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 marker::Send;
  76 use mem::transmute;
  77 use option::Option::{self, Some, None};
  78 use raw::TraitObject;
  79 use intrinsics;
  80 use marker::{Reflect, Sized};
  81
  82 ///////////////////////////////////////////////////////////////////////////////
  83 // Any trait
  84 ///////////////////////////////////////////////////////////////////////////////
  85
  86 /// A type to emulate dynamic typing.
  87 ///
  88 /// Most types implement `Any`. However, any type which contains a non-`'static` reference does not.
  89 /// See the [module-level documentation][mod] for more details.
  90 ///
  91 /// [mod]: index.html
  92 #[stable(feature = "rust1", since = "1.0.0")]
  93 pub trait Any: Reflect + 'static {
  94     /// Gets the `TypeId` of `self`.
  95     ///
  96     /// # Examples
  97     ///
  98     /// ```
  99     /// #![feature(get_type_id)]
 100     ///
 101     /// use std::any::{Any, TypeId};
 102     ///
 103     /// fn is_string(s: &Any) -> bool {
 104     ///     TypeId::of::<String>() == s.get_type_id()
 105     /// }
 106     ///
 107     /// fn main() {
 108     ///     assert_eq!(is_string(&0), false);
 109     ///     assert_eq!(is_string(&"cookie monster".to_owned()), true);
 110     /// }
 111     /// ```
 112     #[unstable(feature = "get_type_id",
 113                reason = "this method will likely be replaced by an associated static",
 114                issue = "27745")]
 115     fn get_type_id(&self) -> TypeId;
 116 }
 117
 118 #[stable(feature = "rust1", since = "1.0.0")]
 119 impl<T: Reflect + 'static + ?Sized > Any for T {
 120     fn get_type_id(&self) -> TypeId { TypeId::of::<T>() }
 121 }
 122
 123 ///////////////////////////////////////////////////////////////////////////////
 124 // Extension methods for Any trait objects.
 125 ///////////////////////////////////////////////////////////////////////////////
 126
 127 #[stable(feature = "rust1", since = "1.0.0")]
 128 impl fmt::Debug for Any {
 129     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 130         f.pad("Any")
 131     }
 132 }
 133
 134 // Ensure that the result of e.g. joining a thread can be printed and
 135 // hence used with `unwrap`. May eventually no longer be needed if
 136 // dispatch works with upcasting.
 137 #[stable(feature = "rust1", since = "1.0.0")]
 138 impl fmt::Debug for Any + Send {
 139     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 140         f.pad("Any")
 141     }
 142 }
 143
 144 impl Any {
 145     /// Returns true if the boxed type is the same as `T`.
 146     ///
 147     /// # Examples
 148     ///
 149     /// ```
 150     /// use std::any::Any;
 151     ///
 152     /// fn is_string(s: &Any) {
 153     ///     if s.is::<String>() {
 154     ///         println!("It's a string!");
 155     ///     } else {
 156     ///         println!("Not a string...");
 157     ///     }
 158     /// }
 159     ///
 160     /// fn main() {
 161     ///     is_string(&0);
 162     ///     is_string(&"cookie monster".to_owned());
 163     /// }
 164     /// ```
 165     #[stable(feature = "rust1", since = "1.0.0")]
 166     #[inline]
 167     pub fn is<T: Any>(&self) -> bool {
 168         // Get TypeId of the type this function is instantiated with
 169         let t = TypeId::of::<T>();
 170
 171         // Get TypeId of the type in the trait object
 172         let boxed = self.get_type_id();
 173
 174         // Compare both TypeIds on equality
 175         t == boxed
 176     }
 177
 178     /// Returns some reference to the boxed value if it is of type `T`, or
 179     /// `None` if it isn't.
 180     ///
 181     /// # Examples
 182     ///
 183     /// ```
 184     /// use std::any::Any;
 185     ///
 186     /// fn print_if_string(s: &Any) {
 187     ///     if let Some(string) = s.downcast_ref::<String>() {
 188     ///         println!("It's a string({}): '{}'", string.len(), string);
 189     ///     } else {
 190     ///         println!("Not a string...");
 191     ///     }
 192     /// }
 193     ///
 194     /// fn main() {
 195     ///     print_if_string(&0);
 196     ///     print_if_string(&"cookie monster".to_owned());
 197     /// }
 198     /// ```
 199     #[stable(feature = "rust1", since = "1.0.0")]
 200     #[inline]
 201     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 202         if self.is::<T>() {
 203             unsafe {
 204                 // Get the raw representation of the trait object
 205                 let to: TraitObject = transmute(self);
 206
 207                 // Extract the data pointer
 208                 Some(&*(to.data as *const T))
 209             }
 210         } else {
 211             None
 212         }
 213     }
 214
 215     /// Returns some mutable reference to the boxed value if it is of type `T`, or
 216     /// `None` if it isn't.
 217     ///
 218     /// # Examples
 219     ///
 220     /// ```
 221     /// use std::any::Any;
 222     ///
 223     /// fn modify_if_u32(s: &mut Any) {
 224     ///     if let Some(num) = s.downcast_mut::<u32>() {
 225     ///         *num = 42;
 226     ///     }
 227     /// }
 228     ///
 229     /// fn main() {
 230     ///     let mut x = 10u32;
 231     ///     let mut s = "starlord".to_owned();
 232     ///
 233     ///     modify_if_u32(&mut x);
 234     ///     modify_if_u32(&mut s);
 235     ///
 236     ///     assert_eq!(x, 42);
 237     ///     assert_eq!(&s, "starlord");
 238     /// }
 239     /// ```
 240     #[stable(feature = "rust1", since = "1.0.0")]
 241     #[inline]
 242     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 243         if self.is::<T>() {
 244             unsafe {
 245                 // Get the raw representation of the trait object
 246                 let to: TraitObject = transmute(self);
 247
 248                 // Extract the data pointer
 249                 Some(&mut *(to.data as *const T as *mut T))
 250             }
 251         } else {
 252             None
 253         }
 254     }
 255 }
 256
 257 impl Any+Send {
 258     /// Forwards to the method defined on the type `Any`.
 259     ///
 260     /// # Examples
 261     ///
 262     /// ```
 263     /// use std::any::Any;
 264     ///
 265     /// fn is_string(s: &(Any + Send)) {
 266     ///     if s.is::<String>() {
 267     ///         println!("It's a string!");
 268     ///     } else {
 269     ///         println!("Not a string...");
 270     ///     }
 271     /// }
 272     ///
 273     /// fn main() {
 274     ///     is_string(&0);
 275     ///     is_string(&"cookie monster".to_owned());
 276     /// }
 277     /// ```
 278     #[stable(feature = "rust1", since = "1.0.0")]
 279     #[inline]
 280     pub fn is<T: Any>(&self) -> bool {
 281         Any::is::<T>(self)
 282     }
 283
 284     /// Forwards to the method defined on the type `Any`.
 285     ///
 286     /// # Examples
 287     ///
 288     /// ```
 289     /// use std::any::Any;
 290     ///
 291     /// fn print_if_string(s: &(Any + Send)) {
 292     ///     if let Some(string) = s.downcast_ref::<String>() {
 293     ///         println!("It's a string({}): '{}'", string.len(), string);
 294     ///     } else {
 295     ///         println!("Not a string...");
 296     ///     }
 297     /// }
 298     ///
 299     /// fn main() {
 300     ///     print_if_string(&0);
 301     ///     print_if_string(&"cookie monster".to_owned());
 302     /// }
 303     /// ```
 304     #[stable(feature = "rust1", since = "1.0.0")]
 305     #[inline]
 306     pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
 307         Any::downcast_ref::<T>(self)
 308     }
 309
 310     /// Forwards to the method defined on the type `Any`.
 311     ///
 312     /// # Examples
 313     ///
 314     /// ```
 315     /// use std::any::Any;
 316     ///
 317     /// fn modify_if_u32(s: &mut (Any+ Send)) {
 318     ///     if let Some(num) = s.downcast_mut::<u32>() {
 319     ///         *num = 42;
 320     ///     }
 321     /// }
 322     ///
 323     /// fn main() {
 324     ///     let mut x = 10u32;
 325     ///     let mut s = "starlord".to_owned();
 326     ///
 327     ///     modify_if_u32(&mut x);
 328     ///     modify_if_u32(&mut s);
 329     ///
 330     ///     assert_eq!(x, 42);
 331     ///     assert_eq!(&s, "starlord");
 332     /// }
 333     /// ```
 334     #[stable(feature = "rust1", since = "1.0.0")]
 335     #[inline]
 336     pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
 337         Any::downcast_mut::<T>(self)
 338     }
 339 }
 340
 341
 342 ///////////////////////////////////////////////////////////////////////////////
 343 // TypeID and its methods
 344 ///////////////////////////////////////////////////////////////////////////////
 345
 346 /// A `TypeId` represents a globally unique identifier for a type.
 347 ///
 348 /// Each `TypeId` is an opaque object which does not allow inspection of what's
 349 /// inside but does allow basic operations such as cloning, comparison,
 350 /// printing, and showing.
 351 ///
 352 /// A `TypeId` is currently only available for types which ascribe to `'static`,
 353 /// but this limitation may be removed in the future.
 354 #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
 355 #[stable(feature = "rust1", since = "1.0.0")]
 356 pub struct TypeId {
 357     t: u64,
 358 }
 359
 360 impl TypeId {
 361     /// Returns the `TypeId` of the type this generic function has been
 362     /// instantiated with.
 363     ///
 364     /// # Examples
 365     ///
 366     /// ```
 367     /// #![feature(get_type_id)]
 368     ///
 369     /// use std::any::{Any, TypeId};
 370     ///
 371     /// fn is_string(s: &Any) -> bool {
 372     ///     TypeId::of::<String>() == s.get_type_id()
 373     /// }
 374     ///
 375     /// fn main() {
 376     ///     assert_eq!(is_string(&0), false);
 377     ///     assert_eq!(is_string(&"cookie monster".to_owned()), true);
 378     /// }
 379     /// ```
 380     #[stable(feature = "rust1", since = "1.0.0")]
 381     pub fn of<T: ?Sized + Reflect + 'static>() -> TypeId {
 382         TypeId {
 383             t: unsafe { intrinsics::type_id::<T>() },
 384         }
 385     }
 386 }