When to write mutable functions

So far, this chapter has indexed heavily on why we prefer to write immutable functions. But there are some instances in which it makes sense to write mutable functions either way. The only real reason is performance. As we saw earlier, the performance implications can often be ignored, but not always. If you are using structs that contain a lot of data, copying that over to each function can negatively impact the performance sufficiently to cripple your application. The only real way of knowing whether this is the case is by adding performance metrics to your application. Even so, a trade-off must be made between more performant code and more maintainable code. Oftentimes, trying to squeeze more performance out of your application hinders long-term maintainability.

Another possible reason to write mutable code using pointers is for resources that need to be singularly unique within your application. If you’re implementing traditional object...