# Stack-allocated `poem` Vectors
This create exports 3 different implementations of a `Vec<T>`-like collection that is:
* able to be parsed like a `Vec<T>` would be, in HTTP route parameters of `poem-openapi` servers.
* allocated on the stack
You would use these to reduce the amount of allocations involved in parsing route parameters, as well as to specify slightly more exact behavior than `Vec<T>`.
## Features
The three vector types exported correspond directly to crate features:
* `PoemArrayVec` is exported by the `arrayvec` feature
* `PoemHeaplessVec` is exported by the `heapless` feature
* `PoemSmallVec` is exported by the `smallvec` feature
## Usage
Use one of the 3 vec types as a route parameter type. Using the `arrayvec` feature, that looks like this:
```rust
use poem_openapi::{OpenApi, param::Query, payload::Json};
use poem_stackvec::PoemArrayVec;
struct Api;
#[OpenApi]
impl Api {
#[oai(path = "/echo", method = "get")]
async fn echo(
&self,
#[oai(explode = false)]
// specifies max of 20 elements
Query(data): Query<PoemArrayVec<u16, 20>>,
) -> poem::Result<Json<Vec<u16>>> {
let slice = data.as_slice();
Ok(Json(Vec::from(slice)))
}
}
```
## Type Differences
The OpenAPI semantics specified by these types differ slightly from the default `Vec<T>` or `[T; SIZE]` in the following way:
### Min/Max Element Counts
both `PoemArrayVec` and `PoemHeaplessVec` specify OpenAPI properties for the minimum (1) & maximum (`SIZE`) for the number of items in an array.
`PoemSmallVec` specifies a minimum of 1 item, unlike `Vec<T>` which has no minimum. It does not specify a maximum.
If you want to make a route parameter optional (allow 0 elements), wrap the type in `Option`:
```rust
Option<PoemArrayVec<T, 8>>
```