Rust by Example

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See also:

Borrowing (&), Bounds (X: Y), enum, impl & self, Overloading, ref, Traits (X for Y), and TupleStructs.

Scoping rules

Scopes play an important part in ownership, borrowing, and lifetimes. That is, they indicate to the compiler when borrows are valid, when resources can be freed, and when variables are created or destroyed.

RAII

Variables in Rust do more than just hold data in the stack: they also own resources, e.g. Box owns memory in the heap. Rust enforces RAII (Resource Acquisition Is Initialization), so whenever an object goes out of scope, its destructor is called and its owned resources are freed.

This behavior shields against resource leak bugs, so you'll never have to manually free memory or worry about memory leaks again! Here's a quick showcase:

// raii.rs

fn create_box() {

// Allocate an integer on the heap

let _box1 = Box::new(3i32);

// `_box1` is destroyed here, and memory gets freed

}

fn main() {

// Allocate an integer on the heap

let _box2 = Box::new(5i32);

// A nested scope:

{

// Allocate an integer on the heap

let _box3 = Box::new(4i32);

// `_box3` is destroyed here, and memory gets freed

}

// Creating lots of boxes just for fun

// There's no need to manually free memory!

for _ in 0u32..1_000 {

create_box();

}

// `_box2` is destroyed here, and memory gets freed

}

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Of course, we can double check for memory errors using valgrind:

$ rustc raii.rs && valgrind ./raii

==26873== Memcheck, a memory error detector

==26873== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.

==26873== Using Valgrind-3.9.0 and LibVEX; rerun with -h for copyright info

==26873== Command: ./raii

==26873==

==26873==

==26873== HEAP SUMMARY:

==26873== in use at exit: 0 bytes in 0 blocks