Creating a "builder" for a struct with a Box<dyn trait>-typed field

1

Hello all,

I'm trying to implement a "builder" for a struct, and am running into a wall because the struct in question uses a Box<dyn trait> type for a field. I've created a (non-functional) playground to illustrate. I'll share an excerpt here:

struct Gizmo {
    buzzer: Box<dyn Buzz>    
}

struct GizmoBuilder {
    buzzer: Box<dyn Buzz>    
}

impl GizmoBuilder {
    pub fn new() -> Self {
        GizmoBuilder {
            buzzer: Box::new(Widget::new())
        }
    }
    
    pub fn build(&self) -> Gizmo {
        Gizmo {
            buzzer: self.buzzer
        }
    }
}

Of course, I cannot move out of self.buzzer which is behind a shared reference. Are there any recommendations for a way that I could achieve this without modifying the underlying Gizmo struct?

2

Usually the build method consumes the builder by having a self param, rather than &self. Have you tried that? You should be able to move out of self.

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3

Conventionally, builder types are consumed when they build the final output. This avoids the whole issue:

impl GizmoBuilder {
    pub fn new() -> Self {
        GizmoBuilder {
            buzzer: Box::new(Widget::new())
        }
    }
    
    pub fn build(self) -> Gizmo {
        Gizmo {
            buzzer: self.buzzer
        }
    }
}

However, the builder is then not reusable. Often, making them Clone is a reasonable way to support reuse, but with your design that's a bit awkward.

Alternately, you can postpone creation of the Widget until it's needed:

struct GizmoBuilder;

impl GizmoBuilder {
    pub fn new() -> Self {
        GizmoBuilder
    }
    
    pub fn build(&self) -> Gizmo {
        Gizmo {
            buzzer: self.make_buzzer(),
        }
    }
    
    fn make_buzzer(&self) -> Box<dyn Buzz> {
        Box::new(Widget::new())
    }
}
1 Like
4

But here's one way how anyhow. Practically speaking, implementors of Buzz have to be Clone with this approach. (Longer explanation.)

5

Very delayed response, but just wanted to say thank you everyone here! The solution that I used was to consume self in the build function as @derspiny and @jumpnbrownweasel suggested. I marked @derspiny's response as the solution only because he included a code snippet. Thanks again!

3 Likes