# heapsz
A crate for calculating the heap usage of a data structure.
It's simple. Only one required method in the `HeapSize` trait, which can be
generated by `#[derive(HeapSize)]`.
It's fast. It estimates an approximate heap size in O(1) time.
## Usage
### Examples
**Calculate `heap_size()` selectively**
<table>
<tr><th> Example </th><th> Expanded example </th></tr>
<tr><td>
```rust,ignore
#[derive(HeapSize)]
struct Foo {
a: usize,
b: Option<u64>,
#[heap_size]
c: Box<[usize; 5]>,
#[heap_size]
d: Vec<usize>,
}
```
</td><td>
```rust,ignore
impl HeapSize for Foo {
fn heap_size(&self) -> usize {
self.c.heap_size() + self.d.heap_size()
}
}
```
</td></tr>
</table>
<details>
<summary><b>Calculate <code>heap_size()</code> of all fields</b></summary>
<table>
<tr><th> Example </th><th> Expanded example </th></tr>
<tr><td>
```rust,ignore
#[derive(HeapSize)]
#[heap_size]
struct Foo {
a: usize,
b: Option<u64>,
c: Box<[usize; 5]>,
d: Vec<usize>,
}
```
</td><td>
```rust,ignore
impl HeapSize for Foo {
fn heap_size(&self) -> usize {
self.a.heap_size()
+ self.b.heap_size()
+ self.c.heap_size()
+ self.d.heap_size()
}
}
```
</td></tr>
</table>
</details>
<details>
<summary><b>Skip irrelative fields</b></summary>
<table>
<tr><th> Example </th><th> Expanded example </th></tr>
<tr><td>
```rust,ignore
#[derive(HeapSize)]
#[heap_size]
struct Foo {
a: usize,
b: Option<u64>,
#[heap_size(skip)]
c: Arc<[usize; 5]>,
d: Vec<usize>,
}
```
</td><td>
```rust,ignore
impl HeapSize for Foo {
fn heap_size(&self) -> usize {
self.a.heap_size()
+ self.b.heap_size()
+ self.d.heap_size()
}
}
```
</td></tr>
</table>
</details>
<details>
<summary><b>Implement HeapSize for third-party structs</b></summary>
<table>
<tr><th> Example </th><th> Expanded example </th></tr>
<tr><td>
```rust,ignore
mod bytes_heap_size {
pub heap_size(
b: &Bytes
) -> usize {
b.len()
}
}
#[derive(HeapSize)]
#[heap_size]
struct Foo {
a: usize,
b: Option<u64>,
c: Box<[usize; 5]>,
d: Bytes,
}
```
</td><td>
```rust,ignore
impl HeapSize for Foo {
fn heap_size(&self) -> usize {
self.a.heap_size()
+ self.b.heap_size()
+ bytes_heap_size::heap_size(&self.d)
}
}
```
</td></tr>
</table>
</details>
## `#[derive(HeapSize)]`
### Field attributes
Apply to one field in a struct or in an enum variant.
* `#[heap_size]`
By default, `#[derive(HeapSize)]` generates an empty implementation which
always return 0. `#[heap_size]` needs to be added to fields that have a heap
allocation.
This is because of 1) most struct does not contain heap allocation at all,
and 2) they often do not implement the `HeapSize` trait.
* `#[heap_size(skip)]`
Skip this field: do not calculate its heap size.
Requires a struct has a container attribute `#[heap_size]`.
* `#[heap_size(with = "module")]`
`#[derive(HeapSize)]` will use `$module::heap_size` as the function to obtain
this field’s heap size.
### Container attributes
Apply to a struct or enum declaration.
* `#[heap_size]`
By default, `#[derive(HeapSize)]` generates an empty implementation that
always return 0. By adding `#[heap_size]`, it sums up `heap_size()` of all
fields in a struct or an enum.
### Variant attributes
Apply to a variant of an enum.
* `#[heap_size]`
Generate
By default, `#[derive(HeapSize)]` generates an empty implementation which
always return 0. By adding `#[heap_size]`, it sums up `heap_size()` of fields
in a variant.
* `#[heap_size(skip)]`
Skip this variant: do not calculate its heap size.
Requires an enum has a `#[heap_size]` container attribute.
## License
This project is licensed under the [MIT license](https://github.com/overvenus/heapsz/blob/main/LICENSE).