PHP constructors and static functions

问题

I'm a bit confused on how constructors work in PHP.

I have a class with a constructor which gets called when I instantiate a new object.

$foo = new Foo($args);

__construct($params) is called in the class Foo and it executes the appropriate initialization code.

However when I use the class to call a static function, the constructor is called again.

$bar = Foo::some_function(); //runs the constructor from Foo

This causes the constructor to execute, running the object initialization code that I intended only for when I create a new Foo object.

Am I missing the point of how constructors work? Or is there a way to prevent __construct() from executing when I use the class to make static function calls?

Should I use a "factory" function instead to do the object initialization? If so, what's the point of the constructor then?

::EDIT:: I have a form where users can upload photos to an album (create_photo.php) and an area where they can view the album (view_photos.php). Upon form submit:

$photo = new Photo($_FILES['photo'], $_POST['arg1'], ect..);

The Photo constructor creates and saves the photo. However in view_photo.php, when I call:

$photo = Photo::find_by_id($_POST['id']) //user-defined function to query database

This is causing Photo's constructor to run!

回答1:

I see nothing that replicates your question.

See Demo: http://codepad.org/h2TMPYUV

Code:

class Foo {
    function __construct(){ 
        echo 'hi!';
    }
    static function bar(){
        return 'there';
    }
}

echo Foo::bar(); //output: "there"

回答2:

Assumption PHP 5.x

Different goals, different path

create a new instance of a class (object)

class myClassA
{
   public $lv;

   public function __construct($par)
   {
       echo "Inside the constructor\n";
       $this->lv = $par;
   }
}

$a = new myClassA(11);
$b = new myClassA(63);

because we create a new object PHP calls:

__construct($par);

of the new object, so:

$a->lv == 11 

$b->lv == 63

use a function of a class
```
class myClassB
{
    public static $sv;

    public static function psf($par)
    {
        self::$sv = $par;
    }
}

myClassB::psf("Hello!");
$rf = &myClassB::$sv;
myClassB::psf("Hi.");
```
now $rf == "Hi."

function or variabiles must defined static to be accessed by ::, no object is created calling "psf", the "class variable" sv has only 1 instance inside the class.

use a singleton created by a Factory (myClassA is above)

class myClassC
{

    private static $singleton;

    public static function getInstance($par){

        if(is_null(self::$singleton)){

            self::$singleton = new myClassA($par);

        }

        return self::$singleton;

    }

}

$g = myClassC::getInstance("gino");
echo "got G\n";

$p = myClassC::getInstance("pino");
echo "got P\n";

Using the factory (getInstance) the first time we construct a new object having $par set to gino.

Using the factory the second time $singleton has already a value that we return. No new object is created (no __construct is called, less memory & cpu is used).

The value of course is an object instanceOf myClassA and don't forget:

myClassC::$singleton->lv == "gino"

Pay attention to singletons:

What is so bad about singletons?

http://www.youtube.com/watch?v=-FRm3VPhseI

By my answer I don't want promote/demote singleton. Simply from the words in the question, I made this calc:

"static"+"__construct"="singleton"!

回答3:

Here is my workaround:

I put method construct() in static class. Notice, it is different than __construct() which I use in regular classes.

Each class is in own file, so I lazy load that file on first use of class. This gives me event of first use of class.

spl_autoload_register(function($class) {

    include_once './' . $class . '.php';

    if (method_exists($class, 'construct')) {
        $class::construct();
    }
});

回答4:

I define class properties as array in a static method and call them via the method. I'm not sure if it's the best solution or not but works great.

Example:

    class Foo
    {
      private static construct_method()
      {
        return [
          'one' => 1,
          'two' => 2
        ];
      }

      public static any_method()
      {
        return self::construct_method()['one'] + self::construct_method()['two'];
      }

    }

    echo Foo::any_method(); // 3

来源：https://stackoverflow.com/questions/8231198/php-constructors-and-static-functions

标签

php

constructor

static-functions