orx-pseudo-default

PseudoDefault is a simple trait for creating a default instance of a type, which is not necessarily useful.

The difference of PseudoDefault from Default is the relaxed expectation of the created instance to be useful.

The main use case of the trait is when we need to create a cheap instance of a type without any arguments to throw away. This sounds exactly the same as the Defaults definition. However, the instance created by pseudo_default does not need to be a decent one, it is totally fine if it is useless.

We would most possibly need such an instance when we need to swap it with another instance or fill a gap in a collection. PseudoDefault allows us to achieve this without requiring to use unsafe code and leaving the memory location uninitialized.

Example

Consider the following fictional type Share which divides a whole into pieces. Without providing the number_of_shares, this type is meaningless; i.e., we cannot justify implementing Default. However, if we need to be able to create dummy Share's, we can simply use pseudo_default. The created share will be useless; however, a valid and safe Share instance.

struct Share {
    number_of_shares: std::num::NonZeroUsize,
}

impl Share {
    fn share_size(&self, whole_amount: usize) -> usize {
        whole_amount / self.number_of_shares
    }
}

impl PseudoDefault for Share {
    fn pseudo_default() -> Self {
        Self {
            number_of_shares: std::num::NonZeroUsize::new(1).unwrap(),
        }
    }
}

License

This library is licensed under MIT license. See LICENSE for details.