Class type hinting in PHP 5

PHP 5’s OO structure is way ahead of PHP 4 and has so many ‘proper’ object oriented concepts built into it that writing well-structured code is much easier than ever. The problem that PHP faces (and may always face) is that being a loosely-typed language (click here for an explanation of the differences between PHP and other languages) there’s no real way to enforce the type of objects that are passed between methods, or to overload methods in the sense of the word used by Java, C# and other OO languages.

One thing that PHP5 does allow, however, is what they call ‘class type hinting’. This allows you to specify the type of object which is passed into a method and, if a variable is passed in incorrectly, then a runtime error is generated. E.g.

class Foo
{
  public function __construct(Bar $bar)
  {
  }
}

$foo = new Foo($bar);

This will throw an error if $bar doesn’t contain an object of class Bar.

This is a step forward in that at least you get a warning about your code breaking closer to where it actually happened than, perhaps, five methods down the line when you actually try and do something with the incorrect object, but there are a couple of parts of the implementation that I still don’t really like.

The first problem is that the error message only gives the line number of the method that’s called, not the callee so there’s still some back-tracking to find the source of the error. Still, beggars can’t be choosers…

The other is that some of the built-in types don’t seem to be recognised. Therefore you can specify your own classes by name but using ‘String $myString’ or ‘Array $myArray’ doesn’t wash with the interpreter. This means you have to leave those untyped, which kind of defeats the purpose in some cases.

The other issues are really to do with a wish-list. What would be really good would be:

  • Overloading: to be able to define more than one method of the same name.
  • Specifying the return type from a method. E.g. Foo $foo = $new Foo(); Bar $bar = $foo->makeBar();

I don’t know whether that’s ever going to be possible given loose typing but I don’t think PHP 5 will be regarded as a true rock-solid OO language without them.

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