What is __construct and _construct in magento2?

Question

In Magento 2, most of classes have these two constructs (__construct and _construct) methods. What is the difference between them?

Answer 1

I'm not completely sure if it has changed between Magento 1 and Magento 2, probably not so I'm gonna go with what I know from Magento 1.

The _construct will be called after the __construct

The PHP native __construct method should not be overwritten or used in your code. If you want to execute code in a safe way on the start of a class use _construct.

Magento will use the native __construct to make sure everything is 'ready' for a class to be used such as defining the right cache tags for a certain model for example.

Answer 2

The _construct method was a "Varien invention" used to wrap some initialization logic in models, helpers and blocks.

So is unusual to change or re-declare the native __construct() method in M1 Models/Blocks or Helpers since we always use the Magento factories. However isn't any problem/bad-practice about using it (if you care of compatibility).

In M2 the _construct() method still present in some parts and is used for the same purposes, but now (in M2) all DI logic is implemented by the __constructor() so you will find a lot of construct declarations in the code base.

BTW there is no more factories like Mage::getModel() in M2.

In Other words:

The _construct() method is implemented by Magento in some classes and it's called automatically inside the __construct function declaration, so if you are extending a Magento class like a Model you can use it to perform some stuff after object creation.

In a Resource Model or Model Class you should define a _construct() method in order to define the table and the primary_key

In the other hand the __construct is a native method of PHP (All OO languages have one), __construct is called every time you instantiate an object. That's all

Example:

Magento\Framework\Model\ResourceModel\AbstractResource

/**
 * Abstract resource model
 */
abstract class AbstractResource
{
    /**
     * Main constructor
     */
    public function __construct()
    {
        /**
         * Please override this one instead of overriding real __construct constructor
         */
        $this->_construct();
    } ...

Magento\Framework\Model\ResourceModel\Db\AbstractDb

/**
 * Class constructor
 *
 * @param \Magento\Framework\Model\ResourceModel\Db\Context $context
 * @param string $connectionName
 */
public function __construct(\Magento\Framework\Model\ResourceModel\Db\Context $context, $connectionName = null)
{
    $this->transactionManager = $context->getTransactionManager();
    $this->_resources = $context->getResources();
    $this->objectRelationProcessor = $context->getObjectRelationProcessor();
    if ($connectionName !== null) {
        $this->connectionName = $connectionName;
    }
    parent::__construct();
}