---
author: Umesh Malik
canonical: "https://umesh-malik.com/blog/rust-dyn-trait-vs-generics-memory-cost"
description: "Rust dyn Trait vs generics: how to switch, and the 16-byte fat-pointer cost dyn Trait pays on every call — the cost generics compile away."
image: "/blog/rust-dyn-trait-vs-generics-memory-cost-cover.svg"
imageAlt: "Dashboard-style cover comparing Rust's 8-byte thin pointer against the 16-byte dyn Trait fat pointer with vtable indirection"
publishDate: "2026-09-06"
category: "Web Engineering"
keywords: rust dyn trait vs generics, rust dyn trait memory cost, rust fat pointer vs thin pointer, rust static dispatch vs dynamic dispatch, rust object safety
primaryKeyword: rust dyn trait vs generics
secondaryKeywords:
- rust dyn trait memory cost
- rust fat pointer vs thin pointer
- rust static dispatch vs dynamic dispatch
- rust object safety rules
featured: false
published: true
readingTime: "8 min read"
tags:
- Rust
- Systems Programming
- Memory Management
- Performance Engineering
- Web Engineering
title: "Rust dyn Trait vs generics: how to switch, and the 16-byte cost"
geoHooks:
  - "What dyn Trait actually costs in memory"
  - "The decision table: dyn Trait vs generics"
  - "Rust dyn Trait vs generics: when should you switch?"
  - "Object safety: why some traits can't become trait objects"
faq:
  - q: "What is a fat pointer in Rust?"
    a: "A fat pointer is a reference that carries two addresses instead of one. &dyn Trait is the most common example: one word points to the value's data, the other points to that type's vtable. A plain reference like &T is a thin pointer — a single 8-byte address — because the compiler already knows T's layout and methods at compile time and has nothing extra to attach."
  - q: "Does Box<dyn Trait> cost more than &dyn Trait?"
    a: "The pointer itself is the same 16 bytes in both cases — a Box is still a fat pointer when it points at a trait object. The difference is ownership: Box<dyn Trait> also heap-allocates and owns the underlying value, while &dyn Trait only borrows a value that lives somewhere else. Neither one makes the fat pointer thinner."
  - q: "Why can't Clone be used as a trait object?"
    a: "Clone::clone returns Self, and behind a &dyn Trait the caller only knows the vtable and a data pointer — it has no way to know how many bytes Self needs to allocate for the returned value. Rust's object-safety rule bans any method that returns Self for exactly this reason. The usual workaround is a second trait with a clone_box(&self) -> Box<dyn Trait> method, which returns a fixed-size, object-safe type instead of Self."
  - q: "Does using generics instead of dyn Trait always make binaries bigger?"
    a: "Only if you call the generic function with many different concrete types — monomorphization compiles one function body per type actually used, so ten call sites with ten types produce ten function bodies. A generic function called with one or two types costs about the same as a non-generic one. dyn Trait keeps exactly one function body no matter how many types implement the trait, which is the trade you're making in the other direction."
  - q: "Is the vtable lookup in dyn Trait actually slow?"
    a: "In isolation, one indirect call through a vtable is a handful of nanoseconds — rarely the bottleneck by itself. The cost that actually shows up in practice is indirect: a Vec<Box<dyn Trait>> scatters its elements across separate heap allocations, so iterating it means chasing a different, unpredictable address on every step, which is what actually hurts cache behavior in a hot loop. A Vec<ConcreteType> keeps its elements contiguous and doesn't pay that price."
  - q: "Can I mix dyn Trait and generics in the same codebase?"
    a: "Yes, and most real Rust codebases do. The choice is made per call site, not per type — the same type can implement a trait and be used generically in one function while being boxed as a dyn Trait in another. Pick generics as the default for a single-type call path and reach for dyn Trait only at the specific boundary where you need one collection or return type to hold genuinely different concrete types."
---

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**TL;DR** Rust dyn Trait vs generics comes down to one number: **dyn Trait** is a 16-byte fat pointer — a data pointer plus a vtable pointer — twice the size of the plain 8-byte reference generics compile down to. Generics pay their cost at compile time (monomorphization: one function body per concrete type you call with), while `dyn Trait` pays it on every call instead (one vtable load plus an indirect jump). Use `dyn Trait` only where you need one collection or return type to hold genuinely different concrete types at once; some traits — anything with a method returning `Self`, or a generic method — can't become trait objects at all, no matter which one you'd prefer.

Every Rust codebase eventually hits the same fork: a `Draw` trait implemented by `Circle`, `Square`, and `Triangle`, and a function that needs to draw any of them. Generics and `dyn Trait` both compile — the compiler is happy to accept either — but they solve different problems, and picking the wrong one shows up either as a binary bigger than it needs to be, or as a `Vec<T>` that won't compile because its elements aren't all the same concrete type.

This is the same trade-off you run into when you're staring at [struct layout and padding](/blog/reduce-rust-struct-memory-footprint) or deciding [how much unsafe is worth a performance win](/blog/rust-safe-gpu-offload-benchmarks) — except here the compiler enforces the boundary whether you understand why or not. So here's the trade-off made explicit: what `dyn Trait` actually costs, measured in bytes, and the rule for when that cost is worth paying.

## What dyn Trait actually costs in memory

A plain Rust reference, `&T`, is a **thin pointer** — 8 bytes on a 64-bit target, holding nothing but the address of the value. Write `&dyn Draw` instead and the compiler hands you a **fat pointer**: 16 bytes, twice the size, because it now carries two addresses instead of one — a pointer to the concrete value's data, and a pointer to that type's **vtable**, a static table of function pointers used to find the right `draw()` implementation for whatever concrete type is actually behind the reference.

You can verify this yourself, no benchmark required: `std::mem::size_of::<&dyn Draw>()` reports 16 on any 64-bit target, against 8 for `std::mem::size_of::<&Circle>()`. `Box<dyn Draw>` costs the same 16 bytes for the pointer, plus whatever the concrete value needs on the heap — boxing a trait object doesn't make the fat pointer thinner, it just adds ownership of whatever it points to.

![Memory layout comparing an 8-byte thin pointer holding one address against a 16-byte dyn Trait fat pointer holding a data pointer and a vtable pointer side by side](/blog/rust-dyn-trait-vs-generics-memory-cost-fat-pointer.svg)

The detail that trips people up: the vtable is keyed on the **(concrete type, trait) pair**, not on the type alone. If `Duck` implements both `Fly` and `Swim`, a `&dyn Fly` and a `&dyn Swim` built from the same duck value carry an identical data pointer but two *different* vtable pointers — one full of `Fly`'s methods, one full of `Swim`'s. There's no single "the vtable" for a type; there's one per trait it's viewed through.

## How static dispatch avoids the cost — and what it costs instead

Write the same function generically — `fn draw_shape<T: Draw>(shape: &T)` — and the compiler does something completely different: it emits a **separate compiled copy of the function for every concrete type you actually call it with**. `draw_shape::<Circle>` and `draw_shape::<Square>` become two distinct functions in the binary, and each one calls `Circle::draw` or `Square::draw` directly, with no lookup at all. This is **monomorphization**, and it's the reason generics in Rust are called a zero-cost abstraction: by the time the program runs, there's no polymorphism left to resolve — it happened at compile time.

The cost doesn't disappear, it moves. Every additional concrete type a generic function gets instantiated with is another compiled function body in your binary — ten call sites with ten different types produce ten function bodies, not one. That's a compile-time and binary-size cost, not a runtime one, which is the opposite trade from `dyn Trait`: one function body, paid for with a vtable load and an indirect call on every use.

![Flowchart contrasting a generic call compiling into three separate direct-call function bodies at build time against a dyn Trait call routing through one shared function and a runtime vtable lookup](/blog/rust-dyn-trait-vs-generics-memory-cost-dispatch.svg)

Rust also draws this line in a different place than C++. In C++, a class either has virtual methods or it doesn't — the choice is baked into the class definition, and every instance carries a vtable pointer whether or not you ever call through it dynamically. In Rust, the same type can be used generically in one function and boxed behind `dyn` in another; the choice is made **per call site** — `&dyn Trait` or `Box<dyn Trait>` — not per type definition.

## The decision table: dyn Trait vs generics

| Axis | Generics (`<T: Trait>` / `impl Trait`) | `dyn Trait` |
| --- | --- | --- |
| Reference size | 8 bytes (thin) | 16 bytes (fat: data + vtable) |
| Dispatch cost per call | None — resolved at compile time | One vtable load + indirect call |
| Binary size | Grows with each concrete type instantiated | One function body, regardless of how many types implement the trait |
| Heterogeneous collections (`Vec<Box<dyn Trait>>`) | Not directly possible — a `Vec<T>` needs one concrete `T` | The whole point — different concrete types in one collection |
| Generic methods on the trait | Supported | Not supported — breaks object safety |
| Compile time | Increases with each instantiation | Unaffected by how many types implement the trait |

Nothing in this table is a tie-breaker by itself — it's the input to the one question that actually decides it.

## Rust dyn Trait vs generics: when should you switch?

Reach for `dyn Trait` when you need **one collection, field, or return type to hold genuinely different concrete types at runtime** — a plugin registry, a list of UI widgets, a set of parsers chosen by content type, a callback registered by code you don't control and can't monomorphize against. That's the case `dyn Trait` exists to solve, and generics can't solve it at all: `Vec<T>` requires every element to be the same concrete `T`.

Reach for generics everywhere else, including the default case of "one call site, one concrete type at a time." You get the same abstraction over the trait's methods with zero per-call cost, and the compiler catches a bound mismatch immediately at the call site — it doesn't wait until you try to build a heterogeneous `Vec` to tell you something doesn't fit. If you're not sure yet whether you'll ever need more than one concrete type behind a given reference, start generic; switching to `dyn Trait` later is a smaller change than the reverse.

## What breaks if you default to dyn Trait everywhere?

The most common mistake is reaching for `Box<dyn Trait>` out of habit and then hitting `E0038: the trait cannot be made into an object` — the compiler refusing to build a vtable for a trait that isn't object-safe (covered next). The fix is almost never "force it"; it's picking generics for that call site instead, or restructuring the trait.

The second mistake is subtler: assuming a `dyn Trait` reference is "basically just a pointer" and forgetting the doubling. A struct with several `Box<dyn Trait>` fields is measurably bigger than the same struct built around an enum of concrete variants, and that adds up across a large collection of such structs.

The third is a cache-locality problem, not a dispatch-cost one. `Vec<Box<dyn Trait>>` scatters its elements across independent heap allocations — iterating it means chasing a different, unpredictable address on every step, on top of the vtable jump itself. `Vec<ConcreteType>` (or an enum, if the type set is closed) keeps its elements contiguous in memory, and that locality is usually worth more in a hot loop than avoiding one indirect call.

## Object safety: why some traits can't become trait objects

Two patterns disqualify a trait from ever becoming `dyn Trait`, and both come down to the same problem: the compiler can't build a fixed-size vtable entry for them.

1. **A method that returns `Self`.** `Clone::clone(&self) -> Self` needs the caller to know the concrete type's size to allocate the returned value — but behind a `&dyn Trait`, all the caller has is a data pointer and a vtable. That's exactly why `Clone` alone can't be a trait object; the standard workaround is a second, object-safe trait with a `clone_box(&self) -> Box<dyn Trait>` method that returns a boxed value instead of `Self` directly.
2. **A generic method.** `fn serialize<T: Write>(&self, out: &mut T)` would need one vtable entry per type `T` the method is ever called with — an unbounded, open-ended set the compiler can't enumerate ahead of time, so it refuses to generate a vtable at all.

Both rules exist for the same reason: a vtable is a **fixed-size table decided once at compile time**, and anything whose shape depends on information only available at the call site can't fit in one.

## Converting a generic function to dyn Trait, step by step

1. **Check object safety first.** Does the trait have any method returning `Self`, or any generic method? If yes, you'll need a second trait (an object-safe subset) before `dyn Trait` will compile.
2. **Change the signature.** `fn draw_shape<T: Draw>(shape: &T)` becomes `fn draw_shape(shape: &dyn Draw)`, or `Box<dyn Draw>` if the function needs to own the value.
3. **Update call sites.** Concrete values now need an explicit `&circle` or `Box::new(circle)` where a bare value used to satisfy a generic bound directly.
4. **Watch the lifetime bound.** `Box<dyn Draw>` implicitly requires `dyn Draw + 'static` unless you write out a shorter lifetime — a common compile error the first time you make this switch.
5. **Re-measure the hot path, don't assume.** If this function runs in a loop that matters, benchmark before and after — the vtable jump itself is rarely the story; a scattered `Vec<Box<dyn Draw>>` replacing a contiguous `Vec<Circle>` usually is.

```rust
// Before: generic, monomorphized per concrete type
fn draw_shape<T: Draw>(shape: &T) {
    shape.draw();
}

// After: dyn Trait, one function body, one vtable jump per call
fn draw_shape(shape: &dyn Draw) {
    shape.draw();
}

// Now the caller can hold a Vec of genuinely different shapes:
let shapes: Vec<Box<dyn Draw>> = vec![Box::new(Circle), Box::new(Square)];
for shape in &shapes {
    draw_shape(shape.as_ref());
}
```

## FAQ

### What is a fat pointer in Rust?

A fat pointer is a reference that carries two addresses instead of one. `&dyn Trait` is the most common example: one word points to the value's data, the other points to that type's vtable. A plain reference like `&T` is a thin pointer — a single 8-byte address — because the compiler already knows `T`'s layout and methods at compile time and has nothing extra to attach.

### Does `Box<dyn Trait>` cost more than `&dyn Trait`?

The pointer itself is the same 16 bytes in both cases — a `Box` is still a fat pointer when it points at a trait object. The difference is ownership: `Box<dyn Trait>` also heap-allocates and owns the underlying value, while `&dyn Trait` only borrows a value that lives somewhere else. Neither one makes the fat pointer thinner.

### Why can't Clone be used as a trait object?

`Clone::clone` returns `Self`, and behind a `&dyn Trait` the caller only knows the vtable and a data pointer — it has no way to know how many bytes `Self` needs to allocate for the returned value. Rust's object-safety rule bans any method that returns `Self` for exactly this reason. The usual workaround is a second trait with a `clone_box(&self) -> Box<dyn Trait>` method, which returns a fixed-size, object-safe type instead of `Self`.

### Does using generics instead of dyn Trait always make binaries bigger?

Only if you call the generic function with many different concrete types — monomorphization compiles one function body per type actually used, so ten call sites with ten types produce ten function bodies. A generic function called with one or two types costs about the same as a non-generic one. `dyn Trait` keeps exactly one function body no matter how many types implement the trait, which is the trade you're making in the other direction.

### Is the vtable lookup in dyn Trait actually slow?

In isolation, one indirect call through a vtable is a handful of nanoseconds — rarely the bottleneck by itself. The cost that actually shows up in practice is indirect: a `Vec<Box<dyn Trait>>` scatters its elements across separate heap allocations, so iterating it means chasing a different, unpredictable address on every step, which is what actually hurts cache behavior in a hot loop. A `Vec<ConcreteType>` keeps its elements contiguous and doesn't pay that price.

### Can I mix dyn Trait and generics in the same codebase?

Yes, and most real Rust codebases do. The choice is made per call site, not per type — the same type can implement a trait and be used generically in one function while being boxed as a `dyn Trait` in another. Pick generics as the default for a single-type call path and reach for `dyn Trait` only at the specific boundary where you need one collection or return type to hold genuinely different concrete types.

## Sources

- [Visualizing Rust's Vtables: How dyn Trait Works In Memory](https://sofiabelen.github.io/projects/visualizing-rusts-vtables-how-dyn-trait-works-in-memory/) — the fat-pointer and per-(type, trait)-vtable details this post builds on.
- [The Rust Reference — Trait objects](https://doc.rust-lang.org/reference/types/trait-object.html) — the formal definition of trait objects and object safety.
- [Rust error code E0038](https://doc.rust-lang.org/error_codes/E0038.html) — the compiler's own explanation of why a trait fails to be object-safe.

If you're weighing this alongside other memory-layout decisions, [cutting a Rust struct's footprint](/blog/reduce-rust-struct-memory-footprint) and [Node.js's pointer-compression trade-off](/blog/nodejs-memory-cut-in-half-pointer-compression) are the same kind of "make the cost explicit, then decide" exercise applied to different problems. And if the LSP you're running to catch these decisions is itself memory-hungry, [Glancer on 8GB of RAM](/blog/rust-glancer-low-memory-lsp) is worth a look.

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