Singleton variable is a data structure widely used in computer programming. It is defined as a static, immutable memory location that holds a single value at any given point in time. The singleton variable allows the programmer to quickly, safely and reliably access a specific, unchanging value without having to declare it as a constant or use other more complex techniques.
Singleton variables are commonly used to store static information such as a hardware configuration or file path. They are also used to track state information, such as whether a network port is open or closed. In some uses, they can contain an address of a callback routine, allowing the program to be provided with a notification if the value changes.
Singleton variables offer a number of advantages over other data structures. Due to its immutability, a singleton variable will always contain the same value, even when the program is running in a shared environment where other programs may be attempting to modify it in the same memory space. As such, it is guaranteed that all code paths within a program will observe the same value. This makes singleton variables particularly useful in applications where concurrent access and synchronization are critical.
Singleton variables are typically used as part of a larger system in which other dynamic memory structures are being used. Unlike regular variables, which are mutable and can be accessed, changed or destroyed at any time, singleton variables can provide a static reference point for a program to track state. This can be especially beneficial in complex data structures, where other dynamic data structures can become out of sync with each other due to unexpected changes.
Despite their advantages, singleton variables are usually considered an anti-pattern and should be used sparingly in code. Many coding standards recommend using constants or other immutable data structures instead of singleton variables whenever possible. This is because their use can often lead to an overly-intricate design that is difficult to debug.
