From 2af6f3b4aecf2f7372719c4cfd5525822a52edcd Mon Sep 17 00:00:00 2001
From: Curtis McEnroe <programble@gmail.com>
Date: Tue, 17 Jan 2017 19:55:55 -0500
Subject: [PATCH] Add concrete wrapper trait draft

---
 _drafts/rust-concrete-wrapper-trait.md | 60 ++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 _drafts/rust-concrete-wrapper-trait.md

diff --git a/_drafts/rust-concrete-wrapper-trait.md b/_drafts/rust-concrete-wrapper-trait.md
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+---
+title: "Rust: Creating Trait Objects for Traits with Generic Methods"
+---
+
+One of the rules
+for being able to create [trait objects]
+in Rust
+is that the trait
+cannot have any generic methods.
+
+For instance,
+the [`Sample`] trait from `rand`
+contains a generic `sample` method.
+
+    pub trait Sample<Support> {
+        fn sample<R: Rng>(&mut self, rng: &mut R) -> Support;
+    }
+
+If we want to do dynamic dispatch
+on this trait,
+we might have something
+like this...
+
+    struct Sampler {
+        sample: Box<Sample<f64>>,
+    }
+
+...which won't work.
+
+    error[E0038]: the trait `rand::distributions::Sample` cannot be made into an object
+     --> src/main.rs:6:5
+      |
+    6 |     sample: Box<Sample<f64>>,
+      |     ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `rand::distributions::Sample` cannot be made into an object
+      |
+      = note: method `sample` has generic type parameters
+
+This makes sense,
+since in order to build a trait object,
+the compiler needs to populate a [vtable]
+with function pointers
+to the code for each method.
+With generic methods,
+the code doesn't even exist
+unless it is called with a concrete type.
+
+Fortunately,
+if we know the concrete type
+we will be passing to `sample`,
+we can create a wrapper trait.
+
+
+    use rand::ThreadRng;
+    trait ThreadRngSample<Support> {
+        fn thread_rng_sample(&mut self, rng: &mut ThreadRng) -> Support;
+    }
+
+[trait objects]: https://doc.rust-lang.org/book/trait-objects.html
+[`Sample`]: https://docs.rs/rand/0.3.15/rand/distributions/trait.Sample.html
+[vtable]: https://en.wikipedia.org/wiki/Virtual_method_table