Embedding a Rust generic with generic functions inside another struct

Question

I have this generic type:

pub struct MyContainer<T, S> {
    array: Vec<T>,
    get_size: S,
    size: u32,
}

impl<T: Default, S> MyContainer<T, S>
where
    S: Fn(&T) -> u32,
{
    pub fn new(size: u32, get_size: S) -> MyContainer<T, S> {
        let array = Vec::with_capacity(size as usize);
        MyContainer {
            array,
            get_size,
            size,
        }
    }
}

I can easily create a container using compiler deduction magic:

pub fn get_size_func(h: &House) -> u32 {
    h.num_rooms
}

let container = MyContainer::new(6, get_size);

However, I'm hitting an issue when I try to instantiate my generic type in another struct:

pub struct City {
    suburbs: MyContainer<House, fn(&House) -> u32>,
}

impl City {
    pub fn new(num_houses: u32) -> City {
        let container = MyContainer::new(num_houses, get_size_func);
        City { suburbs: container }
    }
}

I get

error[E0308]: mismatched types
  --> src/lib.rs:44:25
   |
44 |         City { suburbs: container }
   |                         ^^^^^^^^^ expected fn pointer, found fn item
   |
   = note: expected type `MyContainer<_, for<'r> fn(&'r House) -> u32>`
              found type `MyContainer<_, for<'r> fn(&'r House) -> u32 {get_size_func}>`

Here's the Rust playground that reproduces the problem

Your question may be answered by the answers of Right way to have Function pointers in struct / How do I call a function through a member variable?. If you agree, we can mark this question as already answered. — Shepmaster
– Shepmaster, Commented Nov 2, 2018 at 2:26
Ah, I need fn instead of Fn. But I'm now hitting a case where when I try to call the City new function. Thanks for the other tips BTW, I'll try to create a new Rust playground should I have another question. — NutellaFan
– NutellaFan, Commented Nov 2, 2018 at 3:32

Shepmaster · Accepted Answer · 2018-11-03 23:45:07Z

1

There's two answers to this question:

Specifying the type parameters when creating the the container. I don't know why that works, but it does:
```
let container = MyContainer::<House, fn(&House) -> u32>::new(num_houses, get_size_func);
```
You can also define a generic function that calls a generic trait for an added kick. For example:
```
 let container = MyContainer::<House, fn(&House) -> u32>::new(num_houses, get_size_func2::<House>);
```
get_size_func2 would be a trait bound generic function.

Here's the full playground. For both solutions, type parameters are required and not deduced in the City::new function.

edited Nov 3, 2018 at 23:45

Shepmaster

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answered Nov 2, 2018 at 17:43

NutellaFan

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5 Comments

Shepmaster Over a year ago

What is a function signature and type? — every function has a unique type that is not the function pointer type.

Desty Over a year ago

@Shepmaster can you hint as to why this is? It seems that to have generic functions (e.g. a table of binary operators that take two u64s and return another u64) requires function signatures to be repeated with an as fn(x) -> y cast. What's the benefit?

Shepmaster Over a year ago

@Desty why do you say that?

Desty Over a year ago

@Shepmaster just wondering if you knew the reason that the "every function has a unique type" feature was added to Rust. Like, what problem that solves. I just discovered it while trying to apply a series of functions to some input, and was surprised that it was not possible without casting them to a more generic function pointer with as.

Desty Over a year ago

@Shepmaster oh just noticed your example in the Playground where that isn't necessary. Now I'm more confused than ever! :D

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