PHP中的单例和类实例化

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.

Answer 2

The resources and time used in instantiating a class are usually negligible. The main reason I usually see singleton classes used is to maintain data integrity. For example, if you have a class that represents data in a database, creating multiple objects for it can cause the data to become out of sync. If one object changes and commits data to the DB, the other objects could have old data.

Answer 3

Its rather a simple concept, utilizing singleton-pattern it makes sure that one class is instantiated only once during an application's execution cycle.

This sort of concept apply for libraries more. Lets see a basic example:

class Authenticate {
     public function login($username, $password) {
        ....
     }

     public function logout() {

     }
}

Now, through a execution of a page, there is hardly any case that the object of the above class, needs to be created more than once. The main thing to understand is Utilization of resources

And YES , instantiating same class over and over again will without a doubt add up in the memory, although it might be negligible like in the example I have shown, but it does affect.