library/core/src/any.rs

   1 //! This module implements the `Any` trait, which enables dynamic typing
   2 //! of any `'static` type through runtime reflection.
   3 //!
   4 //! `Any` itself can be used to get a `TypeId`, and has more features when used
   5 //! as a trait object. As `&dyn Any` (a borrowed trait object), it has the `is`
   6 //! and `downcast_ref` methods, to test if the contained value is of a given type,
   7 //! and to get a reference to the inner value as a type. As `&mut dyn Any`, there
   8 //! is also the `downcast_mut` method, for getting a mutable reference to the
   9 //! inner value. `Box<dyn Any>` adds the `downcast` method, which attempts to
  10 //! convert to a `Box<T>`. See the [`Box`] documentation for the full details.
  11 //!
  12 //! Note that `&dyn Any` is limited to testing whether a value is of a specified
  13 //! concrete type, and cannot be used to test whether a type implements a trait.
  14 //!
  15 //! [`Box`]: ../../std/boxed/struct.Box.html
  16 //!
  17 //! # Smart pointers and `dyn Any`
  18 //!
  19 //! One piece of behavior to keep in mind when using `Any` as a trait object,
  20 //! especially with types like `Box<dyn Any>` or `Arc<dyn Any>`, is that simply
  21 //! calling `.type_id()` on the value will produce the `TypeId` of the
  22 //! *container*, not the underlying trait object. This can be avoided by
  23 //! converting the smart pointer into a `&dyn Any` instead, which will return
  24 //! the object's `TypeId`. For example:
  25 //!
  26 //! ```
  27 //! use std::any::{Any, TypeId};
  28 //!
  29 //! let boxed: Box<dyn Any> = Box::new(3_i32);
  30 //!
  31 //! // You're more likely to want this:
  32 //! let actual_id = (&*boxed).type_id();
  33 //! // ... than this:
  34 //! let boxed_id = boxed.type_id();
  35 //!
  36 //! assert_eq!(actual_id, TypeId::of::<i32>());
  37 //! assert_eq!(boxed_id, TypeId::of::<Box<dyn Any>>());
  38 //! ```
  39 //!
  40 //! # Examples
  41 //!
  42 //! Consider a situation where we want to log out a value passed to a function.
  43 //! We know the value we're working on implements Debug, but we don't know its
  44 //! concrete type. We want to give special treatment to certain types: in this
  45 //! case printing out the length of String values prior to their value.
  46 //! We don't know the concrete type of our value at compile time, so we need to
  47 //! use runtime reflection instead.
  48 //!
  49 //! ```rust
  50 //! use std::fmt::Debug;
  51 //! use std::any::Any;
  52 //!
  53 //! // Logger function for any type that implements Debug.
  54 //! fn log<T: Any + Debug>(value: &T) {
  55 //!     let value_any = value as &dyn Any;
  56 //!
  57 //!     // Try to convert our value to a `String`. If successful, we want to
  58 //!     // output the String`'s length as well as its value. If not, it's a
  59 //!     // different type: just print it out unadorned.
  60 //!     match value_any.downcast_ref::<String>() {
  61 //!         Some(as_string) => {
  62 //!             println!("String ({}): {}", as_string.len(), as_string);
  63 //!         }
  64 //!         None => {
  65 //!             println!("{:?}", value);
  66 //!         }
  67 //!     }
  68 //! }
  69 //!
  70 //! // This function wants to log its parameter out prior to doing work with it.
  71 //! fn do_work<T: Any + Debug>(value: &T) {
  72 //!     log(value);
  73 //!     // ...do some other work
  74 //! }
  75 //!
  76 //! fn main() {
  77 //!     let my_string = "Hello World".to_string();
  78 //!     do_work(&my_string);
  79 //!
  80 //!     let my_i8: i8 = 100;
  81 //!     do_work(&my_i8);
  82 //! }
  83 //! ```
  84
  85 #![stable(feature = "rust1", since = "1.0.0")]
  86
  87 use crate::fmt;
  88 use crate::intrinsics;
  89
  90 ///////////////////////////////////////////////////////////////////////////////
  91 // Any trait
  92 ///////////////////////////////////////////////////////////////////////////////
  93
  94 /// A trait to emulate dynamic typing.
  95 ///
  96 /// Most types implement `Any`. However, any type which contains a non-`'static` reference does not.
  97 /// See the [module-level documentation][mod] for more details.
  98 ///
  99 /// [mod]: crate::any
 100 // This trait is not unsafe, though we rely on the specifics of it's sole impl's
 101 // `type_id` function in unsafe code (e.g., `downcast`). Normally, that would be
 102 // a problem, but because the only impl of `Any` is a blanket implementation, no
 103 // other code can implement `Any`.
 104 //
 105 // We could plausibly make this trait unsafe -- it would not cause breakage,
 106 // since we control all the implementations -- but we choose not to as that's
 107 // both not really necessary and may confuse users about the distinction of
 108 // unsafe traits and unsafe methods (i.e., `type_id` would still be safe to call,
 109 // but we would likely want to indicate as such in documentation).
 110 #[stable(feature = "rust1", since = "1.0.0")]
 111 pub trait Any: 'static {
 112     /// Gets the `TypeId` of `self`.
 113     ///
 114     /// # Examples
 115     ///
 116     /// ```
 117     /// use std::any::{Any, TypeId};
 118     ///
 119     /// fn is_string(s: &dyn Any) -> bool {
 120     ///     TypeId::of::<String>() == s.type_id()
 121     /// }
 122     ///
 123     /// assert_eq!(is_string(&0), false);
 124     /// assert_eq!(is_string(&"cookie monster".to_string()), true);
 125     /// ```
 126     #[stable(feature = "get_type_id", since = "1.34.0")]
 127     fn type_id(&self) -> TypeId;
 128 }
 129
 130 #[stable(feature = "rust1", since = "1.0.0")]
 131 impl<T: 'static + ?Sized> Any for T {
 132     fn type_id(&self) -> TypeId {
 133         TypeId::of::<T>()
 134     }
 135 }
 136
 137 ///////////////////////////////////////////////////////////////////////////////
 138 // Extension methods for Any trait objects.
 139 ///////////////////////////////////////////////////////////////////////////////
 140
 141 #[stable(feature = "rust1", since = "1.0.0")]
 142 impl fmt::Debug for dyn Any {
 143     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 144         f.pad("Any")
 145     }
 146 }
 147
 148 // Ensure that the result of e.g., joining a thread can be printed and
 149 // hence used with `unwrap`. May eventually no longer be needed if
 150 // dispatch works with upcasting.
 151 #[stable(feature = "rust1", since = "1.0.0")]
 152 impl fmt::Debug for dyn Any + Send {
 153     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 154         f.pad("Any")
 155     }
 156 }
 157
 158 #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 159 impl fmt::Debug for dyn Any + Send + Sync {
 160     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 161         f.pad("Any")
 162     }
 163 }
 164
 165 impl dyn Any {
 166     /// Returns `true` if the boxed type is the same as `T`.
 167     ///
 168     /// # Examples
 169     ///
 170     /// ```
 171     /// use std::any::Any;
 172     ///
 173     /// fn is_string(s: &dyn Any) {
 174     ///     if s.is::<String>() {
 175     ///         println!("It's a string!");
 176     ///     } else {
 177     ///         println!("Not a string...");
 178     ///     }
 179     /// }
 180     ///
 181     /// is_string(&0);
 182     /// is_string(&"cookie monster".to_string());
 183     /// ```
 184     #[stable(feature = "rust1", since = "1.0.0")]
 185     #[inline]
 186     pub fn is<T: Any>(&self) -> bool {
 187         // Get `TypeId` of the type this function is instantiated with.
 188         let t = TypeId::of::<T>();
 189
 190         // Get `TypeId` of the type in the trait object (`self`).
 191         let concrete = self.type_id();
 192
 193         // Compare both `TypeId`s on equality.
 194         t == concrete
 195     }
 196
 197     /// Returns some reference to the boxed value if it is of type `T`, or
 198     /// `None` if it isn't.
 199     ///
 200     /// # Examples
 201     ///
 202     /// ```
 203     /// use std::any::Any;
 204     ///
 205     /// fn print_if_string(s: &dyn Any) {
 206     ///     if let Some(string) = s.downcast_ref::<String>() {
 207     ///         println!("It's a string({}): '{}'", string.len(), string);
 208     ///     } else {
 209     ///         println!("Not a string...");
 210     ///     }
 211     /// }
 212     ///
 213     /// print_if_string(&0);
 214     /// print_if_string(&"cookie monster".to_string());
 215     /// ```
 216     #[stable(feature = "rust1", since = "1.0.0")]
 217     #[inline]
 218     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 219         if self.is::<T>() {
 220             // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
 221             // that check for memory safety because we have implemented Any for all types; no other
 222             // impls can exist as they would conflict with our impl.
 223             unsafe { Some(&*(self as *const dyn Any as *const T)) }
 224         } else {
 225             None
 226         }
 227     }
 228
 229     /// Returns some mutable reference to the boxed value if it is of type `T`, or
 230     /// `None` if it isn't.
 231     ///
 232     /// # Examples
 233     ///
 234     /// ```
 235     /// use std::any::Any;
 236     ///
 237     /// fn modify_if_u32(s: &mut dyn Any) {
 238     ///     if let Some(num) = s.downcast_mut::<u32>() {
 239     ///         *num = 42;
 240     ///     }
 241     /// }
 242     ///
 243     /// let mut x = 10u32;
 244     /// let mut s = "starlord".to_string();
 245     ///
 246     /// modify_if_u32(&mut x);
 247     /// modify_if_u32(&mut s);
 248     ///
 249     /// assert_eq!(x, 42);
 250     /// assert_eq!(&s, "starlord");
 251     /// ```
 252     #[stable(feature = "rust1", since = "1.0.0")]
 253     #[inline]
 254     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 255         if self.is::<T>() {
 256             // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
 257             // that check for memory safety because we have implemented Any for all types; no other
 258             // impls can exist as they would conflict with our impl.
 259             unsafe { Some(&mut *(self as *mut dyn Any as *mut T)) }
 260         } else {
 261             None
 262         }
 263     }
 264 }
 265
 266 impl dyn Any + Send {
 267     /// Forwards to the method defined on the type `Any`.
 268     ///
 269     /// # Examples
 270     ///
 271     /// ```
 272     /// use std::any::Any;
 273     ///
 274     /// fn is_string(s: &(dyn Any + Send)) {
 275     ///     if s.is::<String>() {
 276     ///         println!("It's a string!");
 277     ///     } else {
 278     ///         println!("Not a string...");
 279     ///     }
 280     /// }
 281     ///
 282     /// is_string(&0);
 283     /// is_string(&"cookie monster".to_string());
 284     /// ```
 285     #[stable(feature = "rust1", since = "1.0.0")]
 286     #[inline]
 287     pub fn is<T: Any>(&self) -> bool {
 288         <dyn Any>::is::<T>(self)
 289     }
 290
 291     /// Forwards to the method defined on the type `Any`.
 292     ///
 293     /// # Examples
 294     ///
 295     /// ```
 296     /// use std::any::Any;
 297     ///
 298     /// fn print_if_string(s: &(dyn Any + Send)) {
 299     ///     if let Some(string) = s.downcast_ref::<String>() {
 300     ///         println!("It's a string({}): '{}'", string.len(), string);
 301     ///     } else {
 302     ///         println!("Not a string...");
 303     ///     }
 304     /// }
 305     ///
 306     /// print_if_string(&0);
 307     /// print_if_string(&"cookie monster".to_string());
 308     /// ```
 309     #[stable(feature = "rust1", since = "1.0.0")]
 310     #[inline]
 311     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 312         <dyn Any>::downcast_ref::<T>(self)
 313     }
 314
 315     /// Forwards to the method defined on the type `Any`.
 316     ///
 317     /// # Examples
 318     ///
 319     /// ```
 320     /// use std::any::Any;
 321     ///
 322     /// fn modify_if_u32(s: &mut (dyn Any + Send)) {
 323     ///     if let Some(num) = s.downcast_mut::<u32>() {
 324     ///         *num = 42;
 325     ///     }
 326     /// }
 327     ///
 328     /// let mut x = 10u32;
 329     /// let mut s = "starlord".to_string();
 330     ///
 331     /// modify_if_u32(&mut x);
 332     /// modify_if_u32(&mut s);
 333     ///
 334     /// assert_eq!(x, 42);
 335     /// assert_eq!(&s, "starlord");
 336     /// ```
 337     #[stable(feature = "rust1", since = "1.0.0")]
 338     #[inline]
 339     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 340         <dyn Any>::downcast_mut::<T>(self)
 341     }
 342 }
 343
 344 impl dyn Any + Send + Sync {
 345     /// Forwards to the method defined on the type `Any`.
 346     ///
 347     /// # Examples
 348     ///
 349     /// ```
 350     /// use std::any::Any;
 351     ///
 352     /// fn is_string(s: &(dyn Any + Send + Sync)) {
 353     ///     if s.is::<String>() {
 354     ///         println!("It's a string!");
 355     ///     } else {
 356     ///         println!("Not a string...");
 357     ///     }
 358     /// }
 359     ///
 360     /// is_string(&0);
 361     /// is_string(&"cookie monster".to_string());
 362     /// ```
 363     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 364     #[inline]
 365     pub fn is<T: Any>(&self) -> bool {
 366         <dyn Any>::is::<T>(self)
 367     }
 368
 369     /// Forwards to the method defined on the type `Any`.
 370     ///
 371     /// # Examples
 372     ///
 373     /// ```
 374     /// use std::any::Any;
 375     ///
 376     /// fn print_if_string(s: &(dyn Any + Send + Sync)) {
 377     ///     if let Some(string) = s.downcast_ref::<String>() {
 378     ///         println!("It's a string({}): '{}'", string.len(), string);
 379     ///     } else {
 380     ///         println!("Not a string...");
 381     ///     }
 382     /// }
 383     ///
 384     /// print_if_string(&0);
 385     /// print_if_string(&"cookie monster".to_string());
 386     /// ```
 387     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 388     #[inline]
 389     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 390         <dyn Any>::downcast_ref::<T>(self)
 391     }
 392
 393     /// Forwards to the method defined on the type `Any`.
 394     ///
 395     /// # Examples
 396     ///
 397     /// ```
 398     /// use std::any::Any;
 399     ///
 400     /// fn modify_if_u32(s: &mut (dyn Any + Send + Sync)) {
 401     ///     if let Some(num) = s.downcast_mut::<u32>() {
 402     ///         *num = 42;
 403     ///     }
 404     /// }
 405     ///
 406     /// let mut x = 10u32;
 407     /// let mut s = "starlord".to_string();
 408     ///
 409     /// modify_if_u32(&mut x);
 410     /// modify_if_u32(&mut s);
 411     ///
 412     /// assert_eq!(x, 42);
 413     /// assert_eq!(&s, "starlord");
 414     /// ```
 415     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 416     #[inline]
 417     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 418         <dyn Any>::downcast_mut::<T>(self)
 419     }
 420 }
 421
 422 ///////////////////////////////////////////////////////////////////////////////
 423 // TypeID and its methods
 424 ///////////////////////////////////////////////////////////////////////////////
 425
 426 /// A `TypeId` represents a globally unique identifier for a type.
 427 ///
 428 /// Each `TypeId` is an opaque object which does not allow inspection of what's
 429 /// inside but does allow basic operations such as cloning, comparison,
 430 /// printing, and showing.
 431 ///
 432 /// A `TypeId` is currently only available for types which ascribe to `'static`,
 433 /// but this limitation may be removed in the future.
 434 ///
 435 /// While `TypeId` implements `Hash`, `PartialOrd`, and `Ord`, it is worth
 436 /// noting that the hashes and ordering will vary between Rust releases. Beware
 437 /// of relying on them inside of your code!
 438 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
 439 #[stable(feature = "rust1", since = "1.0.0")]
 440 pub struct TypeId {
 441     t: u64,
 442 }
 443
 444 impl TypeId {
 445     /// Returns the `TypeId` of the type this generic function has been
 446     /// instantiated with.
 447     ///
 448     /// # Examples
 449     ///
 450     /// ```
 451     /// use std::any::{Any, TypeId};
 452     ///
 453     /// fn is_string<T: ?Sized + Any>(_s: &T) -> bool {
 454     ///     TypeId::of::<String>() == TypeId::of::<T>()
 455     /// }
 456     ///
 457     /// assert_eq!(is_string(&0), false);
 458     /// assert_eq!(is_string(&"cookie monster".to_string()), true);
 459     /// ```
 460     #[stable(feature = "rust1", since = "1.0.0")]
 461     #[rustc_const_unstable(feature = "const_type_id", issue = "77125")]
 462     pub const fn of<T: ?Sized + 'static>() -> TypeId {
 463         TypeId { t: intrinsics::type_id::<T>() }
 464     }
 465 }
 466
 467 /// Returns the name of a type as a string slice.
 468 ///
 469 /// # Note
 470 ///
 471 /// This is intended for diagnostic use. The exact contents and format of the
 472 /// string returned are not specified, other than being a best-effort
 473 /// description of the type. For example, amongst the strings
 474 /// that `type_name::<Option<String>>()` might return are `"Option<String>"` and
 475 /// `"std::option::Option<std::string::String>"`.
 476 ///
 477 /// The returned string must not be considered to be a unique identifier of a
 478 /// type as multiple types may map to the same type name. Similarly, there is no
 479 /// guarantee that all parts of a type will appear in the returned string: for
 480 /// example, lifetime specifiers are currently not included. In addition, the
 481 /// output may change between versions of the compiler.
 482 ///
 483 /// The current implementation uses the same infrastructure as compiler
 484 /// diagnostics and debuginfo, but this is not guaranteed.
 485 ///
 486 /// # Examples
 487 ///
 488 /// ```rust
 489 /// assert_eq!(
 490 ///     std::any::type_name::<Option<String>>(),
 491 ///     "core::option::Option<alloc::string::String>",
 492 /// );
 493 /// ```
 494 #[stable(feature = "type_name", since = "1.38.0")]
 495 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 496 pub const fn type_name<T: ?Sized>() -> &'static str {
 497     intrinsics::type_name::<T>()
 498 }
 499
 500 /// Returns the name of the type of the pointed-to value as a string slice.
 501 /// This is the same as `type_name::<T>()`, but can be used where the type of a
 502 /// variable is not easily available.
 503 ///
 504 /// # Note
 505 ///
 506 /// This is intended for diagnostic use. The exact contents and format of the
 507 /// string are not specified, other than being a best-effort description of the
 508 /// type. For example, `type_name_of_val::<Option<String>>(None)` could return
 509 /// `"Option<String>"` or `"std::option::Option<std::string::String>"`, but not
 510 /// `"foobar"`. In addition, the output may change between versions of the
 511 /// compiler.
 512 ///
 513 /// This function does not resolve trait objects,
 514 /// meaning that `type_name_of_val(&7u32 as &dyn Debug)`
 515 /// may return `"dyn Debug"`, but not `"u32"`.
 516 ///
 517 /// The type name should not be considered a unique identifier of a type;
 518 /// multiple types may share the same type name.
 519 ///
 520 /// The current implementation uses the same infrastructure as compiler
 521 /// diagnostics and debuginfo, but this is not guaranteed.
 522 ///
 523 /// # Examples
 524 ///
 525 /// Prints the default integer and float types.
 526 ///
 527 /// ```rust
 528 /// #![feature(type_name_of_val)]
 529 /// use std::any::type_name_of_val;
 530 ///
 531 /// let x = 1;
 532 /// println!("{}", type_name_of_val(&x));
 533 /// let y = 1.0;
 534 /// println!("{}", type_name_of_val(&y));
 535 /// ```
 536 #[unstable(feature = "type_name_of_val", issue = "66359")]
 537 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 538 pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
 539     type_name::<T>()
 540 }