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| -rw-r--r-- | library/core/src/any.rs | 23 |
1 files changed, 23 insertions, 0 deletions
diff --git a/library/core/src/any.rs b/library/core/src/any.rs index d1951fbbf10..eef8f2046d3 100644 --- a/library/core/src/any.rs +++ b/library/core/src/any.rs @@ -14,6 +14,29 @@ //! //! [`Box`]: ../../std/boxed/struct.Box.html //! +//! # Smart pointers and `dyn Any` +//! +//! One piece of behavior to keep in mind when using `Any` as a trait object, +//! especially with types like `Box<dyn Any>` or `Arc<dyn Any>`, is that simply +//! calling `.type_id()` on the value will produce the `TypeId` of the +//! *container*, not the underlying trait object. This can be avoided by +//! converting the smart pointer into a `&dyn Any` instead, which will return +//! the object's `TypeId`. For example: +//! +//! ``` +//! use std::any::{Any, TypeId}; +//! +//! let boxed: Box<dyn Any> = Box::new(3_i32); +//! +//! // You're more likely to want this: +//! let actual_id = (&*boxed).type_id(); +//! // ... than this: +//! let boxed_id = boxed.type_id(); +//! +//! assert_eq!(actual_id, TypeId::of::<i32>()); +//! assert_eq!(boxed_id, TypeId::of::<Box<dyn Any>>()); +//! ``` +//! //! # Examples //! //! Consider a situation where we want to log out a value passed to a function. |