I believe the concern is ensuring that downcasting is safe is much harder if there are lifetimes, and it's likely infinitely less useful. This function wants to log its parameter out prior to doing work with it. std (default) enables all functionality requiring the standard library ( Downcast::downcast() ). ago rust A non- 'static type is a type that contains non- 'static references, e.g. Logger function for any type that implements Debug. We don't know the concrete type of our value at compile time, so we need to This means you can store arbitrary types somewhere (like on our App object) and retrieve them later. That later is for instance can involve a downcast to the original type. It lets you take a 'static type and box it up as a dyn Any so you can do something with it later. We want to give special treatment to certain types: in thisĬase printing out the length of String values prior to their value. A solution for this issue comes in the form of the Any type. We know the value we're working on implements Debug, but we don't know itsĬoncrete type. The safe way do this is to cast a reference to &dyn Any and use the downcast family of functions to get a reference to our original type back. We have a type with not much information that we want to turn back into our original type. ExamplesĬonsider a situation where we want to log out a value passed to a function. This is where things get a little annoying. This is needed since Rust cannot generate & Anys vtable from &Traits. Note that &Any is limited to testing whether a value is of a specifiedĬoncrete type, and cannot be used to test whether a type implements a trait. Traits to be extended by impldowncast must extend Downcast.
Rust any downcast full#
See the Box documentation for the full details. Box adds the downcast method, which attempts toĬonvert to a Box. Is also the downcast_mut method, for getting a mutable reference to the If the underlying error is not of type T, this will return None. As &Any (a borrowed trait object), it has the is andĭowncast_ref methods, to test if the contained value is of a given type,Īnd to get a reference to the inner value as a type. Attempts to downcast this failure to a concrete type by reference. Of any 'static type through runtime reflection.Īny itself can be used to get a TypeId, and has more features when usedĪs a trait object. This is needed since Rust cannot generate &mut Any’s vtable from &mut Trait’s.
Box
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