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 //! # Examples
  18 //!
  19 //! Consider a situation where we want to log out a value passed to a function.
  20 //! We know the value we're working on implements Debug, but we don't know its
  21 //! concrete type. We want to give special treatment to certain types: in this
  22 //! case printing out the length of String values prior to their value.
  23 //! We don't know the concrete type of our value at compile time, so we need to
  24 //! use runtime reflection instead.
  25 //!
  26 //! ```rust
  27 //! use std::fmt::Debug;
  28 //! use std::any::Any;
  29 //!
  30 //! // Logger function for any type that implements Debug.
  31 //! fn log<T: Any + Debug>(value: &T) {
  32 //!     let value_any = value as &dyn Any;
  33 //!
  34 //!     // Try to convert our value to a `String`. If successful, we want to
  35 //!     // output the String`'s length as well as its value. If not, it's a
  36 //!     // different type: just print it out unadorned.
  37 //!     match value_any.downcast_ref::<String>() {
  38 //!         Some(as_string) => {
  39 //!             println!("String ({}): {}", as_string.len(), as_string);
  40 //!         }
  41 //!         None => {
  42 //!             println!("{:?}", value);
  43 //!         }
  44 //!     }
  45 //! }
  46 //!
  47 //! // This function wants to log its parameter out prior to doing work with it.
  48 //! fn do_work<T: Any + Debug>(value: &T) {
  49 //!     log(value);
  50 //!     // ...do some other work
  51 //! }
  52 //!
  53 //! fn main() {
  54 //!     let my_string = "Hello World".to_string();
  55 //!     do_work(&my_string);
  56 //!
  57 //!     let my_i8: i8 = 100;
  58 //!     do_work(&my_i8);
  59 //! }
  60 //! ```
  61
  62 #![stable(feature = "rust1", since = "1.0.0")]
  63
  64 use crate::fmt;
  65 use crate::intrinsics;
  66
  67 ///////////////////////////////////////////////////////////////////////////////
  68 // Any trait
  69 ///////////////////////////////////////////////////////////////////////////////
  70
  71 /// A trait to emulate dynamic typing.
  72 ///
  73 /// Most types implement `Any`. However, any type which contains a non-`'static` reference does not.
  74 /// See the [module-level documentation][mod] for more details.
  75 ///
  76 /// [mod]: crate::any
  77 // This trait is not unsafe, though we rely on the specifics of it's sole impl's
  78 // `type_id` function in unsafe code (e.g., `downcast`). Normally, that would be
  79 // a problem, but because the only impl of `Any` is a blanket implementation, no
  80 // other code can implement `Any`.
  81 //
  82 // We could plausibly make this trait unsafe -- it would not cause breakage,
  83 // since we control all the implementations -- but we choose not to as that's
  84 // both not really necessary and may confuse users about the distinction of
  85 // unsafe traits and unsafe methods (i.e., `type_id` would still be safe to call,
  86 // but we would likely want to indicate as such in documentation).
  87 #[stable(feature = "rust1", since = "1.0.0")]
  88 pub trait Any: 'static {
  89     /// Gets the `TypeId` of `self`.
  90     ///
  91     /// # Examples
  92     ///
  93     /// ```
  94     /// use std::any::{Any, TypeId};
  95     ///
  96     /// fn is_string(s: &dyn Any) -> bool {
  97     ///     TypeId::of::<String>() == s.type_id()
  98     /// }
  99     ///
 100     /// assert_eq!(is_string(&0), false);
 101     /// assert_eq!(is_string(&"cookie monster".to_string()), true);
 102     /// ```
 103     #[stable(feature = "get_type_id", since = "1.34.0")]
 104     fn type_id(&self) -> TypeId;
 105 }
 106
 107 #[stable(feature = "rust1", since = "1.0.0")]
 108 impl<T: 'static + ?Sized> Any for T {
 109     fn type_id(&self) -> TypeId {
 110         TypeId::of::<T>()
 111     }
 112 }
 113
 114 ///////////////////////////////////////////////////////////////////////////////
 115 // Extension methods for Any trait objects.
 116 ///////////////////////////////////////////////////////////////////////////////
 117
 118 #[stable(feature = "rust1", since = "1.0.0")]
 119 impl fmt::Debug for dyn Any {
 120     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 121         f.pad("Any")
 122     }
 123 }
 124
 125 // Ensure that the result of e.g., joining a thread can be printed and
 126 // hence used with `unwrap`. May eventually no longer be needed if
 127 // dispatch works with upcasting.
 128 #[stable(feature = "rust1", since = "1.0.0")]
 129 impl fmt::Debug for dyn Any + Send {
 130     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 131         f.pad("Any")
 132     }
 133 }
 134
 135 #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 136 impl fmt::Debug for dyn Any + Send + Sync {
 137     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 138         f.pad("Any")
 139     }
 140 }
 141
 142 impl dyn Any {
 143     /// Returns `true` if the boxed type is the same as `T`.
 144     ///
 145     /// # Examples
 146     ///
 147     /// ```
 148     /// use std::any::Any;
 149     ///
 150     /// fn is_string(s: &dyn Any) {
 151     ///     if s.is::<String>() {
 152     ///         println!("It's a string!");
 153     ///     } else {
 154     ///         println!("Not a string...");
 155     ///     }
 156     /// }
 157     ///
 158     /// is_string(&0);
 159     /// is_string(&"cookie monster".to_string());
 160     /// ```
 161     #[stable(feature = "rust1", since = "1.0.0")]
 162     #[inline]
 163     pub fn is<T: Any>(&self) -> bool {
 164         // Get `TypeId` of the type this function is instantiated with.
 165         let t = TypeId::of::<T>();
 166
 167         // Get `TypeId` of the type in the trait object (`self`).
 168         let concrete = self.type_id();
 169
 170         // Compare both `TypeId`s on equality.
 171         t == concrete
 172     }
 173
 174     /// Returns some reference to the boxed value if it is of type `T`, or
 175     /// `None` if it isn't.
 176     ///
 177     /// # Examples
 178     ///
 179     /// ```
 180     /// use std::any::Any;
 181     ///
 182     /// fn print_if_string(s: &dyn Any) {
 183     ///     if let Some(string) = s.downcast_ref::<String>() {
 184     ///         println!("It's a string({}): '{}'", string.len(), string);
 185     ///     } else {
 186     ///         println!("Not a string...");
 187     ///     }
 188     /// }
 189     ///
 190     /// print_if_string(&0);
 191     /// print_if_string(&"cookie monster".to_string());
 192     /// ```
 193     #[stable(feature = "rust1", since = "1.0.0")]
 194     #[inline]
 195     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 196         if self.is::<T>() {
 197             // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
 198             // that check for memory safety because we have implemented Any for all types; no other
 199             // impls can exist as they would conflict with our impl.
 200             unsafe { Some(&*(self as *const dyn Any as *const T)) }
 201         } else {
 202             None
 203         }
 204     }
 205
 206     /// Returns some mutable reference to the boxed value if it is of type `T`, or
 207     /// `None` if it isn't.
 208     ///
 209     /// # Examples
 210     ///
 211     /// ```
 212     /// use std::any::Any;
 213     ///
 214     /// fn modify_if_u32(s: &mut dyn Any) {
 215     ///     if let Some(num) = s.downcast_mut::<u32>() {
 216     ///         *num = 42;
 217     ///     }
 218     /// }
 219     ///
 220     /// let mut x = 10u32;
 221     /// let mut s = "starlord".to_string();
 222     ///
 223     /// modify_if_u32(&mut x);
 224     /// modify_if_u32(&mut s);
 225     ///
 226     /// assert_eq!(x, 42);
 227     /// assert_eq!(&s, "starlord");
 228     /// ```
 229     #[stable(feature = "rust1", since = "1.0.0")]
 230     #[inline]
 231     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 232         if self.is::<T>() {
 233             // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
 234             // that check for memory safety because we have implemented Any for all types; no other
 235             // impls can exist as they would conflict with our impl.
 236             unsafe { Some(&mut *(self as *mut dyn Any as *mut T)) }
 237         } else {
 238             None
 239         }
 240     }
 241 }
 242
 243 impl dyn Any + Send {
 244     /// Forwards to the method defined on the type `Any`.
 245     ///
 246     /// # Examples
 247     ///
 248     /// ```
 249     /// use std::any::Any;
 250     ///
 251     /// fn is_string(s: &(dyn Any + Send)) {
 252     ///     if s.is::<String>() {
 253     ///         println!("It's a string!");
 254     ///     } else {
 255     ///         println!("Not a string...");
 256     ///     }
 257     /// }
 258     ///
 259     /// is_string(&0);
 260     /// is_string(&"cookie monster".to_string());
 261     /// ```
 262     #[stable(feature = "rust1", since = "1.0.0")]
 263     #[inline]
 264     pub fn is<T: Any>(&self) -> bool {
 265         Any::is::<T>(self)
 266     }
 267
 268     /// Forwards to the method defined on the type `Any`.
 269     ///
 270     /// # Examples
 271     ///
 272     /// ```
 273     /// use std::any::Any;
 274     ///
 275     /// fn print_if_string(s: &(dyn Any + Send)) {
 276     ///     if let Some(string) = s.downcast_ref::<String>() {
 277     ///         println!("It's a string({}): '{}'", string.len(), string);
 278     ///     } else {
 279     ///         println!("Not a string...");
 280     ///     }
 281     /// }
 282     ///
 283     /// print_if_string(&0);
 284     /// print_if_string(&"cookie monster".to_string());
 285     /// ```
 286     #[stable(feature = "rust1", since = "1.0.0")]
 287     #[inline]
 288     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 289         Any::downcast_ref::<T>(self)
 290     }
 291
 292     /// Forwards to the method defined on the type `Any`.
 293     ///
 294     /// # Examples
 295     ///
 296     /// ```
 297     /// use std::any::Any;
 298     ///
 299     /// fn modify_if_u32(s: &mut (dyn Any + Send)) {
 300     ///     if let Some(num) = s.downcast_mut::<u32>() {
 301     ///         *num = 42;
 302     ///     }
 303     /// }
 304     ///
 305     /// let mut x = 10u32;
 306     /// let mut s = "starlord".to_string();
 307     ///
 308     /// modify_if_u32(&mut x);
 309     /// modify_if_u32(&mut s);
 310     ///
 311     /// assert_eq!(x, 42);
 312     /// assert_eq!(&s, "starlord");
 313     /// ```
 314     #[stable(feature = "rust1", since = "1.0.0")]
 315     #[inline]
 316     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 317         Any::downcast_mut::<T>(self)
 318     }
 319 }
 320
 321 impl dyn Any + Send + Sync {
 322     /// Forwards to the method defined on the type `Any`.
 323     ///
 324     /// # Examples
 325     ///
 326     /// ```
 327     /// use std::any::Any;
 328     ///
 329     /// fn is_string(s: &(dyn Any + Send + Sync)) {
 330     ///     if s.is::<String>() {
 331     ///         println!("It's a string!");
 332     ///     } else {
 333     ///         println!("Not a string...");
 334     ///     }
 335     /// }
 336     ///
 337     /// is_string(&0);
 338     /// is_string(&"cookie monster".to_string());
 339     /// ```
 340     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 341     #[inline]
 342     pub fn is<T: Any>(&self) -> bool {
 343         Any::is::<T>(self)
 344     }
 345
 346     /// Forwards to the method defined on the type `Any`.
 347     ///
 348     /// # Examples
 349     ///
 350     /// ```
 351     /// use std::any::Any;
 352     ///
 353     /// fn print_if_string(s: &(dyn Any + Send + Sync)) {
 354     ///     if let Some(string) = s.downcast_ref::<String>() {
 355     ///         println!("It's a string({}): '{}'", string.len(), string);
 356     ///     } else {
 357     ///         println!("Not a string...");
 358     ///     }
 359     /// }
 360     ///
 361     /// print_if_string(&0);
 362     /// print_if_string(&"cookie monster".to_string());
 363     /// ```
 364     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 365     #[inline]
 366     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 367         Any::downcast_ref::<T>(self)
 368     }
 369
 370     /// Forwards to the method defined on the type `Any`.
 371     ///
 372     /// # Examples
 373     ///
 374     /// ```
 375     /// use std::any::Any;
 376     ///
 377     /// fn modify_if_u32(s: &mut (dyn Any + Send + Sync)) {
 378     ///     if let Some(num) = s.downcast_mut::<u32>() {
 379     ///         *num = 42;
 380     ///     }
 381     /// }
 382     ///
 383     /// let mut x = 10u32;
 384     /// let mut s = "starlord".to_string();
 385     ///
 386     /// modify_if_u32(&mut x);
 387     /// modify_if_u32(&mut s);
 388     ///
 389     /// assert_eq!(x, 42);
 390     /// assert_eq!(&s, "starlord");
 391     /// ```
 392     #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
 393     #[inline]
 394     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 395         Any::downcast_mut::<T>(self)
 396     }
 397 }
 398
 399 ///////////////////////////////////////////////////////////////////////////////
 400 // TypeID and its methods
 401 ///////////////////////////////////////////////////////////////////////////////
 402
 403 /// A `TypeId` represents a globally unique identifier for a type.
 404 ///
 405 /// Each `TypeId` is an opaque object which does not allow inspection of what's
 406 /// inside but does allow basic operations such as cloning, comparison,
 407 /// printing, and showing.
 408 ///
 409 /// A `TypeId` is currently only available for types which ascribe to `'static`,
 410 /// but this limitation may be removed in the future.
 411 ///
 412 /// While `TypeId` implements `Hash`, `PartialOrd`, and `Ord`, it is worth
 413 /// noting that the hashes and ordering will vary between Rust releases. Beware
 414 /// of relying on them inside of your code!
 415 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
 416 #[stable(feature = "rust1", since = "1.0.0")]
 417 pub struct TypeId {
 418     t: u64,
 419 }
 420
 421 impl TypeId {
 422     /// Returns the `TypeId` of the type this generic function has been
 423     /// instantiated with.
 424     ///
 425     /// # Examples
 426     ///
 427     /// ```
 428     /// use std::any::{Any, TypeId};
 429     ///
 430     /// fn is_string<T: ?Sized + Any>(_s: &T) -> bool {
 431     ///     TypeId::of::<String>() == TypeId::of::<T>()
 432     /// }
 433     ///
 434     /// assert_eq!(is_string(&0), false);
 435     /// assert_eq!(is_string(&"cookie monster".to_string()), true);
 436     /// ```
 437     #[stable(feature = "rust1", since = "1.0.0")]
 438     #[rustc_const_stable(feature = "const_type_id", since = "1.46.0")]
 439     pub const fn of<T: ?Sized + 'static>() -> TypeId {
 440         TypeId { t: intrinsics::type_id::<T>() }
 441     }
 442 }
 443
 444 /// Returns the name of a type as a string slice.
 445 ///
 446 /// # Note
 447 ///
 448 /// This is intended for diagnostic use. The exact contents and format of the
 449 /// string returned are not specified, other than being a best-effort
 450 /// description of the type. For example, amongst the strings
 451 /// that `type_name::<Option<String>>()` might return are `"Option<String>"` and
 452 /// `"std::option::Option<std::string::String>"`.
 453 ///
 454 /// The returned string must not be considered to be a unique identifier of a
 455 /// type as multiple types may map to the same type name. Similarly, there is no
 456 /// guarantee that all parts of a type will appear in the returned string: for
 457 /// example, lifetime specifiers are currently not included. In addition, the
 458 /// output may change between versions of the compiler.
 459 ///
 460 /// The current implementation uses the same infrastructure as compiler
 461 /// diagnostics and debuginfo, but this is not guaranteed.
 462 ///
 463 /// # Examples
 464 ///
 465 /// ```rust
 466 /// assert_eq!(
 467 ///     std::any::type_name::<Option<String>>(),
 468 ///     "core::option::Option<alloc::string::String>",
 469 /// );
 470 /// ```
 471 #[stable(feature = "type_name", since = "1.38.0")]
 472 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 473 pub const fn type_name<T: ?Sized>() -> &'static str {
 474     intrinsics::type_name::<T>()
 475 }
 476
 477 /// Returns the name of the type of the pointed-to value as a string slice.
 478 /// This is the same as `type_name::<T>()`, but can be used where the type of a
 479 /// variable is not easily available.
 480 ///
 481 /// # Note
 482 ///
 483 /// This is intended for diagnostic use. The exact contents and format of the
 484 /// string are not specified, other than being a best-effort description of the
 485 /// type. For example, `type_name_of_val::<Option<String>>(None)` could return
 486 /// `"Option<String>"` or `"std::option::Option<std::string::String>"`, but not
 487 /// `"foobar"`. In addition, the output may change between versions of the
 488 /// compiler.
 489 ///
 490 /// This function does not resolve trait objects,
 491 /// meaning that `type_name_of_val(&7u32 as &dyn Debug)`
 492 /// may return `"dyn Debug"`, but not `"u32"`.
 493 ///
 494 /// The type name should not be considered a unique identifier of a type;
 495 /// multiple types may share the same type name.
 496 ///
 497 /// The current implementation uses the same infrastructure as compiler
 498 /// diagnostics and debuginfo, but this is not guaranteed.
 499 ///
 500 /// # Examples
 501 ///
 502 /// Prints the default integer and float types.
 503 ///
 504 /// ```rust
 505 /// #![feature(type_name_of_val)]
 506 /// use std::any::type_name_of_val;
 507 ///
 508 /// let x = 1;
 509 /// println!("{}", type_name_of_val(&x));
 510 /// let y = 1.0;
 511 /// println!("{}", type_name_of_val(&y));
 512 /// ```
 513 #[unstable(feature = "type_name_of_val", issue = "66359")]
 514 #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
 515 pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
 516     type_name::<T>()
 517 }