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Bug #55268 Traits cannot be used as function/method typehints
Submitted: 2011-07-22 14:22 UTC Modified: 2011-07-22 15:53 UTC
From: php-bugs at majkl578 dot cz Assigned:
Status: Not a bug Package: Class/Object related
PHP Version: 5.4SVN-2011-07-22 (snap) OS: Irrelevant
Private report: No CVE-ID: None
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 [2011-07-22 14:22 UTC] php-bugs at majkl578 dot cz 
Description:
------------
It is not possible to use trait as a typehint in function declaration.

Use case: Extending few different classes to use a trait in them where these classes doesn't have same parent. Then I want to specify typehint, because I require some methods which are declared by that trait.

Test script:
---------------
trait Foo
{}

class Bar {
	use Foo;
}

function test(Foo $obj) {
	echo 'hello';
}

test(new Bar);

Expected result:
----------------
hello

Actual result:
--------------
Catchable fatal error: Argument 1 passed to test() must be an instance of Foo, instance of Bar given...

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 [2011-07-22 14:36 UTC] rasmus@php.net
-Status: Open +Status: Bogus
 [2011-07-22 14:36 UTC] rasmus@php.net 
Three things I don't understand here.

1. $obj is clearly an instance of Bar, even if the type-hint worked, it wouldn't 
be of type Foo

2. An empty trait makes no sense. This is a NOP.

3. A trait is not a type in any way. It is just a code snippet.
 [2011-07-22 15:05 UTC] php-bugs at majkl578 dot cz 
Maybe the example was too brief.

1. It was intended to work the same way as interfaces do: function foo(SomeInterface $foo) where $foo must be any class implementing SomeInterface or any of its subclass.

2. Obviously, it was empty just to make the code short.

3. I know it's just a code snippet, but it'd be still useful for cases mentioned above.
 [2011-07-22 15:53 UTC] rasmus@php.net 
But traits and interfaces are completely different. An interface is a contract a 
class has to meet and carries no implementation, while a trait is purely 
implementation. If you want to specify that a class has to have the implementation 
that a trait provides write an interface that describes the trait.
 
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