Purpose of singletons in programming?

Question

This is admittedly a rather loose question. My current understanding of singletons is that they are a class that you set up in such a way that only one instance is ever created.This sounds a lot like a static class to me. The main difference being that with a static class you don't / can't instance it, you just use it such as `Math.pi()`. With a singleton class, you would still need to do something like singleton firstSingleton = new singleton(); firstSingleton.set_name("foo"); singleton secondSingleton = new singleton();Correct me if i am wrong, but `firstSingleton == secondSingleton` right now, yes? secondSingleston.set_name("bar"); firstSingleton.report_name(); // will output "bar" won't it?Please note, I am asking this language independently, more about the concept. So I am not worried about actually how to code such a class, but more why you would wan't to and what thing you would need to consider.

Answer 1

Yes, it will output "bar" because all the constructor does is return a pointer to the existing instance.

Now, as an example of why such a design, I will share my experience with you, about the objects of an ATM machine that requires singleton classes. The vast majority of such singletons are device objects such as the printer, modem, display, pinpad, keyboard, swiper, and bill dispenser.

First of all, these device objects open up a communications serial port so that the terminal can communicate with the device. Once the communications channel is open, it cannot be opened again. There is one and only one serial port and the device is using it. Therefore it does not make sense to have multiple instances of the object. All there is a GLOBAL instance of the device that gets passed around to whatever process needs the device.

In general, the same can be said about other peripheral devices: security cameras, signature captures, fingerprinting, forensic equipment, medical equipment etc. which are typically found in embedded environments.

It is an uncommon , but not an earth shaking concept