Question

There is a class like this in codeigniter framework ( I edited it to be more clear, full function is here http://pastebin.com/K33amh7r):

function &load_class($class, $directory = 'libraries', $prefix = 'CI_')
    {
        static $_classes = array();

        // Does the class exist?  If so, we're done...
        if (isset($_classes[$class]))
        {
            return $_classes[$class];
        }


        is_loaded($class);

        $_classes[$class] = new $name();
        return $_classes[$class];

    }

So, first time when class is loaded ( passed to this function), it will be saved to this static variable. Next time when the same class is loaded, this function checks if class exists already ( if it’s already assigned to static, cached, I’m not sure how in memory is this stored) and if it exists, it’s loaded ( NOT *instantiated* again )

As far as I can see, the only purpose is to save time or memory and not instantiate the same class twice.

My question here is: Does really instantiating a class can take up memory or consume loading time so it has to be cached like this?

Answer 1

CodeIgniter is is geared for rapid prototyping, and is really not a good example of enterprise patterns in almost any cases. This behavior is related to their design choice of the relationship the “controller” has to almost all other objects; namely that there is exactly one of almost anything (only one instance of controller, only one instance of each library, etc). This design choice is more for rapid development (justified by the developer “not having to keep track of as much” or some such…).

Basically, there is memory saved by not instantiating an object (as much memory as it takes to store the object’s instance variables) and if the object’s constructor tries to do a fair bit of work, you can save time, too.

However, the appropriateness of the single-instance imperative is clearly not universal; sometimes you really do want a new instance. When you can justify this, choose a better framework.