Is there a way to redefine a type hint to a descendant class when extending an abstract class?

by

I wouldn’t expect so, as it can break type hinting contracts. Suppose a function foo took an AbstractFactory and was passed a SimpleFactory. 

function foo(AbstractFactory $maker) {
    $attr = new Attribute();
    $maker->update($attr, 42);
}
...
$packager=new SimpleFactory();
foo($packager);

foo calls update and passes an Attribute to the factory, which it should take because the AbstractFactory::update method signature promises it can take an Attribute. Bam! The SimpleFactory has an object of type it can’t handle properly.

class Attribute {}
class SimpleAttribute extends Attribute {
    public function spin() {...}
}
class SimpleFactory extends AbstractFactory {
    public function update(SimpleAttribute $attr, $data) {
        $attr->spin(); // This will fail when called from foo()
    }
}

In contract terminology, descendent classes must honor the contracts of their ancestors, which means function parameters can get more basal/less specified/offer a weaker contract and return values can be more derived/more specified/offer a stronger contract. The principle is described for Eiffel (arguably the most popular design-by-contract language) in “An Eiffel Tutorial: Inheritance and Contracts“. Weakening and strengthening of types are examples of contravariance and covariance, respectively.

In more theoretical terms, this is an example of LSP violation. No, not that LSP; the Liskov Substitution Principle, which states that objects of a subtype can be substituted for objects of a supertype. SimpleFactory is a subtype of AbstractFactory, and foo takes an AbstractFactory. Thus, according to LSP, foo should take a SimpleFactory. Doing so causes a “Call to undefined method” fatal error, which means LSP has been violated.

More Related Contents:

  1. PHP mail() issue
  2. (PHP) Warning: file_get_contents() [function.file-get-contents]: couldn’t resolve host name
  3. How to drop empty responses in an array of fields?
  4. How to use this code my wordpress theme
  5. How to change mysql to mysqli?
  6. How to check if an email address exists without sending an email?
  7. PHP: Replace umlauts with closest 7-bit ASCII equivalent in an UTF-8 string
  8. Count number of values in array with a given value
  9. Fatal error: Class ‘SoapClient’ not found
  10. Which is faster in PHP, $array[] = $value or array_push($array, $value)?
  11. Upload Multiple Files with PHP and JQuery
  12. Get title of website via link
  13. Remove .php extension (explicitly written) for friendly URL [closed]
  14. mcrypt_encrypt to openssl_encrypt, and OPENSSL_ZERO_PADDING problems
  15. When using Gmail for SMTP, can you set a different “from” address?
  16. Combine PHP prepared statments with LIKE
  17. Add PHP variable inside echo statement as href link address?
  18. PHP – setcookie() not working
  19. Explode string on commas and trim potential spaces from each value
  20. Nullable return types in PHP7
  21. Remove index.php in codeigniter 2.1.0
  22. How do you do multiple SQL statements in one mysql_query?
  23. How to clear previously echoed items in PHP
  24. Setting up PHPMailer with Office365 SMTP
  25. Convert Lat/Longs to X/Y Co-ordinates
  26. What is the difference between the | and || operators?
  27. MongoDB PHP UTF-8 problems
  28. PHP session seemingly not working
  29. adding two time values of similar formats using php
  30. Generate download file link in PHP [duplicate]

Leave a Comment