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Merge pull request #20 from oblique/master

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Fix `size` and `align` of raw vtable
This commit is contained in:
Carl Fredrik Samson
2020-08-23 23:10:28 +02:00
committed by GitHub
parent 4f275f1339 527ec3835c
commit 734763b969
1 changed files with 14 additions and 12 deletions
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26
src/2_waker_context.md
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@@ -56,15 +56,16 @@ trait SomeTrait { }
fn main() { fn main() {
println!("======== The size of different pointers in Rust: ========"); println!("======== The size of different pointers in Rust: ========");
println!("&dyn Trait:-----{}", size_of::<&dyn SomeTrait>()); println!("&dyn Trait:------{}", size_of::<&dyn SomeTrait>());
println!("&[&dyn Trait]:--{}", size_of::<&[&dyn SomeTrait]>()); println!("&[&dyn Trait]:---{}", size_of::<&[&dyn SomeTrait]>());
println!("Box<Trait>:-----{}", size_of::<Box<SomeTrait>>()); println!("Box<Trait>:------{}", size_of::<Box<SomeTrait>>());
println!("&i32:-----------{}", size_of::<&i32>()); println!("Box<Box<Trait>>:-{}", size_of::<Box<Box<SomeTrait>>>());
println!("&[i32]:---------{}", size_of::<&[i32]>()); println!("&i32:------------{}", size_of::<&i32>());
println!("Box<i32>:-------{}", size_of::<Box<i32>>()); println!("&[i32]:----------{}", size_of::<&[i32]>());
println!("&Box<i32>:------{}", size_of::<&Box<i32>>()); println!("Box<i32>:--------{}", size_of::<Box<i32>>());
println!("[&dyn Trait;4]:-{}", size_of::<[&dyn SomeTrait; 4]>()); println!("&Box<i32>:-------{}", size_of::<&Box<i32>>());
println!("[i32;4]:--------{}", size_of::<[i32; 4]>()); println!("[&dyn Trait;4]:--{}", size_of::<[&dyn SomeTrait; 4]>());
println!("[i32;4]:---------{}", size_of::<[i32; 4]>());
} }
``` ```
@@ -98,6 +99,7 @@ Let's explain this in code instead of words by implementing our own trait
object from these parts: object from these parts:
```rust ```rust
# use std::mem::{align_of, size_of};
// A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr) // A reference to a trait object is a fat pointer: (data_ptr, vtable_ptr)
trait Test { trait Test {
fn add(&self) -> i32; fn add(&self) -> i32;
@@ -138,9 +140,9 @@ fn main() {
// format where the three first values has a special meaning like the // format where the three first values has a special meaning like the
// length of the array is encoded in the array itself as the second value. // length of the array is encoded in the array itself as the second value.
let vtable = vec![ let vtable = vec![
0, // pointer to `Drop` (which we're not implementing here) 0, // pointer to `Drop` (which we're not implementing here)
6, // length of vtable size_of::<Data>(), // length of data
8, // alignment align_of::<Data>(), // alignment of data
// we need to make sure we add these in the same order as defined in the Trait. // we need to make sure we add these in the same order as defined in the Trait.
add as usize, // function pointer - try changing the order of `add` add as usize, // function pointer - try changing the order of `add`