new idiom: Default trait

README.md

@@ -20,7 +20,7 @@ language.
 * [Finalisation in destructors](idioms/dtor-finally.md)
 * TODO interior mutability - UnsafeCell, Cell, RefCell
 * [Iterating over an `Option`](idioms/option-iter.md)
-* TODO `Default` trait
+* [`Default` trait](idioms/default.md)
 * [Pass variables to closure](idioms/pass-var-to-closure.md)
 * [`mem::replace(_)` to avoid needless clones](idioms/mem-replace.md)
 

idioms/default.md

@@ -0,0 +1,50 @@
+# The `Default` Trait
+
+## Description
+
+Many types in Rust have a [Constructor]. However, this is *specific* to the
+type; Rust cannot abstract over "everything that has a `new()` method". To
+allow this, the [`Default`] trait was conceived, which can be used with
+containers and other generic types (e.g. see [`Option::unwrap_or_default()`]).
+Notably, some containers already implement it where applicable.
+
+Not only do one-element containers like `Cow`, `Box` or `Arc` implement
+`Default` for contained `Default` types, one can automatically
+`#[derive(Default)]` for structs whose fields all implement it, so the more
+types implement `Default`, the more useful it becomes.
+
+## Example
+
+```rust
+// note that we can simply auto-derive Default here.
+#[derive(Default)]
+struct Interesting {
+    /// integers and floats default to zero
+    cardinality: usize,
+    /// bool defaults to false
+    really: bool,
+    /// collections default to empty
+    see_also: Vec<Interesting>
+}
+
+fn interestingness(i: &Interesting) -> usize {
+    if really {
+        i.cardinality
+    } else {
+        // we can use `Option`'s `unwrap_or_default()` method here because our
+        // `Interesting` struct implements `Default`.
+        i.see_also.first().unwrap_or_default().cardinality
+    }
+}
+```
+
+## See also
+
+- The [Constructor] idiom is another way to generate instances that may or may
+not be "default"
+- The [`Default`] documentation (scroll down for the list of implementors)
+- [`Option::unwrap_or_default()`]
+
+[Constructor]: ctor.md
+[`Default`]: https://docs.rust-lang.org/doc/std/default/trait.Default.html
+[`Option::unwrap_or_default()`]: https://docs.rust-lang.org/doc/std/option/enum.Option.html#method.unwrap_or_default