Erro: mysql_num_rows() expects parameter 1 to be resource, string give

Quando executo o login aparece esta mensagem:

Warning: mysql_num_rows() expects parameter 1 to be resource, string given in C:\Program Files (x86)\EasyPHP-DevServer-13.1VC9\data\localweb\phpteste1\login.php on line 25

**Obs: **linha 25: $c = mysql_num_rows($selecionar);

<?php

class Sql{

	public $host, $user, $pass, $query;

	public function __construct(){
 mysql_connect($this -> host = 'localhost', $this -> user = 'root', $this -> pass = '1234' );
 mysql_select_db('cadastro');
	}
	public function query($qr){
 $this -> query = mysql_query($qr);
 return $this;
	}
	public function retornar(){
 return $this -> query;

}
	}

$sql = new sql;
$login = $_POST{'login'};
$pwd = $_POST{'senha'};
$selecionar = $sql -> query("SELECT * FROM USUARIO WHERE LOGIN = '$login' AND SENHA = '$pwd'");
$c = mysql_num_rows($selecionar);
if($c){
	echo "logado";

	echo "deslogado";
}
?>