The builder pattern assembles a complex value through configurable operations before producing the final value. In Rust, it avoids exposing structures directly and compensates for the absence of optional arguments, while improving encapsulation, convenience, separation of concerns, ergonomics, and safety. section 5.3
A basic Rust builder typically:
- Stores the value under construction inside a separate builder type. section 5.3.1
- Provides configurable methods such as with_make, with_model, with_size, and with_color. section 5.3.1
- Initializes with new, applies settings, and finishes with build. section 5.3.1
- Can consume itself on each setting method and return a new builder, enabling fluent chaining. section 5.4.1
Building a bicycle
let mut builder = BicycleBuilder::new(); builder.with_make("Huffy"); builder.with_model("Radio"); builder.with_size(46); builder.with_color("red"); let bicycle = builder.build(); A fluent variant instead consumes each builder and returns Self, allowing configuration methods to be chained. Its implementation uses owned struct-update syntax, so the previous builder is moved. section 5.3.1 section 5.4.1
Listing 5.4 Code for builder pattern
#[derive(Debug)]
struct Bicycle {
make: String,
model: String,
size: i32,
color: String,
}
impl Bicycle { #1
fn make(&self) -> &String {
&self.make
}
fn model(&self) -> &String {
&self.model
}
fn size(&self) -> i32 {
self.size
}
fn color(&self) -> &String {
&self.color
}
}
struct BicycleBuilder {
bicycle: Bicycle, #2
}
impl BicycleBuilder {
fn new() -> Self { #3
Self {
bicycle: Bicycle {
make: String::new(),
model: String::new(),
size: 0,
color: String::new(),
},
}
}
fn with_make(&mut self, make: &str) { #4
self.bicycle.make = make.into()
}
fn with_model(&mut self, model: &str) {
self.bicycle.model = model.into()
}
fn with_size(&mut self, size: i32) {
self.bicycle.size = size
}
fn with_color(&mut self, color: &str) {
self.bicycle.color = color.into()
}
fn build(self) -> Bicycle { #5
self.bicycle
}
}
Inspect the complete Rust implementation, including the Bicycle value, encapsulated builder state, initialization, setters, and build operation. section 5.3.1
