What is the right patter for a lazy field?

help
1

I have a field that I only want to calculate when is really necessary I access it via a function.

struct MyStruct {
    expensive: Option<i32>,
}

impl MyStruct {

   fn get_expensive(&self)  -> i32 {
        if self.expensive.is_none() {
             self.calculate()
        }
        self.expensive.unwrap()
   }
}

This does not work because calculate() needs to mutate self. But If use mut then it means that I need to add mut to all functions calling get_expensive. Is there a way out of this?

2

Yes! You need "interior mutability" here.

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