Using the visitor pattern

The visitor design pattern is a common pattern used in object-oriented languages. A visitor stores an algorithm that operates over a collection of objects of different types. It allows multiple different algorithms to be written over the same data without having to modify the code of the data's classes (For a more detailed description, see https://en.wikipedia.org/wiki/Visitor_pattern).

Let's implement a visitor to calculate the area of different geometric objects, such as squares and circles.

// see code in Chapter 6/code/visitor_pattern.rs 
// structs: 
struct Point { 
    x: f64, 
    y: f64 
} 
 
struct Circle { 
    center: Point, 
    radius: f64, 
} 
 
struct Square { 
    lowerLeftCorner: Point, 
    side: f64, 
} 
 
// traits: 
trait ShapeVisitor { 
    fn visit_circle(&mut self, c: &Circle); 
    fn visit_square(&mut self, r: &Square); 
} 
 
trait Shape { 
    fn accept<V: ShapeVisitor>(&self, sv: &mut V); 
} 
 
impl Shape for...