Copy-on-write

Normally, when we pass an instance of a value type, such as a structure, we create a new copy of the instance. This means that if we have a large data structure that contains 50,000 elements, every time we pass that instance, we would have to copy all 50,000 elements. This can have a detrimental impact on the performance of our applications especially, if we pass that instance to numerous functions.

To solve this issue, Apple has implemented the COW feature for all the data structures (Array, Dictionary, and Set) within the Swift standard library. With COW, Swift does not make a second copy of the data structure until a change is made to that data structure. Therefore, if we pass an array to 50,000 elements to another part of our code, and that code does not actually make any changes to the array, we will avoid the runtime overhead of copying all the elements.

This is a very nice feature and can greatly increase the performance of our applications, however, our custom value types...