Php Destructors

2019-01-30 17:43发布

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问题:

Please give me some real life examples when you had to use __destruct in your classes.

回答1:

Ok, since my last answer apparently didn't hit the mark, let me try this again. There are plenty of resources and examples on the internet for this topic. Doing a bit of searching and browsing other framework's code and you'll see some pretty good examples...

Don't forget that just because PHP will close resources on termination for you doesn't mean that it's bad to explictly close them when you no longer need them (or good to not close them)... It depends on the use case (is it being used right up to the end, or is there one call early on and then not needed again for the rest of execution)...

Now, we know that __destruct is called when the object is destroyed. Logically, what happens if the object is destroyed? Well, it means it's no longer available. So if it has resources open, doesn't it make sense to close those resources as it's being destroyed? Sure, in the average web page, the page is going to terminate shortly after, so letting PHP close them usually isn't terrible. However, what happens if for some reason the script is long-running? Then you have a resource leak. So why not just close everything when you no longer need it (or considering the scope of the destructor, when it's no longer available)?

Here's some examples in real world frameworks:

Lithium's lithium\net\Socket class
Kohana's Memcached Driver
Joomla's FTP Implementation
Zend Frameworks's SMTP Mail Transport Class
CodeIgniter's TTemplate Class
A Tidy Filter Helper for Cake
A Google-Groups Thread about using Destructors For the Symfony Session Class

The interesting thing is that Kohana keeps track of the tags, so that it can delete by "namespace" later (instead of just clearing the cache). So it uses the destructor to flush those changes to the hard storage.

The CodeIgniter class also does something interesting in that it adds debugging output to the output stream in the destructor. I'm not saying this is good, but it's an example of yet another use...

I personally use destructors whenever I have long running processes on my master controller. In the constructor, I check for a pid file. If that file exists (And its process is still running), I throw an exception. If not, I create a file with the current processes id. Then, in the destructor I remove that file. So it's more about cleaning up after itself than just freeing resources...

回答2:

There is another handy use to generate HTML page

class HTMLgenerator {
  function __construct() {
    echo "<html><body>";
  }
  function __destruct() {
    echo "</body></html>";
  }
}

With this class, you can write

$html = new HTMLgenerator();
echo "Hello, world!";

And the result is

<html><body>Hello, world!</body></html>

回答3:

For example:

<?php
class Session
{
    protected $data = array();

    public function __construct()
    {
        // load session data from database or file
    }

    // get and set functions

    public function __destruct()
    {
        // store session data in database or file
    }
};

This is a good why to use destruct. You prevents reading and writing to a session source all the time and do this only at the start and at the end.

回答4:

I create a php page what will generate a movie information jpg file. This page will have to gather a few information and run inkscape to convert template (an svg file) to a png before converting to jpg. The svg contain relative links to other image which must be a file. So my page download necessary files into a temporary folder, convert the svg file. At the end, the temporary folder must be deleted.

I put the temporary folder deletion into the destructor. Before there can be many reason the page ends unexpected and the only think I can be sure is that destructor will be call when page exit.

Hope this helps.

回答5:

A destructor is extremely useful if you use a custom database connector/wrapper.

In the constructor, you can pass the connection information. Because you can use a destructor (rather than a finalizer, etc.,) you can rely on that to close the connection for you. It's more of a convenience, but it certainly is useful.

For example, when PHP decides to explicitly "free" the object (i.e., it is no longer used,) it will call the destructor at that time. This is more useful in the scenario I describe as you're not waiting for the garbage collector to run and call the finalizer.

$0.02

Ian

回答6:

<?php
class Database
{
    private $connection;
    private $cache = array();

    function __construct([$params])
    {
        //Connection here
    }

    //Query
    public function query(Query $Query)
    {
        if($this->is_cached($Query->checksum))
        {
            return $this->get_cache($Query->checksum);
        }
        //...
    }
    public function __destruct()
    {
        unset($this->connection);
        $this->WriteCache();
        unset($this->cache);
        shutdown_log($this,'Destruction Completed');
    }
}
?>

theres an example that should make you understand.

回答7:

If you use handles returned by fopen() for say, logging, you can use __destruct() to make sure fclose() is called on our resources when your class is destroyed.

回答8:

You are right, __destruct is mostly unnecessary for the short running php scripts. Database connections, file handles and so on close on script exit or sometimes even earlier if variables run out of scope.

One example i can think of is writing logs to the database. Since we didn't want to fire one query per log entry that gets created somewhere in the script we wrote the "write to db" part in the __destruct of the logging class so when the script ends everything gets inserted into the database at one.

Another example: If you allow a user to upload files the destructor is sometimes a nice places to delete the temp file (in case something goes wrong in the script it at least get cleaned up)

But even for filehandles it can be useful. I've worked on a application that did use the old fopen etc. calls wrapped in objects and when using those on large filetrees php would run out of filehandles sooner or later, so cleaning up while the script was running was not only nice but necessary.

回答9:

I use APC caching for large numbers of "low level" objects, that otherwise would use excessive memory; and I have a cacheCollection object that handles the reading and writing of those "low level" objects to and from APC during execution of the script. When the script terminates, the objects must be cleared down from APC, so I use the cacheCollection __destruct method to perform that function.

回答10:

I have used __destruct() in a logging class that wrapped a database connection:

<?php

class anyWrap
{
  private $obj,$calls,$log,$hooks;
  function anyWrap($obj, $logfile = NULL)
  {
       if(is_null($logfile))
       {
         $this->log = dirname(__FILE__) . "/../logs/wrapLog.txt";
       }
       $this->hooks = array();
       $this->dbCalls = 0;
       $this->obj = $obj;
  }

   public function __set($attri, $val) {
       $this->obj->$attri = $val;
   }

   public function __get($attri) {      
       return $this->obj->$attri;
   }
  public function __hook($method)
  {
   $this->hooks[] = $method;
  }


   public function __call($name,$args)
   {
       $this->calls++;
       if(in_array($name,$this->hooks))
       {
           file_put_contents($this->log,var_export($args,TRUE)."\r\n",FILE_APPEND);
       }
       return call_user_func_array(array($this->obj,$name),$args);
   }
   //On destruction log diagnostics
   public function __destruct()
   {
        unset($this->dbReal);
        file_put_contents($this->log,$this->calls."\r\n",FILE_APPEND);
   }
}

The script hooks into the database calls and logs the prepare statements, then when the script has run to an end (I don't always know when) it will finally log the number of calls to the database to the file. This way I can see how many times certain functions has been called on the database and plan my optimization accordingly.

回答11:

If you are creating a view using a PHP script in a MySQL database, you must drop that view at the end of the script. Because if not, the next time that script is executed view will not be created, as there is already a view of similar name in the database. For this purpose you can use destructor.