Table of Contents

Request for Comments: Remove calls with incompatible Context

An RFC for deprecating and removing $this from incompatible context.

Introduction

This RFC proposes deprecating (in PHP 5.5) and removing (in the next version) the “$this from incompatible context” feature.

What this proposal is not about

This proposal is not about removing static calls to instance methods or instance-like calls to static methods.

The feature

To be clear, the feature I'm proposing to remove is this: 

class A {
    function foo() { var_dump(get_class($this)); }
}
class B {
   function bar() { A::foo(); }
}
$b = new B;
$b->bar(); //string(1) "B"

Internal methods cannot be called in this fashion.

The Change

Even though I think an error at call site would be the most useful to the user, the most sensible option is to just have $this === null inside the callee, like when you do: 

class A { function foo() {} }
A::foo(); // E_STRICT

Rationale

Method implementations almost always assume that $this refers to an instance of a compatible type. When they are called with an incompatible $this, there'll a bug most of the time. The only warning is an E_STRICT message, which many people have disabled.

Because this feature is surprising and little-known, a simple error like calling Class::foo() instead of ::fooStatic() (when foo() and fooStatic() have similar names), by not failing on the call site, can be difficult to identify.

When combined with LSB, it can be even more difficult to identify. See https://bugs.php.net/bug.php?id=62446

BC break

The break should be minor. I very much doubt there are many lines of code that rely on this feature. It has been discouraged with an E_STRICT since 2006 (see 6f76b170).

This feature, as noted in the comment, exists only for compatibility with PHP 4. Far more used legacy features, like register globals, have already been axed.

This feature can, however, be used to implement trait-like behavior, and I'm sure someone somewhere did such a thing.

Alternatives

Using traits is perhaps the easiest and cleanest way to replace code that relies on the feature to be removed. Changing the code to use traits implies: 1) refactoring the instance methods called from incompatible contexts into a trait, 2) make the callers from incompatible contexts use the new trait, 3) change the call sites to use $this->method() instead of OrigClass::method(). Example: 

<?php
class A {
	function dumpClass() {
		var_dump(get_class($this));			 
	}
}
class B {
	function test() {
		A::dumpClass();
	}
}
 
$a = new A;
$b = new B;
$a->dumpClass();
$b->test();

would become: 

<?php
trait ATrait {
	function dumpClass() {
		var_dump(get_class($this));			 
	}
}
class A {
	use ATrait;
}
class B {
	use ATrait;
	function test() {
		$this->dumpClass();
	}
}
 
$a = new A;
$b = new B;
$a->dumpClass();
$b->test();

A worse solution, which relies on the possibility of calling instance methods statically would be using an extra parameter: 

<?php
class A {
	function dumpClass($obj=null) {
		if ($obj === null)
			$obj = $this;
		var_dump(get_class($obj));			 
	}
}
class B {
	function test() {
		A::dumpClass($this); //E_STRICT ($this would be NULL on callee)
	}
}
 
$a = new A;
$b = new B;
$a->dumpClass();
$b->test();

Vote

Voting ends not before Monday, January 28th 2013. The PHP language is ultimately changed, so a 2/3 majority is required.

Deprecate calls with incompatible context in 5.5 and disallow them in the version after (be it 5.6 or 6.0)
Real name Yes No
aharvey (aharvey)  
cataphract (cataphract)  
cpriest (cpriest)  
gwynne (gwynne)  
indeyets (indeyets)  
jawed (jawed)  
levim (levim)  
lstrojny (lstrojny)  
mfonda (mfonda)  
nikic (nikic)  
patrickallaert (patrickallaert)  
rasmus (rasmus)  
salathe (salathe)  
stas (stas)  
tyrael (tyrael)  
Final result: 15 0
This poll has been closed.

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