When does a closure implement Fn, FnMut and FnOnce?

by

The traits each represent more and more restrictive properties about closures/functions, indicated by the signatures of their call_... method, and particularly the type of self:

  • FnOnce (self) are functions that can be called once
  • FnMut (&mut self) are functions that can be called if they have &mut access to their environment
  • Fn (&self) are functions that can be called if they only have & access to their environment

A closure |...| ... will automatically implement as many of those as it can.

  • All closures implement FnOnce: a closure that can’t be called once doesn’t deserve the name. Note that if a closure only implements FnOnce, it can be called only once.
  • Closures that don’t move out of their captures implement FnMut, allowing them to be called more than once (if there is unaliased access to the function object).
  • Closures that don’t need unique/mutable access to their captures implement Fn, allowing them to be called essentially everywhere.

These restrictions follow directly from the type of self and the “desugaring” of closures into structs; described in my blog post Finding Closure in Rust.

For information on closures, see Closures: Anonymous Functions that Can Capture Their Environment in The Rust Programming Language.

More Related Contents:

  1. Mutably borrow one struct field while borrowing another in a closure
  2. How do I convert a Rust closure to a C-style callback?
  3. How to use struct self in member method closure
  4. Can’t borrow mutably within two different closures in the same scope
  5. How can I modify self in a closure called from a member function?
  6. Is there another option to share an Arc in multiple closures besides cloning it before each closure?
  7. How do I pass a closure through raw pointers as an argument to a C function?
  8. Why does a mutable borrow of a closure through DerefMut not work?
  9. Return local String as a slice (&str)
  10. How to get a reference to a concrete type from a trait object?
  11. How do I return a reference to something inside a RefCell without breaking encapsulation?
  12. Conditionally iterate over one of several possible iterators
  13. Is there any way to create a type alias for multiple traits?
  14. Do mutable references have move semantics?
  15. How do I store a closure in a struct in Rust?
  16. How do I match enum values with an integer?
  17. How to transform fields during serialization using Serde?
  18. Why is the mutable reference not moved here?
  19. “borrowed value does not live long enough” when using the builder pattern
  20. Is it possible to conditionally enable an attribute like `derive`?
  21. How can I create a function with a variable number of arguments?
  22. Thread ” has overflowed its stack when allocating a large array using Box
  23. Type mismatches resolving a closure that takes arguments by reference
  24. How do you enable a Rust “crate feature”?
  25. How to implement Iterator yielding mutable references [duplicate]
  26. How to swap two variables?
  27. Creating a vector of zeros for a specific size
  28. What is the purpose of the unit type in Rust?
  29. Cannot pass self as callback parameter due to double borrowing
  30. How to print structs and arrays?

Leave a Comment