363 lines
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363 lines
18 KiB
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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="1_background_information.html"><strong aria-hidden="true">1.</strong> Some background information</a></li><li><a href="2_trait_objects.html" class="active"><strong aria-hidden="true">2.</strong> Trait objects and fat pointers</a></li><li><a href="3_generators_pin.html"><strong aria-hidden="true">3.</strong> Generators</a></li><li><a href="4_pin.html"><strong aria-hidden="true">4.</strong> Pin</a></li><li><a href="6_future_example.html"><strong aria-hidden="true">5.</strong> Futures - our main example</a></li><li><a href="8_finished_example.html"><strong aria-hidden="true">6.</strong> Finished example (editable)</a></li><li class="affix"><a href="conclusion.html">Conclusion and exercises</a></li></ol>
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<h1 class="menu-title">Futures Explained in 200 Lines of Rust</h1>
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<h1><a class="header" href="#trait-objects-and-fat-pointers" id="trait-objects-and-fat-pointers">Trait objects and fat pointers</a></h1>
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<blockquote>
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<p><strong>Relevant for:</strong></p>
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<ul>
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<li>Understanding how the Waker object is constructed</li>
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<li>Getting a basic feel for "type erased" objects and what they are</li>
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<li>Learning the basics of dynamic dispatch</li>
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</ul>
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</blockquote>
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<h2><a class="header" href="#trait-objects-and-dynamic-dispatch" id="trait-objects-and-dynamic-dispatch">Trait objects and dynamic dispatch</a></h2>
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<p>One of the most confusing things we encounter when implementing our own <code>Futures</code>
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is how we implement a <code>Waker</code> . Creating a <code>Waker</code> involves creating a <code>vtable</code>
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which allows us to use dynamic dispatch to call methods on a <em>type erased</em> trait
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object we construct our selves.</p>
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<blockquote>
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<p>If you want to know more about dynamic dispatch in Rust I can recommend an article written by Adam Schwalm called <a href="https://alschwalm.com/blog/static/2017/03/07/exploring-dynamic-dispatch-in-rust/">Exploring Dynamic Dispatch in Rust</a>.</p>
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</blockquote>
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<p>Let's explain this a bit more in detail.</p>
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<h2><a class="header" href="#fat-pointers-in-rust" id="fat-pointers-in-rust">Fat pointers in Rust</a></h2>
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<p>Let's take a look at the size of some different pointer types in Rust. If we
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run the following code. <em>(You'll have to press "play" to see the output)</em>:</p>
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<pre><pre class="playpen"><code class="language-rust"># use std::mem::size_of;
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trait SomeTrait { }
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fn main() {
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println!("======== The size of different pointers in Rust: ========");
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println!("&dyn Trait:-----{}", size_of::<&dyn SomeTrait>());
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println!("&[&dyn Trait]:--{}", size_of::<&[&dyn SomeTrait]>());
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println!("Box<Trait>:-----{}", size_of::<Box<SomeTrait>>());
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println!("&i32:-----------{}", size_of::<&i32>());
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println!("&[i32]:---------{}", size_of::<&[i32]>());
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println!("Box<i32>:-------{}", size_of::<Box<i32>>());
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println!("&Box<i32>:------{}", size_of::<&Box<i32>>());
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println!("[&dyn Trait;4]:-{}", size_of::<[&dyn SomeTrait; 4]>());
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println!("[i32;4]:--------{}", size_of::<[i32; 4]>());
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}
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</code></pre></pre>
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<p>As you see from the output after running this, the sizes of the references varies.
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Many are 8 bytes (which is a pointer size on 64 bit systems), but some are 16
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bytes.</p>
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<p>The 16 byte sized pointers are called "fat pointers" since they carry extra
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information.</p>
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<p><strong>Example <code>&[i32]</code> :</strong></p>
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<ul>
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<li>The first 8 bytes is the actual pointer to the first element in the array (or part of an array the slice refers to)</li>
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<li>The second 8 bytes is the length of the slice.</li>
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</ul>
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<p><strong>Example <code>&dyn SomeTrait</code>:</strong></p>
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<p>This is the type of fat pointer we'll concern ourselves about going forward.
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<code>&dyn SomeTrait</code> is a reference to a trait, or what Rust calls a <em>trait object</em>.</p>
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<p>The layout for a pointer to a <em>trait object</em> looks like this:</p>
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<ul>
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<li>The first 8 bytes points to the <code>data</code> for the trait object</li>
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<li>The second 8 bytes points to the <code>vtable</code> for the trait object</li>
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</ul>
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<p>The reason for this is to allow us to refer to an object we know nothing about
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except that it implements the methods defined by our trait. To accomplish this we use <em>dynamic dispatch</em>.</p>
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<p>Let's explain this in code instead of words by implementing our own trait
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object from these parts:</p>
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<blockquote>
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<p>This is an example of <em>editable</em> code. You can change everything in the example
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and try to run it. If you want to go back, press the undo symbol. Keep an eye
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out for these as we go forward. Many examples will be editable.</p>
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</blockquote>
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<pre><pre class="playpen"><code class="language-rust editable">// A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)
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trait Test {
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fn add(&self) -> i32;
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fn sub(&self) -> i32;
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fn mul(&self) -> i32;
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}
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// This will represent our home brewn fat pointer to a trait object
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#[repr(C)]
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struct FatPointer<'a> {
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/// A reference is a pointer to an instantiated `Data` instance
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data: &'a mut Data,
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/// Since we need to pass in literal values like length and alignment it's
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/// easiest for us to convert pointers to usize-integers instead of the other way around.
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vtable: *const usize,
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}
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// This is the data in our trait object. It's just two numbers we want to operate on.
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struct Data {
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a: i32,
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b: i32,
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}
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// ====== function definitions ======
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fn add(s: &Data) -> i32 {
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s.a + s.b
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}
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fn sub(s: &Data) -> i32 {
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s.a - s.b
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}
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fn mul(s: &Data) -> i32 {
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s.a * s.b
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}
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fn main() {
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let mut data = Data {a: 3, b: 2};
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// vtable is like special purpose array of pointer-length types with a fixed
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// format where the three first values has a special meaning like the
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// length of the array is encoded in the array itself as the second value.
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let vtable = vec![
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0, // pointer to `Drop` (which we're not implementing here)
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6, // lenght of vtable
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8, // alignment
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// we need to make sure we add these in the same order as defined in the Trait.
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add as usize, // function pointer - try changing the order of `add`
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sub as usize, // function pointer - and `sub` to see what happens
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mul as usize, // function pointer
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];
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let fat_pointer = FatPointer { data: &mut data, vtable: vtable.as_ptr()};
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let test = unsafe { std::mem::transmute::<FatPointer, &dyn Test>(fat_pointer) };
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// And voalá, it's now a trait object we can call methods on
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println!("Add: 3 + 2 = {}", test.add());
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println!("Sub: 3 - 2 = {}", test.sub());
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println!("Mul: 3 * 2 = {}", test.mul());
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}
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</code></pre></pre>
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<p>The reason we go through this will be clear later on when we implement our own
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<code>Waker</code> we'll actually set up a <code>vtable</code> like we do here to and knowing what
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it is will make this much less mysterious.</p>
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