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Why do owned slices take 16 bytes in rust? (on x64 machine)
Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern)
Data science time! April 2019 and salary with experience
The Ask Question Wizard is Live!Why does Rust borrow checker reject this code?Is &[T] literally an alias of Slice in rust?Why doesn't println! work in Rust unit tests?What is vector slice in Rust?Why are Rust executables so huge?Why are explicit lifetimes needed in Rust?Why is a &str called a slice in Rust?How To Assign Slices While Iterating Over a Vec in Rust without copying?How can I get a single byte from stdin in Rust without waiting for Enter?Creating a slice and appending to it in Rust
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fn main()
// prints 8, 8, 16
println!(
", , ",
std::mem::size_of::<Box<i8>>(),
std::mem::size_of::<Box<&[i8]>>(),
std::mem::size_of::<Box<[i8]>>(),
);
Why do owned slices take 16 bytes but referenced slices take only 8?
rust
add a comment |
fn main()
// prints 8, 8, 16
println!(
", , ",
std::mem::size_of::<Box<i8>>(),
std::mem::size_of::<Box<&[i8]>>(),
std::mem::size_of::<Box<[i8]>>(),
);
Why do owned slices take 16 bytes but referenced slices take only 8?
rust
Playground: play.rust-lang.org/…
– French Boiethios
6 hours ago
2
Previous discussion: i.stack.imgur.com/Xt6L3.png
– hellow
6 hours ago
add a comment |
fn main()
// prints 8, 8, 16
println!(
", , ",
std::mem::size_of::<Box<i8>>(),
std::mem::size_of::<Box<&[i8]>>(),
std::mem::size_of::<Box<[i8]>>(),
);
Why do owned slices take 16 bytes but referenced slices take only 8?
rust
fn main()
// prints 8, 8, 16
println!(
", , ",
std::mem::size_of::<Box<i8>>(),
std::mem::size_of::<Box<&[i8]>>(),
std::mem::size_of::<Box<[i8]>>(),
);
Why do owned slices take 16 bytes but referenced slices take only 8?
rust
rust
edited 6 hours ago
Boann
37.6k1291123
37.6k1291123
asked 7 hours ago
aminamin
1,28812042
1,28812042
Playground: play.rust-lang.org/…
– French Boiethios
6 hours ago
2
Previous discussion: i.stack.imgur.com/Xt6L3.png
– hellow
6 hours ago
add a comment |
Playground: play.rust-lang.org/…
– French Boiethios
6 hours ago
2
Previous discussion: i.stack.imgur.com/Xt6L3.png
– hellow
6 hours ago
Playground: play.rust-lang.org/…
– French Boiethios
6 hours ago
Playground: play.rust-lang.org/…
– French Boiethios
6 hours ago
2
2
Previous discussion: i.stack.imgur.com/Xt6L3.png
– hellow
6 hours ago
Previous discussion: i.stack.imgur.com/Xt6L3.png
– hellow
6 hours ago
add a comment |
2 Answers
2
active
oldest
votes
Box<T>
is basically *const T
(Actually it's a newtype around Unique<T>
, which itself is a NonNull<T>
with PhantomData<T>
(for dropck), but let's stick to *const T
for simplicity).
A pointer in rust normally has the same size as size_of::<usize>()
except when T
is a Dynamically sized type (DST). Currently, a Box<DST>
is 2 * size_of::<usize>()
in size (The exact representation is not stable at the time of writing). A pointer to a DST is called FatPtr
.
Currently, there are two kinds of DSTs: Slices and traits. A FatPtr
to a slice is defined like this:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
For a trait pointer, len
is replaced by a pointer to the vtable
.
Box<i8>
:i8
is a sized type => basically the same as*const i8
=> 8 bytes in size (on 64 bit)Box<[i8]>
:[i8]
is a DST => basically the same asFatPtr<T>
=> 16 bytes in size (on 64 bit)Box<&[i8]>
:&[i8]
is not a DST. It's basically the same as*const FatPtr<T>
=> 8 bytes in size (on 64 bit)
add a comment |
The size of a reference depends on the "sizeness" of the referenced type:
- A reference to a sized type is a single pointer to the memory address.
A reference to an unsized type is a pointer to the memory and the size of the pointed datum. That's what is called a fat pointer:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
A Box
is a special kind of pointer that points to the heap, but it is a pointer anyway.
Knowing that, you understand that:
Box<i8>
is 8 bytes becausei8
is sized,Box<&[i8]>
is 8 bytes because a reference is sized,Box<[i8]>
is 8 bytes because a slice is unsized
Box<[i8]>
is 16 in your own playground link
– Stargateur
7 mins ago
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Box<T>
is basically *const T
(Actually it's a newtype around Unique<T>
, which itself is a NonNull<T>
with PhantomData<T>
(for dropck), but let's stick to *const T
for simplicity).
A pointer in rust normally has the same size as size_of::<usize>()
except when T
is a Dynamically sized type (DST). Currently, a Box<DST>
is 2 * size_of::<usize>()
in size (The exact representation is not stable at the time of writing). A pointer to a DST is called FatPtr
.
Currently, there are two kinds of DSTs: Slices and traits. A FatPtr
to a slice is defined like this:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
For a trait pointer, len
is replaced by a pointer to the vtable
.
Box<i8>
:i8
is a sized type => basically the same as*const i8
=> 8 bytes in size (on 64 bit)Box<[i8]>
:[i8]
is a DST => basically the same asFatPtr<T>
=> 16 bytes in size (on 64 bit)Box<&[i8]>
:&[i8]
is not a DST. It's basically the same as*const FatPtr<T>
=> 8 bytes in size (on 64 bit)
add a comment |
Box<T>
is basically *const T
(Actually it's a newtype around Unique<T>
, which itself is a NonNull<T>
with PhantomData<T>
(for dropck), but let's stick to *const T
for simplicity).
A pointer in rust normally has the same size as size_of::<usize>()
except when T
is a Dynamically sized type (DST). Currently, a Box<DST>
is 2 * size_of::<usize>()
in size (The exact representation is not stable at the time of writing). A pointer to a DST is called FatPtr
.
Currently, there are two kinds of DSTs: Slices and traits. A FatPtr
to a slice is defined like this:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
For a trait pointer, len
is replaced by a pointer to the vtable
.
Box<i8>
:i8
is a sized type => basically the same as*const i8
=> 8 bytes in size (on 64 bit)Box<[i8]>
:[i8]
is a DST => basically the same asFatPtr<T>
=> 16 bytes in size (on 64 bit)Box<&[i8]>
:&[i8]
is not a DST. It's basically the same as*const FatPtr<T>
=> 8 bytes in size (on 64 bit)
add a comment |
Box<T>
is basically *const T
(Actually it's a newtype around Unique<T>
, which itself is a NonNull<T>
with PhantomData<T>
(for dropck), but let's stick to *const T
for simplicity).
A pointer in rust normally has the same size as size_of::<usize>()
except when T
is a Dynamically sized type (DST). Currently, a Box<DST>
is 2 * size_of::<usize>()
in size (The exact representation is not stable at the time of writing). A pointer to a DST is called FatPtr
.
Currently, there are two kinds of DSTs: Slices and traits. A FatPtr
to a slice is defined like this:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
For a trait pointer, len
is replaced by a pointer to the vtable
.
Box<i8>
:i8
is a sized type => basically the same as*const i8
=> 8 bytes in size (on 64 bit)Box<[i8]>
:[i8]
is a DST => basically the same asFatPtr<T>
=> 16 bytes in size (on 64 bit)Box<&[i8]>
:&[i8]
is not a DST. It's basically the same as*const FatPtr<T>
=> 8 bytes in size (on 64 bit)
Box<T>
is basically *const T
(Actually it's a newtype around Unique<T>
, which itself is a NonNull<T>
with PhantomData<T>
(for dropck), but let's stick to *const T
for simplicity).
A pointer in rust normally has the same size as size_of::<usize>()
except when T
is a Dynamically sized type (DST). Currently, a Box<DST>
is 2 * size_of::<usize>()
in size (The exact representation is not stable at the time of writing). A pointer to a DST is called FatPtr
.
Currently, there are two kinds of DSTs: Slices and traits. A FatPtr
to a slice is defined like this:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
For a trait pointer, len
is replaced by a pointer to the vtable
.
Box<i8>
:i8
is a sized type => basically the same as*const i8
=> 8 bytes in size (on 64 bit)Box<[i8]>
:[i8]
is a DST => basically the same asFatPtr<T>
=> 16 bytes in size (on 64 bit)Box<&[i8]>
:&[i8]
is not a DST. It's basically the same as*const FatPtr<T>
=> 8 bytes in size (on 64 bit)
edited 6 hours ago
answered 6 hours ago
Tim DiekmannTim Diekmann
3,56291940
3,56291940
add a comment |
add a comment |
The size of a reference depends on the "sizeness" of the referenced type:
- A reference to a sized type is a single pointer to the memory address.
A reference to an unsized type is a pointer to the memory and the size of the pointed datum. That's what is called a fat pointer:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
A Box
is a special kind of pointer that points to the heap, but it is a pointer anyway.
Knowing that, you understand that:
Box<i8>
is 8 bytes becausei8
is sized,Box<&[i8]>
is 8 bytes because a reference is sized,Box<[i8]>
is 8 bytes because a slice is unsized
Box<[i8]>
is 16 in your own playground link
– Stargateur
7 mins ago
add a comment |
The size of a reference depends on the "sizeness" of the referenced type:
- A reference to a sized type is a single pointer to the memory address.
A reference to an unsized type is a pointer to the memory and the size of the pointed datum. That's what is called a fat pointer:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
A Box
is a special kind of pointer that points to the heap, but it is a pointer anyway.
Knowing that, you understand that:
Box<i8>
is 8 bytes becausei8
is sized,Box<&[i8]>
is 8 bytes because a reference is sized,Box<[i8]>
is 8 bytes because a slice is unsized
Box<[i8]>
is 16 in your own playground link
– Stargateur
7 mins ago
add a comment |
The size of a reference depends on the "sizeness" of the referenced type:
- A reference to a sized type is a single pointer to the memory address.
A reference to an unsized type is a pointer to the memory and the size of the pointed datum. That's what is called a fat pointer:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
A Box
is a special kind of pointer that points to the heap, but it is a pointer anyway.
Knowing that, you understand that:
Box<i8>
is 8 bytes becausei8
is sized,Box<&[i8]>
is 8 bytes because a reference is sized,Box<[i8]>
is 8 bytes because a slice is unsized
The size of a reference depends on the "sizeness" of the referenced type:
- A reference to a sized type is a single pointer to the memory address.
A reference to an unsized type is a pointer to the memory and the size of the pointed datum. That's what is called a fat pointer:
#[repr(C)]
struct FatPtr<T>
data: *const T,
len: usize,
A Box
is a special kind of pointer that points to the heap, but it is a pointer anyway.
Knowing that, you understand that:
Box<i8>
is 8 bytes becausei8
is sized,Box<&[i8]>
is 8 bytes because a reference is sized,Box<[i8]>
is 8 bytes because a slice is unsized
answered 6 hours ago
French BoiethiosFrench Boiethios
11.4k44081
11.4k44081
Box<[i8]>
is 16 in your own playground link
– Stargateur
7 mins ago
add a comment |
Box<[i8]>
is 16 in your own playground link
– Stargateur
7 mins ago
Box<[i8]>
is 16 in your own playground link– Stargateur
7 mins ago
Box<[i8]>
is 16 in your own playground link– Stargateur
7 mins ago
add a comment |
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Playground: play.rust-lang.org/…
– French Boiethios
6 hours ago
2
Previous discussion: i.stack.imgur.com/Xt6L3.png
– hellow
6 hours ago