# mem_dbg
[](https://crates.io/crates/mem_dbg)
[](https://crates.io/crates/mem_dbg/reverse_dependencies)
[](https://github.com/zommiommy/mem_dbg-rs)
[](https://crates.io/crates/mem_dbg)
[](https://docs.rs/mem_dbg)
Traits and associated procedural macros to display recursively the layout and
memory usage of a value.
The trait [`MemDbg`] can be used to display the recursive layout of a value,
together with the size of each part. We provide implementations for most basic
types and a derive macro for structs and enums whose fields implement
[`MemDbg`].
To compute the size, we provide the trait [`MemSize`] and a derive macro that
can be used to compute the size of a value in bytes as the standard library
function [`std::mem::size_of`] returns the stack size of a type in bytes, but it
does not take into consideration heap memory.
# Why `MemSize`
Other traits partially provide the functionality of [`MemSize`], but either they
require implementing manually a trait, which is prone to error, or they do not
provide the flexibility necessary for [`MemDbg`]. Most importantly, [`MemSize`]
uses the type system to avoid iterating over the content of a container (a
vector, etc.) when it is not necessary, making it possible to compute instantly
the size of values occupying hundreds of gigabytes of heap memory.
This is the result of the benchmark `bench_hash_map` contained in the `examples`
directory. It builds a hash map with a hundred million entries and then measure
its heap size:
```test
Allocated: 2281701509
get_size: 1879048240 152477833 ns
deep_size_of: 1879048240 152482000 ns
size_of: 2281701432 152261958 ns
mem_size: 2281701424 209 ns
```
The first line is the number of bytes allocated by the program as returned by
[`cap`]. Then, we display the result of [`get-size`], [`deepsize`], [`size-of`],
and our own [`MemSize`]. Note that the first two crates are just measuring the
space used by the items, and not by the data structure (i.e., they are not
taking into account the load factor and the power-of-two size constraint of the
hash map). Moreover, all other crates are about six orders of magnitude slower
than our implementation, due to the necessity to iterate over all elements.
## Example
```rust
# fn main() -> Result<(), Box<dyn std::error::Error>> {
use mem_dbg::*;
#[derive(MemSize, MemDbg)]
struct Struct<A, B> {
a: A,
b: B,
test: isize,
}
#[derive(MemSize, MemDbg)]
struct Data<A> {
a: A,
b: Vec<i32>,
c: (usize, String)
}
#[derive(MemSize, MemDbg)]
enum TestEnum {
Unit,
Unit2(),
Unit3 {},
Unnamed(usize, u8),
Named { first: usize, second: u8 },
}
let b = Vec::with_capacity(100);
let s = Struct {
a: TestEnum::Unnamed(0, 16),
b: Data {
a: vec![0x42_u8; 700],
b,
c: (1, "foo".to_owned()),
},
test: -0xbadf00d,
};
println!("size: {}", s.mem_size(SizeFlags::default()));
println!("capacity: {}", s.mem_size(SizeFlags::CAPACITY));
s.mem_dbg(DbgFlags::default())?;
// Different flags can be combined
// s.mem_dbg(DbgFlags::default() | DbgFlags::CAPACITY | DbgFlags::HUMANIZE)?;
# Ok(())
# }
```
The previous program prints:
```text
size: 815
capacity: 1215
985 B 100.00% ⏺: example::Struct<example::TestEnum, example::Data<alloc::vec::Vec<u8>>>
16 B 1.62% ├╴a: example::TestEnum
│ ├╴Variant: Unnamed
8 B 0.81% │ ├╴0: usize
1 B 0.10% │ ╰╴1: u8
823 B 83.55% ├╴b: example::Data<alloc::vec::Vec<u8>>
724 B 73.50% │ ├╴a: alloc::vec::Vec<u8>
64 B 6.50% │ ├╴b: alloc::vec::Vec<i32>
35 B 3.55% │ ╰╴c: (usize, alloc::string::String)
8 B 0.81% │ ├╴0: usize
27 B 2.74% │ ╰╴1: alloc::string::String
8 B 0.81% ├╴test: isize
138 B 14.01% ╰╴s: std::collections::hash::set::HashSet<usize>
```
If we add the flags [`DbgFlags::CAPACITY`] and [`DbgFlags::HUMANIZE`] it prints:
```text
size: 815
capacity: 1215
2_407 B 100.00% ⏺: example::Struct<example::TestEnum, example::Data<alloc::vec::Vec<u8>>>
16 B 0.66% ├╴a: example::TestEnum
│ ├╴Variant: Unnamed
8 B 0.33% │ ├╴0: usize
1 B 0.04% │ ╰╴1: u8
1_183 B 49.15% ├╴b: example::Data<alloc::vec::Vec<u8>>
724 B 30.08% │ ├╴a: alloc::vec::Vec<u8>
424 B 17.62% │ ├╴b: alloc::vec::Vec<i32>
35 B 1.45% │ ╰╴c: (usize, alloc::string::String)
8 B 0.33% │ ├╴0: usize
27 B 1.12% │ ╰╴1: alloc::string::String
8 B 0.33% ├╴test: isize
1_200 B 49.85% ╰╴s: std::collections::hash::set::HashSet<usize>
```
If we use [`DbgFlags::empty()`] it prints:
```text
size: 815
capacity: 1215
985 B ⏺
16 B ├╴a
│ ├╴Variant: Unnamed
8 B │ ├╴0
1 B │ ╰╴1
823 B ├╴b
724 B │ ├╴a
64 B │ ├╴b
35 B │ ╰╴c
8 B │ ├╴0
27 B │ ╰╴1
8 B ├╴test
138 B ╰╴s
```
## Caveats
* We support out-of-the-box most basic types, and tuples up to size ten. The
derive macros `MemSize`/`MemDbg` will generate implementations for structs and
enums whose fields implement the associated interface: if this is not the case
(e.g., because of the orphan rule) one can implement the traits manually.
* Computation of the size of arrays, slices, and vectors will be performed by
iterating over their elements unless the type is a copy type that does not
contain references and it is declared as such using the attribute
`#[copy_type]`. See [`CopyType`] for more details.
* The content of vectors and slices is not expanded recursively as the output
might be too complex; this might change in the future (e.g., via a flag)
should interesting use cases arise.
* `BTreeMap`, and `BTreeSet`, are not currently supported as we still have to
figure out a way to precisely measure their memory size and capacity.
[`MemDbg`]: https://docs.rs/mem_dbg/latest/mem_dbg/trait.MemDbg.html
[`MemSize`]: https://docs.rs/mem_dbg/latest/mem_dbg/trait.MemSize.html
[`std::mem::size_of`]: https://doc.rust-lang.org/std/mem/fn.size_of.html
[`DbgFlags::CAPACITY`]: https://docs.rs/mem_dbg/latest/mem_dbg/struct.DbgFlags.html#associatedconstant.CAPACITY
[`DbgFlags::HUMANIZE`]: https://docs.rs/mem_dbg/latest/mem_dbg/struct.DbgFlags.html#associatedconstant.HUMANIZE
[`DbgFlags::empty()`]: https://docs.rs/mem_dbg/latest/mem_dbg/struct.DbgFlags.html#method.empty
[`CopyType`]: https://docs.rs/mem_dbg/latest/mem_dbg/trait.CopyType.html
[`cap`]: (https:/crates.io/crates/cap)
[`get-size`]: (https://crates.io/crates/get_size)
[`deepsize`]: (https://crates.io/crates/deepsize)
[`size-of`]: (https://crates.io/crates/size_of)