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36 lines
1.6 KiB
Markdown
36 lines
1.6 KiB
Markdown
---
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layout: post
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title: "Summary: What are the Allocation Rules?"
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description: "A synopsis and reference."
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category:
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tags: [rust, understanding-allocations]
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---
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While there's a lot of interesting detail captured in this series, it's often helpful to have a
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document that answers some "yes/no" questions. You may not care about what an `Iterator` looks like
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in assembly, you just need to know whether it allocates an object on the heap or not. And while Rust
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will prioritize the fastest behavior it can, here are the rules for each memory type:
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**Heap Allocation**:
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- Smart pointers (`Box`, `Rc`, `Mutex`, etc.) allocate their contents in heap memory.
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- Collections (`HashMap`, `Vec`, `String`, etc.) allocate their contents in heap memory.
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- Some smart pointers in the standard library have counterparts in other crates that don't need heap
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memory. If possible, use those.
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**Stack Allocation**:
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- Everything not using a smart pointer will be allocated on the stack.
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- Structs, enums, iterators, arrays, and closures are all stack allocated.
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- Cell types (`RefCell`) behave like smart pointers, but are stack-allocated.
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- Inlining (`#[inline]`) will not affect allocation behavior for better or worse.
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- Types that are marked `Copy` are guaranteed to have their contents stack-allocated.
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**Global Allocation**:
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- `const` is a fixed value; the compiler is allowed to copy it wherever useful.
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- `static` is a fixed reference; the compiler will guarantee it is unique.
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 --
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[Raph Levien](https://docs.google.com/presentation/d/1q-c7UAyrUlM-eZyTo1pd8SZ0qwA_wYxmPZVOQkoDmH4/edit?usp=sharing)
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