AbstractClass
{
public function prefixName($name, $separator = ".") {
if ($name == "Pacman") {
$prefix = "Mr";
} elseif ($name == "Pacwoman") {
$prefix = "Mrs";
} else {
$prefix = "";
}
return "{$prefix}{$separator} {$name}";
}
}
$class = new ConcreteClass;
echo $class->prefixName("Pacman"), "<br/>";
echo $class->prefixName("Pacwoman"), "<br/>";
//output: Mr. Pacman
// Mrs. Pacwoman
7. Abstract class doesn't support multiple inheritance:Abstract class can extends another abstract class,Abstract class can provide the implementation of interface.But it doesn't support multiple inheritance.
interface MyInterface{
public function foo();
public function bar();
}
abstract class MyAbstract1{
abstract public function baz();
}
abstract class MyAbstract2 extends MyAbstract1 implements MyInterface{
public function foo(){ echo "foo"; }
public function bar(){ echo "bar"; }
public function baz(){ echo "baz"; }
}
class MyClass extends MyAbstract2{
}
$obj=new MyClass;
$obj->foo();
$obj->bar();
$obj->baz();
//output: foobarbaz
Note: Please note order or positioning of the classes in your code can affect the interpreter and can cause a Fatal error.
So, when using multiple levels of abstraction, be careful of the positioning of the classes within the source code.
below example will cause Fatal error: Class 'horse' not found
class cart extends horse {
public function get_breed() { return "Wood"; }
}
abstract class horse extends animal {
public function get_breed() { return "Jersey"; }
}
abstract class animal {
public abstract function get_breed();
}
$cart = new cart();
print($cart->get_breed());
An abstract class is a class that is
only partially implemented by the
programmer. It may contain one or more
abstract methods. An abstract method
is simply a function definition that
serves to tell the programmer that the
method must be implemented in a child
class.
There is good explanation of that here.
Abstract Class
1. Contains an abstract method
2. Cannot be directly initialized
3. Cannot create an object of abstract class
4. Only used for inheritance purposes
Abstract Method
1. Cannot contain a body
2. Cannot be defined as private
3. Child classes must define the methods declared in abstract class
Example Code:
abstract class A {
public function test1() {
echo 'Hello World';
}
abstract protected function f1();
abstract public function f2();
protected function test2(){
echo 'Hello World test';
}
}
class B extends A {
public $a = 'India';
public function f1() {
echo "F1 Method Call";
}
public function f2() {
echo "F2 Method Call";
}
}
$b = new B();
echo $b->test1() . "<br/>";
echo $b->a . "<br/>";
echo $b->test2() . "<br/>";
echo $b->f1() . "<br/>";
echo $b->f2() . "<br/>";
Output:
Hello World
India
Hello World test
F1 Method Call
F2 Method Call
Abstract Class contains only declare the method's signature, they can't define the implementation.
Abstraction class are defined using the keyword abstract .
Abstract Class is not possible to implement multiple inheritance.
Latest version of PHP 5 has introduces abstract classes and methods.
Classes defined as abstract , we are unable to create the object ( may not instantiated )
An abstract class is like the normal class it contains variables it contains protected variables functions it contains constructor only one thing is different it contains abstract method.
The abstract method means an empty method without definition so only one difference in abstract class we can not create an object of abstract class
Abstract must contains the abstract method and those methods must be defined in its inheriting class.
class absclass {
abstract public function fuc();
}
reports:
PHP Fatal error: Class absclass
contains 1 abstract method and must
therefore be declared abstract or
implement the remaining methods
(absclass::fuc)
I want to know what it means by implement the remaining methods,how?
See the chapter on Class Abstraction in the PHP manual:
PHP 5 introduces abstract classes and methods. Classes defined as abstract may not be instantiated, and any class that contains at least one abstract method must also be abstract. Methods defined as abstract simply declare the method's signature - they cannot define the implementation.
It means you either have to
abstract class absclass { // mark the entire class as abstract
abstract public function fuc();
}
or
class absclass {
public function fuc() { // implement the method body
// which means it won't be abstract anymore
};
}
I presume that remaining methods actually refers to the abstract methods you're trying to define (in this case, fuc()), since the non-abstract methods that might exist are okay anyway. It's probably an error message that could use a more precise wording: where it says remaining it could have said abstract.
The fix is pretty straightforward (that part of the error message is fine): you need to change this:
abstract public function fuc();
... into a proper implementation:
public function fuc(){
// Code comes here
}
... or, alternatively and depending your needs, make the whole class abstract:
abstract class absclass {
abstract public function fuc();
}
An abstract class cannot be directly instantiated, but it can contain both abstract and non-abstract methods.
If you extend an abstract class, you have to either implement all its abstract functions, or make the subclass abstract.
You cannot override a regular method and make it abstract, but you must (eventually) override all abstract methods and make them non-abstract.
<?php
abstract class Dog {
private $name = null;
private $gender = null;
public function __construct($name, $gender) {
$this->name = $name;
$this->gender = $gender;
}
public function getName() {return $this->name;}
public function setName($name) {$this->name = $name;}
public function getGender() {return $this->gender;}
public function setGender($gender) {$this->gender = $gender;}
abstract public function bark();
}
// non-abstract class inheritting from an abstract class - this one has to implement all inherited abstract methods.
class Daschund extends Dog {
public function bark() {
print "bowowwaoar" . PHP_EOL;
}
}
// this class causes a compilation error, because it fails to implement bark().
class BadDog extends Dog {
// boom! where's bark() ?
}
// this one succeeds in compiling,
// it's passing the buck of implementing it's inheritted abstract methods on to sub classes.
abstract class PassTheBuckDog extends Dog {
// no boom. only non-abstract subclasses have to bark().
}
$dog = new Daschund('Fred', 'male');
$dog->setGender('female');
print "name: " . $dog->getName() . PHP_EOL;
print "gender: ". $dog->getGender() . PHP_EOL;
$dog->bark();
?>
That program bombs with:
PHP Fatal error: Class BadDog
contains 1 abstract method and must
therefore be declared abstract or
implement the remaining methods
(Dog::bark)
If you comment out the BadDog class, then the output is:
name: Fred
gender: female
bowowwaoar
If you try to instantiate a Dog or a PassTheBuckDog directly, like this:
$wrong = new Dog('somma','it');
$bad = new PassTheBuckDog('phamous','monster');
..it bombs with:
PHP Fatal error: Cannot instantiate
abstract class Dog
or (if you comment out the $wrong line)
PHP Fatal error: Cannot instantiate
abstract class PassTheBuckDog
You can, however, call a static function of an abstract class:
abstract class Dog {
..
public static function getBarker($classname, $name, $gender) {
return new $classname($name, $gender);
}
..
}
..
$other_dog = Dog::getBarker('Daschund', 'Wilma', 'female');
$other_dog->bark();
That works just fine.
You're being slightly led astray by this error message. In this case, since it is within this class that fuc is being defined, it wouldn't really make sense to implement it in this class. What the error is trying to tell you is that a non-abstract class cannot have abstract methods. As soon as you put an abstract method in the definition of a class, you must also mark the class itself as abstract.
Abstract keywords are used to label classes or methods as patterns. It's similar to interfaces but can contain variables and implementations of methods.
There are a lot of misunderstandings concerning abstract classes. Here is an example of an abstract Dog class. If a developer wants to create some basic Dog class for other developers or for himself to extend he declares the class as abstract. You can't instantiate the Dog class directly (nobody can), but you can extend Dog by your own class. SmartDog extends Dog etc.
All methods that are declared to be abstract by the Dog class must be implemented manually in each class that extends Dog.
For example, the abstract class Dog has an abstract method Dog::Bark(). But all Dogs bark differently. So in each Dog-subclasses you must describe HOW that dog barks concretely, so you must define eg SmartDog::Bark().
It means that the proper of an abstract class is having at least one abstract method. So your class has either to implement the method (non abstract), or to be declared abstract.
I wanted to use an abstract method within a non-abstract class (normal class?) and found that I could wrap the method's contents in an 'if' statement with get_parent_class() like so:
if (get_parent_class($this) !== false) {
Or, in action (tested in a file on cmd line: php -f "abstract_method_normal_class_test.php"):
<?php
class dad {
function dad() {
if (get_parent_class($this) !== false) {
// implements some logic
echo "I'm " , get_class($this) , "\n";
} else {
echo "I'm " , get_class($this) , "\n";
}
}
}
class child extends dad {
function child() {
parent::dad();
}
}
$foo = new dad();
$bar = new child();
?>
Output:
I'm dad
I'm child
PHP get_parent_class() Documentation