/
any.rs
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/
any.rs
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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.
//! Traits for dynamic typing of any `'static` type (through runtime reflection)
//!
//! 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
//! `as_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 `as_mut` method, for getting a mutable reference to the inner value.
//! `Box<Any>` adds the `move` method, which will unwrap a `Box<T>` from the
//! object. See the extension traits (`*Ext`) 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.
//!
//! # 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 Show, 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::Show;
//! use std::any::{Any, AnyRefExt};
//!
//! // Logger function for any type that implements Show.
//! fn log<T: Any+Show>(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: Show+'static>(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]
use mem::{transmute};
use option::Option;
use option::Option::{Some, None};
use raw::TraitObject;
use intrinsics::TypeId;
///////////////////////////////////////////////////////////////////////////////
// Any trait
///////////////////////////////////////////////////////////////////////////////
/// The `Any` trait is implemented by all `'static` types, and can be used for
/// dynamic typing
///
/// Every type with no non-`'static` references implements `Any`, so `Any` can
/// be used as a trait object to emulate the effects dynamic typing.
#[stable]
pub trait Any: 'static {
/// Get the `TypeId` of `self`
#[experimental = "this method will likely be replaced by an associated static"]
fn get_type_id(&self) -> TypeId;
}
impl<T: 'static> Any for T {
fn get_type_id(&self) -> TypeId { TypeId::of::<T>() }
}
///////////////////////////////////////////////////////////////////////////////
// Extension methods for Any trait objects.
// Implemented as three extension traits so that the methods can be generic.
///////////////////////////////////////////////////////////////////////////////
/// Extension methods for a referenced `Any` trait object
#[unstable = "this trait will not be necessary once DST lands, it will be a \
part of `impl Any`"]
pub trait AnyRefExt<'a> {
/// Returns true if the boxed type is the same as `T`
#[stable]
fn is<T: 'static>(self) -> bool;
/// Returns some reference to the boxed value if it is of type `T`, or
/// `None` if it isn't.
#[unstable = "naming conventions around acquiring references may change"]
fn downcast_ref<T: 'static>(self) -> Option<&'a T>;
}
#[stable]
impl<'a> AnyRefExt<'a> for &'a Any {
#[inline]
fn is<T: 'static>(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
}
#[inline]
fn downcast_ref<T: 'static>(self) -> Option<&'a T> {
if self.is::<T>() {
unsafe {
// Get the raw representation of the trait object
let to: TraitObject = transmute(self);
// Extract the data pointer
Some(transmute(to.data))
}
} else {
None
}
}
}
/// Extension methods for a mutable referenced `Any` trait object
#[unstable = "this trait will not be necessary once DST lands, it will be a \
part of `impl Any`"]
pub trait AnyMutRefExt<'a> {
/// Returns some mutable reference to the boxed value if it is of type `T`, or
/// `None` if it isn't.
#[unstable = "naming conventions around acquiring references may change"]
fn downcast_mut<T: 'static>(self) -> Option<&'a mut T>;
}
#[stable]
impl<'a> AnyMutRefExt<'a> for &'a mut Any {
#[inline]
fn downcast_mut<T: 'static>(self) -> Option<&'a mut T> {
if self.is::<T>() {
unsafe {
// Get the raw representation of the trait object
let to: TraitObject = transmute(self);
// Extract the data pointer
Some(transmute(to.data))
}
} else {
None
}
}
}