Add init-struct pattern

src/SUMMARY.md

@@ -32,6 +32,7 @@
     - [Visitor](./patterns/behavioural/visitor.md)
   - [Creational](./patterns/creational/intro.md)
     - [Builder](./patterns/creational/builder.md)
+    - [Init-Struct](./patterns/creational/init-struct.md)
     - [Fold](./patterns/creational/fold.md)
   - [Structural](./patterns/structural/intro.md)
     - [Compose Structs](./patterns/structural/compose-structs.md)

src/patterns/creational/init-struct.md

@@ -0,0 +1,66 @@
+# Init-Struct
+
+## Description
+
+Construct an object relying on defaults for omitted fields.
+
+## Example
+
+```rust
+#[derive(Debug, Default, PartialEq)]
+pub struct Foo {
+    pub foo: Option<u32>,
+    pub bar: String,
+    // Private members.
+    baz: Vec<u16>
+    // A lot more fields.
+}
+
+
+#[test]
+fn init_struct_test() {
+    let foo = Foo {
+        bar: "Some string".to_string(),
+        ..Default::default
+    };
+}
+```
+
+## Motivation
+
+Useful when you have a struct with a lot of fields that have well-defined/useful defaults.
+
+## Advantages
+
+On complex/deeply nested structs only the fields that require explicit initalization have to be
+touched.
+
+Prevents proliferation of constructors.
+
+Can be used for one-liner initialisation as well as more complex construction.
+
+Avoids having to write a lot of boilerplate for the common alternative, the [Builder](builder.md)
+pattern.
+
+Can still be combined with the Builder pattern if initializtion requires transformation of some
+sort.
+
+Members can still be private as long as they implement [`Default`](https://doc.rust-lang.org/std/default/trait.Default.html).
+
+## Disadvantages
+
+Requires each field of the struct to implement `Default`.
+
+I.e. if a field is missing `Default`, `default()` has to be implemented and all fields have to be set manualy to their defaults.
+
+## Discussion
+
+This pattern is seen frequently in Rust since you can only have a single method with a given name.
+Having multiple constructors is thus less nice in Rust than it is in C++, Java, or others.
+
+The common way to solve this is the [Builder](builder.md) pattern but this often requires a lot of
+boilerplate.
+
+## See Also
+
+- [Init Struct Pattern](https://xaeroxe.github.io/init-struct-pattern/)