Deref polymorphism anti-pattern

Author: nrc

README.md

@@ -19,6 +19,7 @@ language.
 * [Finalisation in destructors](idioms/dtor-finally.md)
 * TODO interior mutability - UnsafeCell, Cell, RefCell
 * TODO treating Option like a list
+* TODO `Default` trait
 
 ### Design patterns
 
@@ -40,14 +41,15 @@ language.
 * TODO 'shadow' borrowed version of struct - e.g., double buffering, Niko's parser generator
 * TODO composition of structs to please the borrow checker
 * TODO `Error` traits and `Result` forwarding
+* TODO graphs
 
 
 
 ### Anti-patterns
 
 * TODO thread + catch_panic for exceptions
 * TODO Clone to satisfy the borrow checker
-* TODO Deref polymorphism
+* [Deref polymorphism](anti_patterns/deref.md)
 * TODO Matching all fields of a struct (back compat)
 * TODO wildcard matches
 * TODO taking an enum rather than having multiple functions

anti_patterns/deref.md

@@ -0,0 +1,128 @@
+# `Deref` polymorphism
+
+## Description
+
+Abuse the `Deref` trait to emulate inheritance between structs, and thus reuse
+methods.
+
+
+## Example
+
+Sometimes we want to emulate the following common pattern from OO languages such
+as Java:
+
+```java
+class Foo {
+    void m() { ... }
+}
+
+class Bar extends Foo {}
+
+public static void main(String[] args) {
+    Bar b = new Bar();
+    b.m();
+}
+```
+
+We can use the deref polymorphism anti-pattern to do so:
+
+```rust
+struct Foo {}
+
+impl Foo {
+    fn m(&self) { ... }
+}
+
+struct Bar {
+    f: Foo
+}
+
+impl Deref for Bar {
+    type Target = Foo;
+    fn deref(&self) -> &Foo {
+        &self.f
+    }
+}
+
+fn main() {
+    let b = Bar { Foo {} };
+    b.m();
+}
+```
+
+There is no struct inheritance in Rust. Instead we use composition and include
+an instance of `Foo` in `Bar` (since the field is a value, it is stored inline,
+so if there were fields, they would have the same layout in memory as the Java
+version (probably, you should use `#[repr(C)]` if you want to be sure)).
+
+In order to make the method call work we implement `Deref` for `Bar` with `Foo`
+as the target (returning the embedded `Foo` field). That means that when we
+dereference a `Foo` (for example, using `*`) then we will get a `Bar`. That is
+pretty weird. Dereferencing usually gives a `T` from a reference to `T`, here we
+have two unrelated types. However, since the dot operator does implicit
+dereferencing, it means that the method call will search for methods on `Foo` as
+well as `Bar`.
+
+
+## Advantages
+
+You save a little boilerplate, e.g.,
+
+```rust
+impl Bar {
+    fn m(&self) { 
+        self.f.m()
+    }
+}
+```
+
+
+## Disadvantages
+
+Most importantly this is a surprising idiom - future programmers reading this in
+code will not expect this to happen. That's because we are abusing the `Deref`
+trait rather than using it as intended (and documented, etc.). It's also because
+the mechanism here is completely implicit.
+
+This pattern does not introduce subtyping between `Foo` and `Bar` like
+inheritance in Java or C++ does. Furthermore, traits implemented by `Foo` are
+not automatically implemented for `Bar`, so this pattern interacts badly with
+bounds checking and thus generic programming.
+
+Using this pattern gives subtly different semantics from most OO languages with
+regards to `self`. Usually it remains a reference to the sub-class, with this
+pattern it will be the 'class' where the method is defined.
+
+Finally, this pattern only supports single inheritance, and has no notion of
+interfaces, class-based privacy, or other inheritance-related features. So, it
+gives an experience that will be subtly surprising to programmers used to Java
+inheritance, etc.
+
+
+## Discussion
+
+There is no one good alternative. Depending on the exact circumstances it might
+be better to re-implement using traits or to write out the facade methods to
+dispatch to `Foo` manually. We do intend to add a mechanism for inheritance
+similar to this to Rust, but it is likely to be some time before it reaches
+stable Rust. See these [blog](http://aturon.github.io/blog/2015/09/18/reuse/)
+[posts](http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/)
+and this [RFC issue](https://github.com/rust-lang/rfcs/issues/349) for more details.
+
+The `Deref` trait is designed for the implementation of custom pointer types.
+The intention is that it will take a pointer-to-`T` to a `T`, not convert
+between different types. It is a shame that this isn't (probably cannot be)
+enforced by the trait definition.
+
+Rust tries to strike a careful balance between explicit and implicit mechanisms,
+favouring explicit conversions between types. Automatic dereferencing in the dot
+operator is a case where the ergonomics strongly favour an implicit mechanism,
+but the intention is that this is limited to degrees of indirection, not
+conversion between arbitrary types.
+
+
+## See also
+
+[Collections are smart pointers idiom](../idioms/deref.md).
+
+[Documentation for `Deref` trait](https://doc.rust-lang.org/std/ops/trait.Deref.html).

idioms/deref.md

@@ -76,6 +76,6 @@ slicing syntax. The target will be the borrowed view.
 
 ## See also
 
-Deref polymorphism anti-pattern.
+[Deref polymorphism anti-pattern](../anti_patterns/deref.md).
 
 [Documentation for `Deref` trait](https://doc.rust-lang.org/std/ops/trait.Deref.html).

patterns/RAII.md

@@ -1,4 +1,4 @@
-# RAII guards
+# RAII with guards
 
 ## Description
 

patterns/entry.md

@@ -36,3 +36,6 @@ TODO vs insert_or_update etc.
 ## See also
 
 [RFC](https://github.com/rust-lang/rfcs/blob/master/text/0216-collection-views.md)
+[RFC](https://github.com/rust-lang/rfcs/blob/8e2d3a3341da533f846f61f10335b72c9a9f4740/text/0921-entry_v3.md)
+
+[Hashmap::entry docs](https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.entry)