src/libcore/any.rs

   1 // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! This module implements the `Any` trait, which enables dynamic typing
  12 //! of any `'static` type through runtime reflection.
  13 //!
  14 //! `Any` itself can be used to get a `TypeId`, and has more features when used
  15 //! as a trait object. As `&Any` (a borrowed trait object), it has the `is` and
  16 //! `downcast_ref` methods, to test if the contained value is of a given type,
  17 //! and to get a reference to the inner value as a type. As `&mut Any`, there
  18 //! is also the `downcast_mut` method, for getting a mutable reference to the
  19 //! inner value. `Box<Any>` adds the `downcast` method, which attempts to
  20 //! convert to a `Box<T>`. See the [`Box`] documentation for the full details.
  21 //!
  22 //! Note that &Any is limited to testing whether a value is of a specified
  23 //! concrete type, and cannot be used to test whether a type implements a trait.
  24 //!
  25 //! [`Box`]: ../../std/boxed/struct.Box.html
  26 //!
  27 //! # Examples
  28 //!
  29 //! Consider a situation where we want to log out a value passed to a function.
  30 //! We know the value we're working on implements Debug, but we don't know its
  31 //! concrete type.  We want to give special treatment to certain types: in this
  32 //! case printing out the length of String values prior to their value.
  33 //! We don't know the concrete type of our value at compile time, so we need to
  34 //! use runtime reflection instead.
  35 //!
  36 //! ```rust
  37 //! use std::fmt::Debug;
  38 //! use std::any::Any;
  39 //!
  40 //! // Logger function for any type that implements Debug.
  41 //! fn log<T: Any + Debug>(value: &T) {
  42 //!     let value_any = value as &Any;
  43 //!
  44 //!     // try to convert our value to a String.  If successful, we want to
  45 //!     // output the String's length as well as its value.  If not, it's a
  46 //!     // different type: just print it out unadorned.
  47 //!     match value_any.downcast_ref::<String>() {
  48 //!         Some(as_string) => {
  49 //!             println!("String ({}): {}", as_string.len(), as_string);
  50 //!         }
  51 //!         None => {
  52 //!             println!("{:?}", value);
  53 //!         }
  54 //!     }
  55 //! }
  56 //!
  57 //! // This function wants to log its parameter out prior to doing work with it.
  58 //! fn do_work<T: Any + Debug>(value: &T) {
  59 //!     log(value);
  60 //!     // ...do some other work
  61 //! }
  62 //!
  63 //! fn main() {
  64 //!     let my_string = "Hello World".to_string();
  65 //!     do_work(&my_string);
  66 //!
  67 //!     let my_i8: i8 = 100;
  68 //!     do_work(&my_i8);
  69 //! }
  70 //! ```
  71
  72 #![stable(feature = "rust1", since = "1.0.0")]
  73
  74 use fmt;
  75 use intrinsics;
  76
  77 ///////////////////////////////////////////////////////////////////////////////
  78 // Any trait
  79 ///////////////////////////////////////////////////////////////////////////////
  80
  81 /// A type to emulate dynamic typing.
  82 ///
  83 /// Most types implement `Any`. However, any type which contains a non-`'static` reference does not.
  84 /// See the [module-level documentation][mod] for more details.
  85 ///
  86 /// [mod]: index.html
  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     /// #![feature(get_type_id)]
  95     ///
  96     /// use std::any::{Any, TypeId};
  97     ///
  98     /// fn is_string(s: &Any) -> bool {
  99     ///     TypeId::of::<String>() == s.get_type_id()
 100     /// }
 101     ///
 102     /// fn main() {
 103     ///     assert_eq!(is_string(&0), false);
 104     ///     assert_eq!(is_string(&"cookie monster".to_string()), true);
 105     /// }
 106     /// ```
 107     #[unstable(feature = "get_type_id",
 108                reason = "this method will likely be replaced by an associated static",
 109                issue = "27745")]
 110     fn get_type_id(&self) -> TypeId;
 111 }
 112
 113 #[stable(feature = "rust1", since = "1.0.0")]
 114 impl<T: 'static + ?Sized > Any for T {
 115     fn get_type_id(&self) -> TypeId { TypeId::of::<T>() }
 116 }
 117
 118 ///////////////////////////////////////////////////////////////////////////////
 119 // Extension methods for Any trait objects.
 120 ///////////////////////////////////////////////////////////////////////////////
 121
 122 #[stable(feature = "rust1", since = "1.0.0")]
 123 impl fmt::Debug for Any {
 124     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 125         f.pad("Any")
 126     }
 127 }
 128
 129 // Ensure that the result of e.g. joining a thread can be printed and
 130 // hence used with `unwrap`. May eventually no longer be needed if
 131 // dispatch works with upcasting.
 132 #[stable(feature = "rust1", since = "1.0.0")]
 133 impl fmt::Debug for Any + Send {
 134     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 135         f.pad("Any")
 136     }
 137 }
 138
 139 impl Any {
 140     /// Returns true if the boxed type is the same as `T`.
 141     ///
 142     /// # Examples
 143     ///
 144     /// ```
 145     /// use std::any::Any;
 146     ///
 147     /// fn is_string(s: &Any) {
 148     ///     if s.is::<String>() {
 149     ///         println!("It's a string!");
 150     ///     } else {
 151     ///         println!("Not a string...");
 152     ///     }
 153     /// }
 154     ///
 155     /// fn main() {
 156     ///     is_string(&0);
 157     ///     is_string(&"cookie monster".to_string());
 158     /// }
 159     /// ```
 160     #[stable(feature = "rust1", since = "1.0.0")]
 161     #[inline]
 162     pub fn is<T: Any>(&self) -> bool {
 163         // Get TypeId of the type this function is instantiated with
 164         let t = TypeId::of::<T>();
 165
 166         // Get TypeId of the type in the trait object
 167         let boxed = self.get_type_id();
 168
 169         // Compare both TypeIds on equality
 170         t == boxed
 171     }
 172
 173     /// Returns some reference to the boxed value if it is of type `T`, or
 174     /// `None` if it isn't.
 175     ///
 176     /// # Examples
 177     ///
 178     /// ```
 179     /// use std::any::Any;
 180     ///
 181     /// fn print_if_string(s: &Any) {
 182     ///     if let Some(string) = s.downcast_ref::<String>() {
 183     ///         println!("It's a string({}): '{}'", string.len(), string);
 184     ///     } else {
 185     ///         println!("Not a string...");
 186     ///     }
 187     /// }
 188     ///
 189     /// fn main() {
 190     ///     print_if_string(&0);
 191     ///     print_if_string(&"cookie monster".to_string());
 192     /// }
 193     /// ```
 194     #[stable(feature = "rust1", since = "1.0.0")]
 195     #[inline]
 196     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 197         if self.is::<T>() {
 198             unsafe {
 199                 Some(&*(self as *const Any as *const T))
 200             }
 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 Any) {
 215     ///     if let Some(num) = s.downcast_mut::<u32>() {
 216     ///         *num = 42;
 217     ///     }
 218     /// }
 219     ///
 220     /// fn main() {
 221     ///     let mut x = 10u32;
 222     ///     let mut s = "starlord".to_string();
 223     ///
 224     ///     modify_if_u32(&mut x);
 225     ///     modify_if_u32(&mut s);
 226     ///
 227     ///     assert_eq!(x, 42);
 228     ///     assert_eq!(&s, "starlord");
 229     /// }
 230     /// ```
 231     #[stable(feature = "rust1", since = "1.0.0")]
 232     #[inline]
 233     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 234         if self.is::<T>() {
 235             unsafe {
 236                 Some(&mut *(self as *mut Any as *mut T))
 237             }
 238         } else {
 239             None
 240         }
 241     }
 242 }
 243
 244 impl Any+Send {
 245     /// Forwards to the method defined on the type `Any`.
 246     ///
 247     /// # Examples
 248     ///
 249     /// ```
 250     /// use std::any::Any;
 251     ///
 252     /// fn is_string(s: &(Any + Send)) {
 253     ///     if s.is::<String>() {
 254     ///         println!("It's a string!");
 255     ///     } else {
 256     ///         println!("Not a string...");
 257     ///     }
 258     /// }
 259     ///
 260     /// fn main() {
 261     ///     is_string(&0);
 262     ///     is_string(&"cookie monster".to_string());
 263     /// }
 264     /// ```
 265     #[stable(feature = "rust1", since = "1.0.0")]
 266     #[inline]
 267     pub fn is<T: Any>(&self) -> bool {
 268         Any::is::<T>(self)
 269     }
 270
 271     /// Forwards to the method defined on the type `Any`.
 272     ///
 273     /// # Examples
 274     ///
 275     /// ```
 276     /// use std::any::Any;
 277     ///
 278     /// fn print_if_string(s: &(Any + Send)) {
 279     ///     if let Some(string) = s.downcast_ref::<String>() {
 280     ///         println!("It's a string({}): '{}'", string.len(), string);
 281     ///     } else {
 282     ///         println!("Not a string...");
 283     ///     }
 284     /// }
 285     ///
 286     /// fn main() {
 287     ///     print_if_string(&0);
 288     ///     print_if_string(&"cookie monster".to_string());
 289     /// }
 290     /// ```
 291     #[stable(feature = "rust1", since = "1.0.0")]
 292     #[inline]
 293     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 294         Any::downcast_ref::<T>(self)
 295     }
 296
 297     /// Forwards to the method defined on the type `Any`.
 298     ///
 299     /// # Examples
 300     ///
 301     /// ```
 302     /// use std::any::Any;
 303     ///
 304     /// fn modify_if_u32(s: &mut (Any+ Send)) {
 305     ///     if let Some(num) = s.downcast_mut::<u32>() {
 306     ///         *num = 42;
 307     ///     }
 308     /// }
 309     ///
 310     /// fn main() {
 311     ///     let mut x = 10u32;
 312     ///     let mut s = "starlord".to_string();
 313     ///
 314     ///     modify_if_u32(&mut x);
 315     ///     modify_if_u32(&mut s);
 316     ///
 317     ///     assert_eq!(x, 42);
 318     ///     assert_eq!(&s, "starlord");
 319     /// }
 320     /// ```
 321     #[stable(feature = "rust1", since = "1.0.0")]
 322     #[inline]
 323     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 324         Any::downcast_mut::<T>(self)
 325     }
 326 }
 327
 328
 329 ///////////////////////////////////////////////////////////////////////////////
 330 // TypeID and its methods
 331 ///////////////////////////////////////////////////////////////////////////////
 332
 333 /// A `TypeId` represents a globally unique identifier for a type.
 334 ///
 335 /// Each `TypeId` is an opaque object which does not allow inspection of what's
 336 /// inside but does allow basic operations such as cloning, comparison,
 337 /// printing, and showing.
 338 ///
 339 /// A `TypeId` is currently only available for types which ascribe to `'static`,
 340 /// but this limitation may be removed in the future.
 341 #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 342 #[stable(feature = "rust1", since = "1.0.0")]
 343 pub struct TypeId {
 344     t: u64,
 345 }
 346
 347 impl TypeId {
 348     /// Returns the `TypeId` of the type this generic function has been
 349     /// instantiated with.
 350     ///
 351     /// # Examples
 352     ///
 353     /// ```
 354     /// use std::any::{Any, TypeId};
 355     ///
 356     /// fn is_string<T: ?Sized + Any>(_s: &T) -> bool {
 357     ///     TypeId::of::<String>() == TypeId::of::<T>()
 358     /// }
 359     ///
 360     /// fn main() {
 361     ///     assert_eq!(is_string(&0), false);
 362     ///     assert_eq!(is_string(&"cookie monster".to_string()), true);
 363     /// }
 364     /// ```
 365     #[stable(feature = "rust1", since = "1.0.0")]
 366     pub fn of<T: ?Sized + 'static>() -> TypeId {
 367         TypeId {
 368             t: unsafe { intrinsics::type_id::<T>() },
 369         }
 370     }
 371 }