I have a struct member vector that might be presented and might not. The obvious choice is to put it behind Option.
But if I do that, when I'll set the option to None, I'll free the vector. The next time it will have to be re-allocated.
This is bad. I want to re-use the capacity from the previous time.
I can build a container that does that, but is there a built-in way to do this?
You can use Option::take() to get the old value of the vector and store it somewhere for reuse, and Vec::clear() to empty the vector without reducing its capacity.
How do you plan on using the Vector?
Exactly like the previous time; I process input and in some parts of it I need access to the vector.
I can also store the vector without Option and also store a boolean indicating whether the vector is accessible; However, I don't want to manually deal with storing/restoring the vector.
Have you considered having a different type for each state your algorithm can be in that only has the fields valid in that state?
Well, that’s the global allocator’s job; if you don’t like how it behaves, you either need to implement the alternative manually or find a crate that does.
This doesn't work this way. I don't have other data accessible for each state (then I could define an enum), but instead data that is always available and data that's available based on certain conditions (that may overlap).
I don't understand what you say, I replied to your idea:
And said that using this way I'll have to manually store the vector in a different field and keep track on where it's available.
My point was that, aside from exceptional situations, giving the vector’s memory back to the allocator when you’re not using it and allocating a new one when you need it again is performant enough.
If your situation is one of the exceptional ones, it’s unreasonable to expect to not write at least some memory-management code specific to your use case. And writing a thin wrapper around a 2-field struct isn’t that much effort in the grand scheme of things.
Got it 