Are there pure virtual functions in PHP like with C++

Question

I would have thought lots of people would have wondered whether this is possible but I can't find any duplicate questions... do correct me.

I just want to know whether PHP offers pure virtual functions. I want the following

class Parent {
   // no implementation given
   public function foo() {
      // nothing 
   }
}

class Child extends Parent {
   public function foo() {
      // implementation of foo goes here
   }
}

Thanks very much.

score 21 · Accepted Answer · answered Sep 29 '11 at 17:03

21

You can create abstract functions, but you need to declare the parent class as abstract, too:

abstract class Parent {
   // no implementation given
   abstract public function foo();
}

class Child extends Parent {
   public function foo() {
      // implementation of foo goes here
   }
}

answered Sep 29 '11 at 17:03

daiscog

11,441
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1

A pure virtual function requires that is defined in a sub-class. If not an error should occur. Will an error occur if foo() is not defined. This is key. – Jul 14 '12 at 19:34
Yes, if you extend `Parent` but do not define `foo` you will get an error (unless your subclass is also abstract, in which case it cannot be instantiated and any non-abstract subclasses must define `foo` themselves). For more information, see [the PHP manual](http://php.net/manual/en/language.oop5.abstract.php), which states that "all methods marked abstract in the parent's class declaration must be defined by the child". – daiscog Jul 16 '12 at 16:38
And when a class is declared `abstract` does it mean all it's functions must be `abstract` (that is, `pure virtual`), or you may create virtual and final functions (I mean functions not prepended with the `abstract` keyword)? – Silidrone Jan 14 '21 at 17:47
@Silidrone An abstract class may have abstract or concrete functions. See [the docs](https://www.php.net/manual/en/language.oop5.abstract.php) for more details. – daiscog Jan 15 '21 at 08:43

score 4 · Answer 2 · answered Sep 29 '11 at 17:02

4

Declare the method as abstract in the Parent class:

abstract public function foo();

answered Sep 29 '11 at 17:02

stivlo

83,644
31
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199

score 3 · Answer 3 · answered Sep 29 '11 at 17:04

3

There are abstract classes!

abstract class Parent {
   // no implementation given
   abstract public function foo();
   }
}

class Child extends Parent {
   public function foo() {
      // implementation of foo goes here
   }
}

answered Sep 29 '11 at 17:04

Davide

2,329
1
14
12

score 1 · Answer 4 · answered Sep 29 '11 at 17:01

1

Yes, that type of solution is possible, it's called polymorphism, you can do it without declaring an abstract class or an interface.

answered Sep 29 '11 at 17:01

Madara's Ghost

172,118
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Creating an empty method without declaring the method as abstract (like in the question) does not reflect C++ pure-virtual method behaviour (which is the same as abstract methods in PHP). – daiscog Sep 29 '11 at 17:07
1

This is the best answer in the world for anything ever. I hope that one day when my kids grow up I can show them this answer and they can be impressed as much as I am right now. Generations of programmers are better because of it, you probably single handedly saved millions of lives here - well done sir, well done. Have a +1. – Benjamin Gruenbaum Feb 13 '15 at 18:15

Are there pure virtual functions in PHP like with C++

4 Answers4

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