Rust function to return generic trait - rust

Question

I have two structs implementing same trait. I need a single fuction that returs object of any of these structure.
trait
pub trait DbfFile{
fn new(fields: HashMap<String, FieldValue>) -> Self;
}
The structures are
pub struct InsertableAttachable {
pub f1: String,
pub f2: String,
pub f3: String,
pub f4: i8,
}
impl DbfFile for InsertableAttachable{
fn new(fields: HashMap<String, FieldValue, RandomState>) -> Self {
InsertableAttachable {
// fields
}
}
}
// Converting this to generic
impl InsertableAttachable{
pub fn get_data_vector(mut iter: DatabaseRecordIterator) -> Vec<InsertableAttachable>{
let mut db_data: Vec<InsertableAttachable> = vec!();
while let Some(table_row) = iter.next() {
let fields = table_row.fields;
db_data.push(InsertableAttachable::new(fields));
};
db_data
}
}
And another structure
pub struct InsertableAttached {
pub f1: String,
pub f2: i32,
pub f3: i32,
}
impl DbfFile for InsertableAttached {
fn new(fields: HashMap<String, FieldValue, RandomState>) -> Self {
InsertableAttached {
// fields
}
}
}
// Converting this to generic
impl InsertableAttached{
pub fn get_data_vector(mut iter: DatabaseRecordIterator) -> Vec<InsertableAttached>{
let mut db_data: Vec<InsertableAttached> = vec!();
while let Some(table_row) = iter.next() {
let fields = table_row.fields;
db_data.push(InsertableAttached::new(fields));
};
db_data
}
}
Here are example like this returns box / struct and this one returns an option.
The following function has compile time error
error[E0277]: the size for values of type `(dyn models::traits::DbfFile + 'static)` cannot be known at compilation time
--> src/dbf.rs:75:1
|
75 | / pub fn get_data_vector(&mut iter: DatabaseRecordIterator) -> Vec<DbfFile>{
76 | | let mut db_data: Vec<DbfFile> = vec!();
77 | | while let Some(table_row) = iter.next() {
78 | | let fields = table_row.fields;
... |
81 | | db_data
82 | | }
| |_^ doesn't have a size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `(dyn models::traits::DbfFile + 'static)`
= note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
= note: required by `std::vec::Vec`
error[E0038]: the trait `models::traits::DbfFile` cannot be made into an object
--> src/dbf.rs:75:1
|
75 | pub fn get_data_vector(&mut iter: DatabaseRecordIterator) -> Vec<DbfFile>{
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `models::traits::DbfFile` cannot be made into an object

Answer 1

You would need to box the results in order to be able to return a heterogenous collection via dynamic dispatch:
pub fn get_data_vector(&mut iter: DatabaseRecordIterator) -> Vec<Box<dynDbfFile>>
However you will then get the following error:
| pub fn get_data_vector(&mut iter: DatabaseRecordIterator) -> Vec<Box<dyn DbfFile>> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `DbfFile` cannot be made into an object
|
The reason for this is that your function returns Self - which is different for each implementation. You would need to change it to a common type to make the trait object-safe (allow dynamic dispatch).
An alternative could be to return multiple types via an enumeration:
enum GetDataVectorResult {
Attachable(InsertableAttachable),
Attached(InsertableAttached),
}
pub fn get_data_vector(&mut iter: DatabaseRecordIterator) -> Vec<GetDataVectorResult>