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137 lines
5.4 KiB
Plaintext
137 lines
5.4 KiB
Plaintext
Revamped object model using object handles
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===========================================
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Background
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----------
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In the Zend Engine 1.0 (and its predecessor the PHP 3 scripting
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engine) the object model's design is that instantiated objects are
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language values. This means that when programmers are performing
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operations, such variable assignment and passing parameters to
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functions, objects are handled very similarly to the way other
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primitive types are handled such as integers and strings.
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Semantically this means that the whole object is being copied. The
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approach Java takes is different where one refers to objects by handle
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and not by value (one can think of a handle as an objects' ID).
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Need
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----
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Unfortunately, the approach taken up to now has severely limited the
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Zend Engine's object oriented model, both feature and simplicity
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wise. One of the main problems with the former approach is that object
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instantiation and duplication is very hard to control, a problem which
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can not only lead to inefficient development but also often to strange
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run-time behavior. Changing the object model to a handle oriented
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model will allow the addressing of many needs such as destructors,
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de-referencing method return values, tight control of object
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duplication and more.
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Overview
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--------
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The proposed object model is very much influenced by the Java
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model. In general, when you create a new object you will be getting a
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handle to the object instead of the object itself. When this handle is
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sent to functions, assigned and copied it is only the handle which is
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copied/sent/assigned. The object itself is never copied nor
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duplicated. This results in all handles of this object to always point
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at the same object making it a very consistent solution and saving
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unnecessary duplication and confusing behavior.
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Functionality
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-------------
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After this change the basic use of objects will be almost identical to
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previous versions of the scripting engine. However, you won't bump
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into awkward and confusing copying & destructing of objects. In order
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to create and use a new object instance you will do the following:
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$object = new MyClass(); $object->method();
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The previous code will assign $object the handle of a new instance of
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the class MyClass and call one of its methods.
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Consider the following code:
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1 class MyClass
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2 {
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3 function setMember($value)
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4 {
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5 $this->member = $value;
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6 }
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7
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8 function getMember()
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9 {
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10 return $this->member;
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11 }
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12 }
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13
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14 function foo($obj)
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15 {
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16 $obj->setMember("foo");
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17 }
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18
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19 $object = new MyClass();
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20 $object->setMember("bar");
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21 foo($object);
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22 print $object->getMember();
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Without the new Java-like handles, at line 20 the objects' data member
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member is set to the string value of "bar". Because of the internal
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representation of objects in the Zend Engine 1.0, the object is marked
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as a reference, and when it is sent by value to the function foo, it
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is duplicated (!). Therefore, the call to foo() on line 21 will
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result in the $obj->setMember("foo") call being called on a duplicate
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of $object. Line 22 will then result in "bar" being printed.
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This is how the scripting engine has worked until today. Most
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developers are probably unaware of the fact that they aren't always
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talking to the same object but often duplicates; others may have
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realized this can usually be solved by always passing objects by
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reference (unless a replica is actually desired, which is uncommon).
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The new object model will allow for a much more intuitive
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implementation of the code. On line 21, the object's handle (ID) is
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passed to foo() by value. Inside foo(), the object is fetched
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according to this handle and, therefore, the setMember() method is
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called on the originally instantiated object and not a copy. Line 22
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will therefore result in "foo" being printed. This approach gives
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developers tighter control of when objects are created and duplicated.
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An additional not-as-important benefit is that the object handle will
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be passed to foo() by value, which most probably will also save
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unnecessary duplication of the value containing the ID itself and thus
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additionally improving run-time performance.
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This was just a simple description of why the new object model solves
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awkward behavior and makes object handling much easier, intuitive and
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efficient. The importance of this change goes far beyond what is
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mentioned in this section as you will see in further sections which
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describe new features with a majority of them being based on this
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change.
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Compatibility Notes
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--------------------
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Many PHP programmers aren't even aware of the copying quirks of the
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current object model and, therefore, there is a relatively good chance
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that the amount of PHP applications that will work out of the box or
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after a very small amount of modifications would be high.
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To simplify migration, version 2.0 will support an optional
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'auto-clone' feature, which will perform a cloning of the object
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whenever it would have been copied in version 1.0. Optionally, it
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will also be possible to request that the engine will emit an E_NOTICE
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message whenever such an automatic clone occurs, in order to allow
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developers to gradually migrate to the version 2.0-style behavior
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(without automatic clones).
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Dependencies
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------------
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The new object model is not dependent on other features. Many of the
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other Zend Engine 2.0 features, such as the $foo->bar()->barbara()
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syntax, destructors and others completely rely on this new object
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model.
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