Add the constructor idiom.

README.md

@@ -10,7 +10,7 @@ language.
 
 ### Idioms
 
-* Constructor
+* [Constructor](idioms/ctor.md)
 
 ### Design patterns
 

idioms/ctor.md

@@ -0,0 +1,37 @@
+# Contructors
+
+## Description
+
+Rust does not have constructors as a language construct. Instead, the convention
+is to use a static `new` method to create an object.
+
+
+## Example
+
+```rust
+// A Rust vector, see libcollections/vec.rs
+pub struct Vec<T> {
+    buf: RawVec<T>,
+    len: usize,
+}
+
+impl<T> Vec<T> {
+    // Constructs a new, empty `Vec<T>`.
+    // Note this is a static method - no self.
+    // This constructor doesn't take any arguments, but some might in order to
+    // properly initialise an object
+    pub fn new() -> Vec<T> {
+        // Create a new Vec with fields properly initialised.
+        Vec {
+            // Note that here we are calling RawVec's constructor.
+            buf: RawVec::new(),
+            len: 0,
+        }
+    }
+}
+```
+
+## See also
+
+The builder pattern for constructing objects where there are multiple
+configurations.