Operators and Overloading

Introduction#

Most operators in Rust can be defined (“overloaded”) for user-defined types. This can be achieved by implementing the respective trait in std::ops module.

Overloading the addition operator (+)

Overloading the addition operator (+) requires implement the std::ops::Add trait.

From the documentation, the full definition of the trait is:

pub trait Add<RHS = Self> {
    type Output;
    fn add(self, rhs: RHS) -> Self::Output;
}

How does it work?

• the trait is implemented for the Left Hand Side type
• the trait is implemented for one Right Hand Side argument, unless specified it defaults to having the same type as the Left Hand Side one
• the type of the result of the addition is specified in the associated type Output

Thus, having 3 different types is possible.

Note: the trait consumes is left-hand side and right-hand side arguments, you may prefer to implement it for references to your type rather than the bare types.

Implementing + for a custom type:

use std::ops::Add;

#[derive(Clone)]
struct List<T> {
    data: Vec<T>,
}
//  Implementation which consumes both LHS and RHS
impl<T> Add for List<T> {
    type Output = List<T>;

    fn add(self, rhs: List<T>) -> List<T> {
        self.data.extend(rhs.data.drain(..));
        self
    }
}

//  Implementation which only consumes RHS (and thus where LHS != RHS)
impl<'a, T: Clone> Add<List<T>> for &'a List<T> {
    type Output = List<T>;

    fn add(self, rhs: List<T>) -> List<T> {
        self.clone() + rhs
    }
}