2019-02-09 23:21:25 -05:00
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---
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layout: post
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title: "Summary: What Are the 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
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to have a document that answers some "yes/no" questions. You may not care about
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what an `Iterator` looks like in assembly, you just need to know whether it allocates
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an object on the heap or not.
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To that end, it should be said once again: if you care about memory behavior,
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use an allocator to verify the correct behavior. Tools like
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[`alloc_counter`](https://crates.io/crates/alloc_counter) are designed to make
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testing this behavior simple easy.
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Finally, a summary of the content that's been covered. Rust will prioritize
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the fastest behavior it can, but here are the ground rules for understanding
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the memory model in Rust:
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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
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don't need heap memory. If possible, use those.
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**Stack Allocation**:
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- Everything not using a smart pointer type 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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2019-02-09 23:31:11 -05:00
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---
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And if you've read through this series: thanks. I've enjoyed the process that went
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into writing this, both in building new tools and forcing myself to understand
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the content well enough to explain it. I hope this is valuable as a reference to you
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as well.
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