Version: 1.1.0

Lifetimes, origins, and references

A reference is only valid as long as the value it points at. Mojo calls that span an origin and checks that you do not keep the reference too long.

Essentials

• An argument reference lives for the function call.
• Returning a reference to a local is rejected.
• A struct method can return a reference tied to self.
• Origins compose: a reference derived from another reference cannot outlive it.
• When the checker cannot see the relationship, store an owned value instead.

Example

struct Box:
    var text: String

    fn borrow(ref self) -> ref [self] String:
        return self.text

Keep in mind

Origins are the compile-time answer to dangling pointers. If a signature becomes hard to read, the data model is usually asking for an owned copy.