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// This is largely taken from the Rust distribution, with only comparatively // minor additions and alterations. Therefore, their copyright notice follows: // // 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. // // I have kept my additions under the same terms (being rather fond of MIT/Apache-2.0 myself). //! **MOPA: My Own Personal Any.** A macro to implement all the `Any` methods on your own trait. //! //! You like `Any`—its ability to store any `'static` type as a trait object and then downcast it //! back to the original type is very convenient, and in fact you need it for whatever misguided //! reason. But it’s not enough. What you *really* want is your own trait object type with `Any`’s //! functionality glued onto it. Maybe you have a `Person` trait and you want your people to be //! able to do various things, but you also want to be able to conveniently downcast the person to //! its original type, right? Alas, you can’t write a type like `Box<Person + Any>` (at present, //! anyway). So what do you do instead? Do you give up? No, no! No, no! Enter MOPA. //! //! > There once was a quite friendly trait //! > Called `Person`, with much on its plate. //! > “I need to be `Any` //! > To downcast to `Benny`— //! > But I’m not, so I guess I’ll just wait.” //! //! A pitiful tale, isn’t it? Especially given that there was a bear chasing it with intent to eat //! it. Fortunately now you can *mopafy* `Person` in three simple steps: //! //! 1. Add the `mopa` crate to your `Cargo.toml` as usual and your crate root like so: //! //! ```rust,ignore //! #[macro_use] #[no_link] //! extern crate mopa; //! ``` //! //! 2. Make `Any` a supertrait of `Person`; //! //! 3. `mopafy!(Person);`. //! //! And lo, you can now write `person.is::<Benny>()` and `person.downcast_ref::<Benny>()` and so on //! to your heart’s content. Simple, huh? //! //! Oh, by the way, it was actually the person on the bear’s plate. There wasn’t really anything on //! `Person`’s plate after all. //! //! ```rust //! #![feature(core)] //! #[macro_use] #[no_link] //! extern crate mopa; //! //! use std::any::Any; //! //! struct Bear { //! // This might be a pretty fat bear. //! fatness: u16, //! } //! //! impl Bear { //! fn eat(&mut self, person: Box<Person>) { //! self.fatness = (self.fatness as i16 + person.weight()) as u16; //! } //! } //! //! trait Person: Any { //! fn panic(&self); //! fn yell(&self) { println!("Argh!"); } //! fn sleep(&self); //! fn weight(&self) -> i16; //! } //! //! mopafy!(Person); //! //! struct Benny { //! // (Benny is not a superhero. He can’t carry more than 256kg of food at once.) //! kilograms_of_food: u8, //! } //! //! impl Person for Benny { //! fn panic(&self) { self.yell() } //! fn sleep(&self) { /* ... */ } //! fn weight(&self) -> i16 { //! // Who’s trying to find out? I’m scared! //! self.yell(); //! self.kilograms_of_food as i16 + 60 //! } //! } //! //! struct Chris; //! //! impl Chris { //! // Normal people wouldn’t be brave enough to hit a bear but Chris might. //! fn hit(&self, bear: &mut Bear) { //! println!("Chris hits the bear! How brave! (Or maybe stupid?)"); //! // Meh, boundary conditions, what use are they in examples? //! // Chris clearly hits quite hard. Poor bear. //! bear.fatness -= 1; //! } //! } //! //! impl Person for Chris { //! fn panic(&self) { /* ... */ } //! fn sleep(&self) { /* ... */ } //! fn weight(&self) -> i16 { -5 /* antigravity device! cool! */ } //! } //! //! fn simulate_simulation(person: Box<Person>, bear: &mut Bear) { //! if person.is::<Benny>() { //! // None of the others do, but Benny knows this particular //! // bear by reputation and he’s *really* going to be worried. //! person.yell() //! } //! // If it happens to be Chris, he’ll hit the bear. //! person.downcast_ref::<Chris>().map(|chris| chris.hit(bear)); //! bear.eat(person); //! } //! //! fn main() { //! let mut bear = Bear { fatness: 10 }; //! simulate_simulation(Box::new(Benny { kilograms_of_food: 5 }), &mut bear); //! simulate_simulation(Box::new(Chris), &mut bear); //! } //! ``` //! //! Now *should* you do something like this? Probably not. Enums are probably a better solution for //! this particular case as written; frankly I believe that almost the only time you should //! downcast an Any trait object (or a mopafied trait object) is with a generic parameter, when //! producing something like `AnyMap`, for example. If you control *all* the code, `Any` trait //! objects are probably not the right solution; they’re good for cases with user-defined //! types across a variety of libraries. But the question of purpose and suitability is open, and I //! don’t have a really good example of such a use case here at present. TODO. /// The macro for implementing all the `Any` methods on your own trait. /// /// # Instructions for use /// /// 1. Make sure your trait extends `Any` (e.g. `trait Trait: Any { }`) /// /// 2. Mopafy your trait (see the next subsection for specifics). /// /// 3. … /// /// 4. Profit! /// /// ## Mopafication techniques /// /// There are three ways of mopafying traits, depending on what libraries you are using. /// /// 1. If you are a **normal person**: /// /// ```rust /// # #![feature(core)] /// # #[macro_use] #[no_link] extern crate mopa; /// # trait Trait: std::any::Any { } /// mopafy!(Trait); /// # fn main() { } /// ``` /// /// 2. If you are using **libcore** but not libstd (`#![no_std]`) or liballoc: /// /// ```rust /// # #![feature(core)] /// # #[macro_use] #[no_link] extern crate mopa; /// # extern crate core; /// # trait Trait: core::any::Any { } /// mopafy!(Trait, core = core); /// # fn main() { } /// ``` /// /// (This is akin to `mopafy!(Trait, core = std)` if you were using libstd.) /// /// Unlike the other two techniques, this only gets you the `&Any` and `&mut Any` methods; the /// `Box<Any>` methods require liballoc. /// /// 3. If you are using **libcore and liballoc** but not libstd (`#![nostd]`): /// /// ```rust /// # #![feature(core, alloc)] /// # #[macro_use] #[no_link] extern crate mopa; /// # extern crate core; /// # extern crate alloc; /// # trait Trait: core::any::Any { } /// mopafy!(Trait, core = core, alloc = alloc); /// # fn main() { } /// ``` /// /// (This is akin to `mopafy!(Trait, core = std, alloc = std)` if you were using libstd; in /// fact, the first form is just sugar for this very thing.) /// /// This gets you all the methods. #[macro_export] macro_rules! mopafy { // Using libstd like a normal person? Here’s what you want, just a simple `mopafy!(Trait)`. ($trait_:ty) => { mopafy!($trait_, core = std, alloc = std); }; // Not using libstd or liballoc? You can get the &Any and &mut Any methods by specifying what // libcore is here, e.g. `mopafy!(Trait, core = core)`, but you won’t get the `Box<Any>` // methods. ($trait_:ty, core = $core:ident) => { impl $trait_ { /// Returns true if the boxed type is the same as `T` #[inline] pub fn is<T: 'static>(&self) -> bool { use $core::any::Any; // Get TypeId of the type this function is instantiated with let t = ::$core::any::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. #[inline] pub fn downcast_ref<T: 'static>(&self) -> ::$core::option::Option<&T> { if self.is::<T>() { unsafe { ::$core::option::Option::Some(self.downcast_ref_unchecked()) } } else { ::$core::option::Option::None } } /// Returns a reference to the boxed value, blindly assuming it to be of type `T`. /// If you are not *absolutely certain* of `T`, you *must not* call this. #[inline] pub unsafe fn downcast_ref_unchecked<T: 'static>(&self) -> &T { // Get the raw representation of the trait object let to: ::$core::raw::TraitObject = ::$core::mem::transmute(self); // Extract the data pointer ::$core::mem::transmute(to.data) } /// Returns some mutable reference to the boxed value if it is of type `T`, or /// `None` if it isn't. #[inline] pub fn downcast_mut<T: 'static>(&mut self) -> ::$core::option::Option<&mut T> { if self.is::<T>() { unsafe { ::$core::option::Option::Some(self.downcast_mut_unchecked()) } } else { ::$core::option::Option::None } } /// Returns a mutable reference to the boxed value, blindly assuming it to be of type `T`. /// If you are not *absolutely certain* of `T`, you *must not* call this. #[inline] pub unsafe fn downcast_mut_unchecked<T: 'static>(&mut self) -> &mut T { // Get the raw representation of the trait object let to: ::$core::raw::TraitObject = ::$core::mem::transmute(self); // Extract the data pointer ::$core::mem::transmute(to.data) } } }; // Not using libstd? You can get the Box<Any> methods by specifying what liballoc is here, // e.g. `mopafy!(Trait, alloc = alloc)` ($trait_:ty, core = $core:ident, alloc = $alloc:ident) => { mopafy!($trait_, core = $core); impl $trait_ { /// Returns the boxed value if it is of type `T`, or `Err(Self)` if it isn't. #[inline] pub fn downcast<T: 'static>(self: ::$alloc::boxed::Box<Self>) -> ::$core::result::Result<::$alloc::boxed::Box<T>, ::$alloc::boxed::Box<Self>> { if self.is::<T>() { unsafe { ::$core::result::Result::Ok(self.downcast_unchecked()) } } else { ::$core::result::Result::Err(self) } } /// Returns the boxed value, blindly assuming it to be of type `T`. /// If you are not *absolutely certain* of `T`, you *must not* call this. #[inline] pub unsafe fn downcast_unchecked<T: 'static>(self: ::$alloc::boxed::Box<Self>) -> ::$alloc::boxed::Box<T> { // Get the raw representation of the trait object let to: ::$core::raw::TraitObject = ::$core::mem::transmute(self); // Extract the data pointer ::$core::mem::transmute(to.data) } } }; }