Visualizing Rust's Vtables: How dyn Trait Works In Memory
- ID
- 22044
- Status
- summarized
- Published
- 05 Sep 2026, 9:31 PM
- Fetched
- 07 Sep 2026, 8:42 PM
- Provider
- Hacker News
- Category
- dev-community
- Original URL
- https://sofiabelen.github.io/projects/visualizing-rusts-vtables-how-dyn-trait-works-in-memory/
- Source URL
- https://hnrss.org/best
Summary
- Score
- 6.0
- Created
- 07 Sep 2026, 9:50 PM
- Tags
- Audience
- developers
What happened
A detailed visual walkthrough of how Rust's `dyn Trait` implements runtime polymorphism in memory, comparing it to C++ virtual functions and CRTP. The author dissects vtable layout and dispatch mechanics, with code and experiments on GitHub.
Why it matters
If you're writing or debugging Rust code that uses trait objects, understanding the fat-pointer (data ptr + vtable ptr) layout helps you reason about memory overhead and dispatch cost versus monomorphization. Use this to decide when `dyn Trait` vs generics is the right tradeoff for your codebase.
Discussion angle
Compare the practical tradeoffs of `dyn Trait` vs monomorphized generics in Rust—when does the vtable overhead actually matter, and how does this map to choices Rust builders face in real projects?