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POdestructing an object with references to itself
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copied!<p>While debugging a piece of code that was exhausting the memory, I found a very interesting problem, and most importantly I don't know how to fix it.</p> <p>The application consists roughly of a single <code>Survey</code> object, which contains a number of <code>Question</code> objects. The Question objects contain a reference to the Survey they're in, this is needed to be able to fetch answers from other Questions for instance.</p> <hr> <p>The following loop was causing the memory overflow:</p> <pre><code>foreach ( $survey_ids_arr as $survey_id ) { $Survey = new Survey( $survey_id ); } </code></pre> <p>Nothing really exotic is happening in the Survey constructor; </p> <ul> <li>fetching its properties from the database</li> <li>fetching all questions' properties from the database</li> <li>creating a Question object for each question (passing a reference to $this)</li> <li>adding all Question objects to an internal array</li> </ul> <p>and from looking at the code, you would say that in each iteration the object is cleared from memory because the $Survey variable is overwritten. Right?? Wrong :)</p> <p>The memory is piling up as the script goes through the loop - adding <code>memory_get_usage()</code> calls shows that the memory used by the Survey object isn't freed as expected, at the moment another object is assigned to the <code>$Survey</code> variable. Even calling <code>unset( $Survey )</code> at the end of the loop does not free the memory.</p> <hr> <p>The culprit are the references to <code>$this</code> that are passed to the Question objects upon creation. These references prevent the object to be cleared from memory - as the manual on php.net states: </p> <blockquote> <p>The destructor method will be called as soon as all references to a particular object are removed</p> </blockquote> <p>So what prevents the object from being cleaned up, is the references it has in it to itself. Nice, huh? :)</p> <p>So, the problem is my object is a memory killer. Unfortunately, I can't think of a solution (other than writing an ugly method which clears the questions and calling that from the loop). The destructor in Survey is not an option; as stated above this is not called <em>because</em> the Question objects still have references.</p> <p>Any ideas? Someone must have run into this problem already - the parent-containing-child-objects is not an uncommon architecture, is it?</p>

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