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

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    //! 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 `&dyn 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 dyn Any`, there
    //! is also the `downcast_mut` method, for getting a mutable reference to the
    //! inner value. `Box<dyn Any>` adds the `downcast` method, which attempts to
    //! convert to a `Box<T>`. See the [`Box`] documentation for the full details.
    //!
    //! Note that `&dyn 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 &dyn 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 crate::fmt;
    use crate::intrinsics;

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

    /// A trait 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
    // This trait is not unsafe, though we rely on the specifics of it's sole impl's
    // `type_id` function in unsafe code (e.g., `downcast`). Normally, that would be
    // a problem, but because the only impl of `Any` is a blanket implementation, no
    // other code can implement `Any`.
    //
    // We could plausibly make this trait unsafe -- it would not cause breakage,
    // since we control all the implementations -- but we choose not to as that's
    // both not really necessary and may confuse users about the distinction of
    // unsafe traits and unsafe methods (i.e., `type_id` would still be safe to call,
    // but we would likely want to indicate as such in documentation).
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Any: 'static {
        /// Gets the `TypeId` of `self`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::{Any, TypeId};
        ///
        /// fn is_string(s: &dyn Any) -> bool {
        ///     TypeId::of::<String>() == s.type_id()
        /// }
        ///
        /// assert_eq!(is_string(&0), false);
        /// assert_eq!(is_string(&"cookie monster".to_string()), true);
        /// ```
        #[stable(feature = "get_type_id", since = "1.34.0")]
        fn type_id(&self) -> TypeId;
    }

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

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

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

    #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
    impl fmt::Debug for dyn Any + Send + Sync {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("Any")
        }
    }

    impl dyn Any {
        /// Returns `true` if the boxed type is the same as `T`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn is_string(s: &dyn Any) {
        ///     if s.is::<String>() {
        ///         println!("It's a string!");
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// 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 (`self`).
            let concrete = self.type_id();

            // Compare both `TypeId`s on equality.
            t == concrete
        }

        /// 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: &dyn Any) {
        ///     if let Some(string) = s.downcast_ref::<String>() {
        ///         println!("It's a string({}): '{}'", string.len(), string);
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// 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>() {
                // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
                // that check for memory safety because we have implemented Any for all types; no other
                // impls can exist as they would conflict with our impl.
                unsafe { Some(&*(self as *const dyn 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 dyn Any) {
        ///     if let Some(num) = s.downcast_mut::<u32>() {
        ///         *num = 42;
        ///     }
        /// }
        ///
        /// 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>() {
                // SAFETY: just checked whether we are pointing to the correct type, and we can rely on
                // that check for memory safety because we have implemented Any for all types; no other
                // impls can exist as they would conflict with our impl.
                unsafe { Some(&mut *(self as *mut dyn Any as *mut T)) }
            } else {
                None
            }
        }
    }

    impl dyn Any + Send {
        /// Forwards to the method defined on the type `Any`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn is_string(s: &(dyn Any + Send)) {
        ///     if s.is::<String>() {
        ///         println!("It's a string!");
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// 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: &(dyn Any + Send)) {
        ///     if let Some(string) = s.downcast_ref::<String>() {
        ///         println!("It's a string({}): '{}'", string.len(), string);
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// 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 (dyn Any + Send)) {
        ///     if let Some(num) = s.downcast_mut::<u32>() {
        ///         *num = 42;
        ///     }
        /// }
        ///
        /// 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)
        }
    }

    impl dyn Any + Send + Sync {
        /// Forwards to the method defined on the type `Any`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        ///
        /// fn is_string(s: &(dyn Any + Send + Sync)) {
        ///     if s.is::<String>() {
        ///         println!("It's a string!");
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// is_string(&0);
        /// is_string(&"cookie monster".to_string());
        /// ```
        #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
        #[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: &(dyn Any + Send + Sync)) {
        ///     if let Some(string) = s.downcast_ref::<String>() {
        ///         println!("It's a string({}): '{}'", string.len(), string);
        ///     } else {
        ///         println!("Not a string...");
        ///     }
        /// }
        ///
        /// print_if_string(&0);
        /// print_if_string(&"cookie monster".to_string());
        /// ```
        #[stable(feature = "any_send_sync_methods", since = "1.28.0")]
        #[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 (dyn Any + Send + Sync)) {
        ///     if let Some(num) = s.downcast_mut::<u32>() {
        ///         *num = 42;
        ///     }
        /// }
        ///
        /// 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 = "any_send_sync_methods", since = "1.28.0")]
        #[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.
    ///
    /// While `TypeId` implements `Hash`, `PartialOrd`, and `Ord`, it is worth
    /// noting that the hashes and ordering will vary between Rust releases. Beware
    /// of relying on them inside of your code!
    #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, 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>()
        /// }
        ///
        /// assert_eq!(is_string(&0), false);
        /// assert_eq!(is_string(&"cookie monster".to_string()), true);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[rustc_const_unstable(feature = "const_type_id", issue = "41875")]
        pub const fn of<T: ?Sized + 'static>() -> TypeId {
            TypeId { t: intrinsics::type_id::<T>() }
        }
    }

    /// Returns the name of a type as a string slice.
    ///
    /// # Note
    ///
    /// This is intended for diagnostic use. The exact contents and format of the
    /// string are not specified, other than being a best-effort description of the
    /// type. For example, `type_name::<Option<String>>()` could return the
    /// `"Option<String>"` or `"std::option::Option<std::string::String>"`, but not
    /// `"foobar"`. In addition, the output may change between versions of the
    /// compiler.
    ///
    /// The type name should not be considered a unique identifier of a type;
    /// multiple types may share the same type name.
    ///
    /// The current implementation uses the same infrastructure as compiler
    /// diagnostics and debuginfo, but this is not guaranteed.
    ///
    /// # Examples
    ///
    /// ```rust
    /// assert_eq!(
    ///     std::any::type_name::<Option<String>>(),
    ///     "core::option::Option<alloc::string::String>",
    /// );
    /// ```
    #[stable(feature = "type_name", since = "1.38.0")]
    #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
    pub const fn type_name<T: ?Sized>() -> &'static str {
        intrinsics::type_name::<T>()
    }

    /// Returns the name of the type of the pointed-to value as a string slice.
    /// This is the same as `type_name::<T>()`, but can be used where the type of a
    /// variable is not easily available.
    ///
    /// # Note
    ///
    /// This is intended for diagnostic use. The exact contents and format of the
    /// string are not specified, other than being a best-effort description of the
    /// type. For example, `type_name_of_val::<Option<String>>(None)` could return
    /// `"Option<String>"` or `"std::option::Option<std::string::String>"`, but not
    /// `"foobar"`. In addition, the output may change between versions of the
    /// compiler.
    ///
    /// This function does not resolve trait objects,
    /// meaning that `type_name_of_val(&7u32 as &dyn Debug)`
    /// may return `"dyn Debug"`, but not `"u32"`.
    ///
    /// The type name should not be considered a unique identifier of a type;
    /// multiple types may share the same type name.
    ///
    /// The current implementation uses the same infrastructure as compiler
    /// diagnostics and debuginfo, but this is not guaranteed.
    ///
    /// # Examples
    ///
    /// Prints the default integer and float types.
    ///
    /// ```rust
    /// #![feature(type_name_of_val)]
    /// use std::any::type_name_of_val;
    ///
    /// let x = 1;
    /// println!("{}", type_name_of_val(&x));
    /// let y = 1.0;
    /// println!("{}", type_name_of_val(&y));
    /// ```
    #[unstable(feature = "type_name_of_val", issue = "66359")]
    #[rustc_const_unstable(feature = "const_type_name", issue = "63084")]
    pub const fn type_name_of_val<T: ?Sized>(_val: &T) -> &'static str {
        type_name::<T>()
    }