Initializing variables outside of PHP constructor

Question

I was looking through the PHP documentation and saw several comments where a variable was initialized outside of a class's constructor, similar to the following:

classMyClass {
    private $count = 0;

    public function __construct() {
        //Do stuff
    }
}

In PHP Objects, Patterns, and Practice, the author recommends using constructs only for the initialization of properties, deferring any heavy lifting or complex logic to specialized methods. This tutorial (a quick example that I found on Google) also recommends using constructors to initialize properties: http://www.killerphp.com/tutorials/object-oriented-php/php-objects-page-3.php.

Why would you want to initialize a variable outside the constructor? Is this just sloppy coding, or is there a reason to do something like this? I have to say that until recently, I initialized default values outside the constructor, and there doesn't seem to be any programmatic advantage of one way over the other.

when you just want to initialize simple variables it's quite clear doing it outside the constructor, for more complex initializations use the constructor — Packet Tracer
– Packet Tracer, Commented Jun 28, 2012 at 16:07
The example you have posted is providing a default value for the $count variable. It is a good idea to provide a default value - this way you'll know what to test against to find out whether assignment in your constructor or elsewhere has succeeded. — blearn
– blearn, Commented Jun 28, 2012 at 16:07
With this the property will be declared and initialized even before the constructor was called. — WolvDev
– WolvDev, Commented Jun 28, 2012 at 16:07

Explosion Pills · Accepted Answer · 2012-06-28 16:07:56Z

7

When you initialize a variable outside of the constructor, it must be initialized as a constant. You can't do any operation to initialize it. Thus, the initial value of that member is actually a part of the class signature.

For example, this is invalid:

private $var = $othervar;
private $var = func();

You could do it in the constructor as well, but it would be a bit more verbose and add some clutter to the constructor unless there was some sort of logic going on.

answered Jun 28, 2012 at 16:07

Explosion Pills

192k56 gold badges341 silver badges417 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Nate Over a year ago

So there's no reason to not initialize a variable outside of the constructor if they are assigned a known value, such as a string?

hakre · Accepted Answer · 2012-06-28 16:16:09Z

2

More a comment than an answer, but please elaborate here a little:

Since it is recommended to use constructors for property initialization only,

Who says this and why? I assume the only relates to something else than property definitions with default values.

An answer part:

By default in PHP variables do not need to be defined because variables are then defined when first accessed in a write context. All variables, including undefined ones contain NULL (Demo):

class A {}

$a = new A;

var_dump($a->property); # NULL

Introducing class variables (properties) PHP then allowed to actually define variables. Those still return NULL by default, but they are defined (Demo):

class A {
    public $property;
}

$a = new A;

var_dump($a->property); # NULL

In the next step of the evolution, this language construct also allows to specify a constant expression. That is constant because definition is compile-time (not run-time as the when the constructor is invoked). An example (Demo):

class A {
    public $property = 'hello';
}

$a = new A;

var_dump($a->property); # string(5) "hello"

As this is compile- but your constructor run-time, I find it hard to compare both feature with another. Also it's not clear why you say that initializing via the constructor is recommended.

edited Jun 28, 2012 at 16:16

answered Jun 28, 2012 at 16:08

hakre

200k55 gold badges454 silver badges868 bronze badges

3 Comments

Andrew Over a year ago

Please see my updates to my question - I added the source I had in mind for that claim.

hakre Over a year ago

Okay: A site with the name "Killer PHP" and the tagline "PHP Tutorials for Web Designers" might not go that much into detail as you would like to know. I can not see anything wrong in defining object properties. You can initialize later if you want, but you should define them. If their initial value should be something different than NULL but a constant expression, define them with the initial value. That is nothing wrong. Indeed, it makes your constructor less complicated which is a good thing. Constructors should be dumb, should not do much.

Andrew Over a year ago

Thanks. I was mainly confused because I have seen good code primarily written with initialization outside the constructor, but it seems that most of the things that I have read have recommended initializing within the constructor. I can see how it is less cluttered (which is reason enough to do it), but I haven't found any other practical advantages in PHP for having setting a default value for instance property outside the constructor. THank you for your well thought-out answer.

Brian · Accepted Answer · 2012-06-28 16:08:25Z

0

Far from sloppy... it's good programming practice. As you would also do in Java/C++, it just sets them up, and then you can do any initialisation in the constructor - usually to sent them to non-defaults as such.

answered Jun 28, 2012 at 16:08

Brian

8,6212 gold badges27 silver badges32 bronze badges

Collectives™ on Stack Overflow

Initializing variables outside of PHP constructor

3 Answers 3

1 Comment

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related