Downcasting
Downcasting is converting a value from a general type to a more specific compatible type, typically subject to runtime validation.
Learn Rust in a Month of Lunches
Downcasting is the attempted conversion of a dynamically typed or trait-object value back to a specific concrete type.
- How it is used
- Uses downcasting to recover specific error types from Box<dyn Error> values when generic formatting is insufficient.
- How deep it goes
- Explains trait-object type erasure and demonstrates downcast_ref across two error types, while not developing downcast ownership details.
- Examples
- Uses boxed MyError and RecvError values, then checks them with downcast_ref and prints the matching type.
- What you can do after
- Diagnose boxed errors by testing one known concrete type at a time and handling successful matches.
Read how Learn Rust in a Month of Lunches explains downcasting →
Idiomatic Rust
- How it is used
- Recovers a concrete type from a trait object when trait methods alone are insufficient.
- How deep it goes
- Explains `Any`, `as_any`, and borrowed versus consuming downcasts, while cautioning against overuse.
- Examples
- Demonstrates checking a vector of boxed trait objects for `MyStruct1` or `MyStruct2` and calling type-specific methods.
- What you can do after
- Apply it via `Any` and an `as_any` trait method to obtain a reference to a matching concrete type.
Related concepts
- dynamic_castaddressed by · dynamic_cast is a C++ run-time checked cast that converts pointers or references within an inheritance hierarchy and safely indicates incompatible conversions.
- trait objectused by · A Rust trait object is a value accessed through a trait interface with its concrete implementing type hidden and methods dispatched dynamically.
- instanceof operatorused by · The instanceof operator tests whether an object or reference is compatible with a specified runtime type or constructor according to the programming language’s rules.
- precisionuses · Numerical precision is the fineness with which a numerical representation or computation distinguishes and retains values.
- downcast_refalternative to · downcast_ref is an operation that attempts to obtain a shared reference to an interface value as a specified concrete type.
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