To inject or to new?

Question

With regards to using class objects within another class what is the best practice? To pass the class objects in the class _construct statement or create a new class object?

Example 1:

class Foo {
    private $bar;

    public function __construct($bar){
       $this->bar = $bar;
   }
}

Or Example 2 :

class Foo {

    private $bar;

    public function __construct(){
        $this->bar= NEW bar;
    }    
}

I'm aware that obviously it's taken for granted that the class file must already be included somewhere else, and in the first instance a class object of this type would need to exist already, but I want to know what the advantages are each method are, as I have a lot of classes I need to code that use a database object and I need the best way to pass this into the classes. Is there a third option that's better than these two?

From what I can understand, the advantage of the first one could be a few less lines of coding and in the case of a DB, not having a new connection created. The second one might be better however because it's more self contained? Anyhow I thought I'd ask the experts.

Answer 1

The first. (This approach is called Dependency Injection).

The constructor asks for whatever the object in questions needs in order to work. This way, it's pretty clear from the methods alone (what they need, and what they return), what it does. Without even looking at the source code.

A way to improve your code would be to introduce type hinting into your method:

class Foo {
    private $bar;

    public function __construct(Bar $bar){
       $this->bar = $bar;
   }
}

So that only Bar objects may be passed in.

---

Advantages of Dependency Injection

• Very readable.
• Ability to tell the method's dependencies without viewing the source code.
• Makes Unit Testing possible.
• ^*Saves kittens from God's wrath.

_{* Disclaimer: No kittens were harmed during the manifestation of this answer}

Answer 2

You should go for option 1, as this is the simplest form of dependency injection.

In option 1:

• classes are independent of each other
• classes can be tested independent, using a mock for the bar class