Question

Suppose I have this:

class Validator {
  private $db;

  public checkIfUsernameAlreadyExists($username) {
    if (!$this->db)
      return false;

  // Queries
  }
}

Assume the $db-object was created in the constructor (or a $db-object was given as a parameter in the constructor).

The problem is that all methods which have to use the $db-object need to check first if this object really exists. A database-connection may fail for several reasons. If it doesn’t exist and no check was made, the script will crash (“method on non-object”-error).

Is there a way to work around this issue? Checking the object in every method doesn’t sound the correct way. Or is it?

Thank you

Answer 1

Checking the object in every method is definitely not right.

The purpose of constructor arguments is to have a valid object to work with after it is instantiated:

class Validator {
  private $db;

  public function __construct(PDO $db) {
     // validate $db here
  }

  public checkIfUsernameAlreadyExists($username) {
     $this->db->query('SELECT * FROM table'); // exception thrown here
     // never reaches here
  }
}

try {
    $validator = new Validator(new PDO('mysql:dbname=db', 'user', 'pass'));
    $validator->checkIfUsernameAlreadyExists('foo');
} catch (PDOException $e) {
    echo 'Database error occured: ', $e->getMessage();
    exit(1);
}

So, you validate $db once in the constructor and that’s it. If the database connection fails, then the $db object should (and will if it’s PDO) throw an exception which will halt the execution of the method anyway.

Note: I’d discourage the use of Singleton pattern or global variables for a lot of reasons that can easily be found on Stack Overflow or Google.