我们必须在构造函数中声明所有属性吗？

Question

I am new to OOP. Do we have to declare all the properties in constructor? For example:

  private $_conn;
  private $_limit;
  private $_page;
  private $_query;
  private $_total;

public function __construct($conn, $query){

$this->_conn = $conn;
$this->_query = $query;

$rs = $this->_conn->prepare($this->_query);
$rs->execute();
$this->_total = $rs->rowCount();
}

public function getData($page = 1, $limit = 10){

$this->_limit = $limit;
$this->_page  = $page;

if ($this->_limit == 'all'){
    $query = $this->_query;
} else {
    $query = $this->_query." LIMIT ".(($this->_page - 1) * $this->_limit).", $this->_limit";
}
$rs = $this->_conn->prepare($query);
$rs-> execute();

while ($row = $rs->fetch(PDO::FETCH_ASSOC)){
    $results[] = $row;
}

I got this from somewhere on internet. The coder didn't declare $_limit and $_page properties in constructor. Is that for not to create the properties if we don't use the getData method? and can we use this logic for other programming languages like Java, C#, C++?

Answer 1

every method/function (including constructor of the object) should declare only the variables it's going to use.