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< ol class = "chapter" > < li class = "chapter-item expanded " > < a href = "../intro.html" > < strong aria-hidden = "true" > 1.< / strong > Introduction< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/index.html" > < strong aria-hidden = "true" > 2.< / strong > Idioms< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../idioms/coercion-arguments.html" > < strong aria-hidden = "true" > 2.1.< / strong > Use borrowed types for arguments< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/concat-format.html" > < strong aria-hidden = "true" > 2.2.< / strong > Concatenating Strings with format!< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/ctor.html" > < strong aria-hidden = "true" > 2.3.< / strong > Constructor< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/default.html" > < strong aria-hidden = "true" > 2.4.< / strong > The Default Trait< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/deref.html" > < strong aria-hidden = "true" > 2.5.< / strong > Collections Are Smart Pointers< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/dtor-finally.html" > < strong aria-hidden = "true" > 2.6.< / strong > Finalisation in Destructors< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/mem-replace.html" > < strong aria-hidden = "true" > 2.7.< / strong > mem::replace(_)< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/on-stack-dyn-dispatch.html" > < strong aria-hidden = "true" > 2.8.< / strong > On-Stack Dynamic Dispatch< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/option-iter.html" > < strong aria-hidden = "true" > 2.9.< / strong > Iterating over an Option< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/pass-var-to-closure.html" > < strong aria-hidden = "true" > 2.10.< / strong > Pass Variables to Closure< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/priv-extend.html" > < strong aria-hidden = "true" > 2.11.< / strong > Privacy For Extensibility< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/rustdoc-init.html" > < strong aria-hidden = "true" > 2.12.< / strong > Easy doc initialization< / a > < / li > < li class = "chapter-item expanded " > < a href = "../idioms/temporary-mutability.html" > < strong aria-hidden = "true" > 2.13.< / strong > Temporary mutability< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/index.html" > < strong aria-hidden = "true" > 3.< / strong > Design Patterns< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../patterns/builder.html" > < strong aria-hidden = "true" > 3.1.< / strong > Builder< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/compose-structs.html" > < strong aria-hidden = "true" > 3.2.< / strong > Compose Structs< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/fold.html" > < strong aria-hidden = "true" > 3.3.< / strong > Fold< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/newtype.html" > < strong aria-hidden = "true" > 3.4.< / strong > Newtype< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/RAII.html" > < strong aria-hidden = "true" > 3.5.< / strong > RAII Guards< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/small-crates.html" > < strong aria-hidden = "true" > 3.6.< / strong > Prefer Small Crates< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/unsafe-mods.html" > < strong aria-hidden = "true" > 3.7.< / strong > Contain unsafety in small modules< / a > < / li > < li class = "chapter-item expanded " > < a href = "../patterns/visitor.html" > < strong aria-hidden = "true" > 3.8.< / strong > Visitor< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../anti_patterns/index.html" > < strong aria-hidden = "true" > 4.< / strong > Anti-patterns< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../anti_patterns/deny-warnings.html" > < strong aria-hidden = "true" > 4.1.< / strong > #[deny(warnings)]< / a > < / li > < li class = "chapter-item expanded " > < a href = "../anti_patterns/deref.html" class = "active" > < strong aria-hidden = "true" > 4.2.< / strong > Deref Polymorphism< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../functional/index.html" > < strong aria-hidden = "true" > 5.< / strong > Functional Programming< / a > < / li > < li class = "chapt
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< h1 > < a class = "header" href = "#deref-polymorphism" id = "deref-polymorphism" > < code > Deref< / code > polymorphism< / a > < / h1 >
< h2 > < a class = "header" href = "#description" id = "description" > Description< / a > < / h2 >
< p > Abuse the < code > Deref< / code > trait to emulate inheritance between structs, and thus reuse
methods.< / p >
< h2 > < a class = "header" href = "#example" id = "example" > Example< / a > < / h2 >
< p > Sometimes we want to emulate the following common pattern from OO languages such
as Java:< / p >
< pre > < code class = "language-java" > class Foo {
void m() { ... }
}
class Bar extends Foo {}
public static void main(String[] args) {
Bar b = new Bar();
b.m();
}
< / code > < / pre >
< p > We can use the deref polymorphism anti-pattern to do so:< / p >
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< pre > < code class = "language-rust ignore" > use std::ops::Deref;
struct Foo {}
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impl Foo {
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fn m(& self) {
//..
}
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}
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();
}
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< / code > < / pre >
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< p > There is no struct inheritance in Rust. Instead we use composition and include
an instance of < code > Foo< / code > in < code > Bar< / code > (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 < code > #[repr(C)]< / code > if you want to be sure)).< / p >
< p > In order to make the method call work we implement < code > Deref< / code > for < code > Bar< / code > with < code > Foo< / code >
as the target (returning the embedded < code > Foo< / code > field). That means that when we
dereference a < code > Bar< / code > (for example, using < code > *< / code > ) then we will get a < code > Foo< / code > . That is
pretty weird. Dereferencing usually gives a < code > T< / code > from a reference to < code > T< / code > , 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 < code > Foo< / code > as
well as < code > Bar< / code > .< / p >
< h2 > < a class = "header" href = "#advantages" id = "advantages" > Advantages< / a > < / h2 >
< p > You save a little boilerplate, e.g.,< / p >
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< pre > < code class = "language-rust ignore" > impl Bar {
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fn m(& self) {
self.f.m()
}
}
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< / code > < / pre >
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< h2 > < a class = "header" href = "#disadvantages" id = "disadvantages" > Disadvantages< / a > < / h2 >
< p > 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 < code > Deref< / code >
trait rather than using it as intended (and documented, etc.). It's also because
the mechanism here is completely implicit.< / p >
< p > This pattern does not introduce subtyping between < code > Foo< / code > and < code > Bar< / code > like
inheritance in Java or C++ does. Furthermore, traits implemented by < code > Foo< / code > are
not automatically implemented for < code > Bar< / code > , so this pattern interacts badly with
bounds checking and thus generic programming.< / p >
< p > Using this pattern gives subtly different semantics from most OO languages with
regards to < code > self< / code > . Usually it remains a reference to the sub-class, with this
pattern it will be the 'class' where the method is defined.< / p >
< p > 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.< / p >
< h2 > < a class = "header" href = "#discussion" id = "discussion" > Discussion< / a > < / h2 >
< p > 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 < code > Foo< / code > 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 < a href = "http://aturon.github.io/blog/2015/09/18/reuse/" > blog< / a >
< a href = "http://smallcultfollowing.com/babysteps/blog/2015/10/08/virtual-structs-part-4-extended-enums-and-thin-traits/" > posts< / a >
and this < a href = "https://github.com/rust-lang/rfcs/issues/349" > RFC issue< / a > for more details.< / p >
< p > The < code > Deref< / code > trait is designed for the implementation of custom pointer types.
The intention is that it will take a pointer-to-< code > T< / code > to a < code > T< / code > , not convert
between different types. It is a shame that this isn't (probably cannot be)
enforced by the trait definition.< / p >
< p > 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.< / p >
< h2 > < a class = "header" href = "#see-also" id = "see-also" > See also< / a > < / h2 >
< p > < a href = "../idioms/deref.html" > Collections are smart pointers idiom< / a > .< / p >
< p > < a href = "https://doc.rust-lang.org/std/ops/trait.Deref.html" > Documentation for < code > Deref< / code > trait< / a > .< / p >
< / main >
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