Why does PHP allow “incompatible” constructors?

by

To understand why they are treated differently, you have to understand Liskov’s Substitution Principle, which states

If for each object o1 of type S there is an object o2 of type T such that for all programs P defined in terms of T, the behavior of P is unchanged when o1 is substituted for o2 then S is a subtype of T.” – BarbaraLiskov, Data Abstraction and Hierarchy, SIGPLAN Notices, 23,5 (May, 1988).

In a nutshell this means any class using your Lion or Cat should be able to reliably call doSomething on it, regardless of the class being one or the other. If you change the method signature, this is no longer guaranteed (you may widen it, but not narrow it though).

Very simple Example

public function doSomethingWithFeline(Cat $feline)
{
    $feline->doSomething(42);
}

Since Lion extends Cat, you established an is-a relationship, meaning doSomethingWithFeline will accept a Lion for a Cat. Now imagine you add a required argument to doSomething in Lion. The code above would break because it is not passing that new param. Hence, the need for compatible signatures.

LSP does not apply to constructors though, because subtypes might have different dependencies. For instance if you have a FileLogger and a DBLogger, the ctors (constructors) of the first would require a filename, while the latter would require a db adapter. As such, ctors are about concrete implementations and not part of the contract between classes.

More Related Contents:

  1. Returning a value in constructor function of a class
  2. How do I get a PHP class constructor to call its parent’s parent’s constructor?
  3. When should you make an interface/contract, and when not? [closed]
  4. When should I use ‘self’ over ‘$this’?
  5. What does the variable $this mean in PHP?
  6. How to use PDO connection in other classes?
  7. MVC for advanced PHP developers [closed]
  8. What does it mean to start a PHP function with an ampersand?
  9. Get PHP class property by string
  10. What is the point of interfaces in PHP?
  11. Best practice on PHP singleton classes [duplicate]
  12. Override method parameter with child interface as a new parameter
  13. PHP OOP core framework
  14. Multiple Inheritance in PHP
  15. Accessing Class Properties with Spaces
  16. Is it possible to create static classes in PHP (like in C#)?
  17. When should I declare variables in a PHP class?
  18. Autoload classes from different folders
  19. When should I use stdClass and when should I use an array in php oo code?
  20. Chaining Static Methods in PHP?
  21. Dynamically create PHP object based on string
  22. Nested or Inner Class in PHP
  23. Performance of static methods vs. functions
  24. PHP dynamic name for object property
  25. Why is type hinting necessary in PHP?
  26. Fatal error: Uncaught ArgumentCountError: Too few arguments to function
  27. Cannot use concatenation when declaring default class properties in PHP?
  28. How do I use a class method as a callback function?
  29. unexpected ‘use’ (T_USE) when trying to use composer
  30. Expression is not allowed as field default value

Leave a Comment