Where is class $tableClass defined?

Question

I have very little knowledge about joomla core and PHP. In order to better handle my website, I am trying to read a small part of joomla core, when I come across this function JTable:getInstance, (libararies\joomla\table\table.php line268), it returns

    return new $tableClass($db);

To understand what getInstance returns, I need to know how this $tableClass is defined, so I searched it in all php files in my website, although there are lots of references, even some so-called 'definitions' as this page points out, none of them is what I am looking for. I am expecting something like

class tableClass{...}

Furthermore, from other php files, there are these lines:

$row = JTable::getInstance('K2Item', 'Table');
$row->hit($id);

So it looks to me, what is returned by getInstance should be an object that has a member hit(), so I am expecting something like

class tableClass{...
    function hit(){
    .....
    }
}

But this kind of code is nowhere to be found, so I am stuck here and definitely need help.

Some of my thoughts: Is $tableClass really a class? I notice that it has a $, which all other classes don't have? If it is not a class, then why it could be invoked like this new $tableClass? I really need to understand these basics but it is kind of hard to google keyword $.

Answer 1

$tableClass is a variable. You won't find a class named "tableClass". It's just a placeholder for the real class name which will be generated at the beginning of the getInstance method.

Take a look at lines 268-272 where getInstance begins:

public static function getInstance($type, $prefix = 'JTable', $config = array())
{
    // Sanitize and prepare the table class name.
    $type       = preg_replace('/[^A-Z0-9_\.-]/i', '', $type);
    $tableClass = $prefix . ucfirst($type);

The first line of this defines which parameters you can pass into getInstance later.

 $type    The type (name) of the JTable class to get an instance of.
 $prefix  An optional prefix for the table class name.
 $config  An optional array of configuration values for the JTable object.

So if you look at the example you gave,

$row = JTable::getInstance('K2Item', 'Table');

"K2Item" is the name of the JTable class ($type) and "Table" is the prefix ($prefix). [$config was not set as it is only optional and probably not needed here]

Line 271 ($type = preg_replace('/[^A-Z0-9_\.-]/i', '', $type);) will remove all bad characters from the $type parameter which means every character except letters (A-Z), numbers (0-9) and the characters '_', '.', and '-'. It uses a regular expression for that.

In line 272 $tableClass (which will be the final class name to look after) is generated. It takes $prefix and concatenates the $type from the previous line. ucfirst() is there to convert the first letter of $type to uppercase.

Some examples:

JTable::getInstance('K2Item', 'Table');

$tableClass will become "TableK2Item"

JTable::getInstance('C#!ont&ent');

$tableClass will become JTableContent, because of the bad character removal and $prefix is "JTable" by default.