any.rs.html -- source
Description: Source to the Rust file `src/libcore/any.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2013-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! This module implements the `Any` trait, which enables dynamic typing
    //! of any `'static` type through runtime reflection.
    //!
    //! `Any` itself can be used to get a `TypeId`, and has more features when used
    //! as a trait object. As `&Any` (a borrowed trait object), it has the `is` and
    //! `downcast_ref` methods, to test if the contained value is of a given type,
    //! and to get a reference to the inner value as a type. As `&mut Any`, there
    //! is also the `downcast_mut` method, for getting a mutable reference to the
    //! inner value. `Box<Any>` adds the `downcast` method, which attempts to
    //! convert to a `Box<T>`. See the [`Box`] documentation for the full details.
    //!
    //! Note that &Any is limited to testing whether a value is of a specified
    //! concrete type, and cannot be used to test whether a type implements a trait.
    //!
    //! [`Box`]: ../../std/boxed/struct.Box.html
    //!
    //! # Examples
    //!
    //! Consider a situation where we want to log out a value passed to a function.
    //! We know the value we're working on implements Debug, but we don't know its
    //! concrete type.  We want to give special treatment to certain types: in this
    //! case printing out the length of String values prior to their value.
    //! We don't know the concrete type of our value at compile time, so we need to
    //! use runtime reflection instead.
    //!
    //! ```rust
    //! use std::fmt::Debug;
    //! use std::any::Any;
    //!
    //! // Logger function for any type that implements Debug.
    //! fn log<T: Any + Debug>(value: &T) {
    //!     let value_any = value as &Any;
    //!
    //!     // try to convert our value to a String.  If successful, we want to
    //!     // output the String's length as well as its value.  If not, it's a
    //!     // different type: just print it out unadorned.
    //!     match value_any.downcast_ref::<String>() {
    //!         Some(as_string) => {
    //!             println!("String ({}): {}", as_string.len(), as_string);
    //!         }
    //!         None => {
    //!             println!("{:?}", value);
    //!         }
    //!     }
    //! }
    //!
    //! // This function wants to log its parameter out prior to doing work with it.
    //! fn do_work<T: Any + Debug>(value: &T) {
    //!     log(value);
    //!     // ...do some other work
    //! }
    //!
    //! fn main() {
    //!     let my_string = "Hello World".to_string();
    //!     do_work(&my_string);
    //!
    //!     let my_i8: i8 = 100;
    //!     do_work(&my_i8);
    //! }
    //! ```

    #![stable(feature = "rust1", since = "1.0.0")]

    use fmt;
    use intrinsics;

    ///////////////////////////////////////////////////////////////////////////////
    // Any trait
    ///////////////////////////////////////////////////////////////////////////////

    /// A type to emulate dynamic typing.
    ///
    /// Most types implement `Any`. However, any type which contains a non-`'static` reference does not.
    /// See the [module-level documentation][mod] for more details.
    ///
    /// [mod]: index.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Any: 'static {
        /// Gets the `TypeId` of `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(get_type_id)]
        ///
        /// use std::any::{Any, TypeId};
        ///
        /// fn is_string(s: &Any) -> bool {
        ///     TypeId::of::<String>() == s.get_type_id()
        /// }
        ///
        /// fn main() {
        ///     assert_eq!(is_string(&0), false);
        ///     assert_eq!(is_string(&"cookie monster".to_string()), true);
        /// }
        /// ```
        #[unstable(feature = "get_type_id",
                   reason = "this method will likely be replaced by an associated static",
                   issue = "27745")]
        fn get_type_id(&self) -> TypeId;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: 'static + ?Sized > Any for T {
        fn get_type_id(&self) -> TypeId { TypeId::of::<T>() }
    }

    ///////////////////////////////////////////////////////////////////////////////
    // Extension methods for Any trait objects.
    ///////////////////////////////////////////////////////////////////////////////

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Debug for Any {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Any")
        }
    }

    // Ensure that the result of e.g. joining a thread can be printed and
    // hence used with `unwrap`. May eventually no longer be needed if
    // dispatch works with upcasting.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Debug for Any + Send {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("Any")
        }
    }

    impl Any {
        /// Returns true if the boxed type is the same as `T`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn is_string(s: &Any) {
        ///     if s.is::<String>() {
        ///         println!("It's a string!");
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// fn main() {
        ///     is_string(&0);
        ///     is_string(&"cookie monster".to_string());
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn is<T: Any>(&self) -> bool {
            // Get TypeId of the type this function is instantiated with
            let t = TypeId::of::<T>();

            // Get TypeId of the type in the trait object
            let boxed = self.get_type_id();

            // Compare both TypeIds on equality
            t == boxed
        }

        /// Returns some reference to the boxed value if it is of type `T`, or
        /// `None` if it isn't.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn print_if_string(s: &Any) {
        ///     if let Some(string) = s.downcast_ref::<String>() {
        ///         println!("It's a string({}): '{}'", string.len(), string);
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// fn main() {
        ///     print_if_string(&0);
        ///     print_if_string(&"cookie monster".to_string());
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
            if self.is::<T>() {
                unsafe {
                    Some(&*(self as *const Any as *const T))
                }
            } else {
                None
            }
        }

        /// Returns some mutable reference to the boxed value if it is of type `T`, or
        /// `None` if it isn't.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn modify_if_u32(s: &mut Any) {
        ///     if let Some(num) = s.downcast_mut::<u32>() {
        ///         *num = 42;
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let mut x = 10u32;
        ///     let mut s = "starlord".to_string();
        ///
        ///     modify_if_u32(&mut x);
        ///     modify_if_u32(&mut s);
        ///
        ///     assert_eq!(x, 42);
        ///     assert_eq!(&s, "starlord");
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
            if self.is::<T>() {
                unsafe {
                    Some(&mut *(self as *mut Any as *mut T))
                }
            } else {
                None
            }
        }
    }

    impl Any+Send {
        /// Forwards to the method defined on the type `Any`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn is_string(s: &(Any + Send)) {
        ///     if s.is::<String>() {
        ///         println!("It's a string!");
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// fn main() {
        ///     is_string(&0);
        ///     is_string(&"cookie monster".to_string());
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn is<T: Any>(&self) -> bool {
            Any::is::<T>(self)
        }

        /// Forwards to the method defined on the type `Any`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn print_if_string(s: &(Any + Send)) {
        ///     if let Some(string) = s.downcast_ref::<String>() {
        ///         println!("It's a string({}): '{}'", string.len(), string);
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// fn main() {
        ///     print_if_string(&0);
        ///     print_if_string(&"cookie monster".to_string());
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
            Any::downcast_ref::<T>(self)
        }

        /// Forwards to the method defined on the type `Any`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn modify_if_u32(s: &mut (Any+ Send)) {
        ///     if let Some(num) = s.downcast_mut::<u32>() {
        ///         *num = 42;
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let mut x = 10u32;
        ///     let mut s = "starlord".to_string();
        ///
        ///     modify_if_u32(&mut x);
        ///     modify_if_u32(&mut s);
        ///
        ///     assert_eq!(x, 42);
        ///     assert_eq!(&s, "starlord");
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn downcast_mut<T: Any>(&mut self) -> Option<&mut T> {
            Any::downcast_mut::<T>(self)
        }
    }

    ///////////////////////////////////////////////////////////////////////////////
    // TypeID and its methods
    ///////////////////////////////////////////////////////////////////////////////

    /// A `TypeId` represents a globally unique identifier for a type.
    ///
    /// Each `TypeId` is an opaque object which does not allow inspection of what's
    /// inside but does allow basic operations such as cloning, comparison,
    /// printing, and showing.
    ///
    /// A `TypeId` is currently only available for types which ascribe to `'static`,
    /// but this limitation may be removed in the future.
    #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct TypeId {
        t: u64,
    }

    impl TypeId {
        /// Returns the `TypeId` of the type this generic function has been
        /// instantiated with.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::{Any, TypeId};
        ///
        /// fn is_string<T: ?Sized + Any>(_s: &T) -> bool {
        ///     TypeId::of::<String>() == TypeId::of::<T>()
        /// }
        ///
        /// fn main() {
        ///     assert_eq!(is_string(&0), false);
        ///     assert_eq!(is_string(&"cookie monster".to_string()), true);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn of<T: ?Sized + 'static>() -> TypeId {
            TypeId {
                t: unsafe { intrinsics::type_id::<T>() },
            }
        }
    }

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