Question

I was reading a blog post when I came across this code:

<?php
include_once 'config.php';
class User
{
//Database connect 
public function __construct() 
{
$db = new DB_Class();
}

In the comments someone posted the following:

NEVER INITIATE db connection in the constructor

But as with all comment warriors they never give a reason why? Why is this wrong or a bad practice to do?

Answer 1

Constructors should not do any real work:

Work in the constructor such as: creating/initializing collaborators, communicating with other services, and logic to set up its own state removes seams needed for testing, forcing subclasses/mocks to inherit unwanted behavior. Too much work in the constructor prevents instantiation or altering collaborators in the test.

Instead, use Dependency Injection and pass any collaborators to the constructor. This will create code that is easier to test. When you new something in the ctor, it is much more difficult to mock/stub that with a Test Double.

Also, by injecting collaborators, you are making it easier to swap out collaborators with different implementations, thus reducing coupling, fostering code reuse and coding towards interfaces instead of concrete implementations.