使用方法、关联函数和自由函数在 Rust 中初始化函数指针之间的区别

这个问题是在这question个问题之后提出的.

作为Rust的新手，我正在try 了解函数指针(FN)的初始化方式.以下是对我正在编写的应用程序的简短且简单得多的描述.

Controller是用route: i32值初始化的.稍后，该值可以被修改，并且所有旧值被存储在列表中，作为history of changes.

下面是一个‘lib.rs’示例内容:

use futures::future;
use random_number::random;



pub struct Controller {
    pub route: i32,
    pub running_history_modified_routes: Vec<i32>
}

impl Controller {

     // a 'route' is initialized with an integer
     pub fn new(p: i32) -> Controller {
         Controller { route: p, running_history_modified_routes: Vec::new()}
     }

     // Subsequently, the 'route' may get a new value.
     // Here, we are generating a random new value and storing that in
     // vector, as history of changes to route.
     pub fn compute_a_new_route (&mut self, using_seed: i32) -> &mut Controller {

            // My confusion is around this initialization!
            let as_function_pointer: fn(i32) -> i32 = free_function_generate_random_route_value;

            let a_fresh_route = self.get_a_route_afresh(using_seed,as_function_pointer);
            self.running_history_modified_routes.push(a_fresh_route);
            self

     }

     fn get_a_route_afresh(&self, seed_as_higher_limit: i32, f:fn(i32) -> i32) -> i32 {
             f(seed_as_higher_limit)
     }

     fn method_generate_random_route_value(&self,seed_as_higher_limit: i32) -> i32 {
         random!(0 as i32, seed_as_higher_limit)
     }

    fn assoc_function_generate_random_route_value(seed_as_higher_limit: i32) -> i32 {
        random!(0 as i32, seed_as_higher_limit)
    }
}

fn free_function_generate_random_route_value(seed_as_higher_limit: i32) -> i32 {
    random!(0 as i32, seed_as_higher_limit)
}

fn get_a_route_afresh(..)接收函数指针并调用它以获得新的路由值(在本例中，这当然是一个随机数:-).

我有三个不同的候选函数指针(在上面的代码中注释):

• 控制器的实现方法method_generate_random_route_value
• 控制器的关联功能assoc_function_generate_random_route_value
• 模块的自由功能free_function_generate_random_route_value

我的理解是，它们中的每一个都可以用来在调用same way！但是，当我这样做时，编译器不同意:

let as_function_pointer: fn(i32) -> i32 = self.method_generate_random_route_value;

他告诉我:

error[E0615]: attempted to take value of method `method_generate_random_route_value` on type `&mut Controller`
  --> src/lib.rs:20:60
   |
20 |             let as_function_pointer: fn(i32) -> i32 = self.method_generate_random_route_value;
   |                                                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ method, not a field
   |
help: use parentheses to call the method
   |
20 |             let as_function_pointer: fn(i32) -> i32 = self.method_generate_random_route_value(_);
   |                                                                                              +++

当然，当我使用关联或自由函数时，编译器和我都很高兴.

当需要函数指针时，我在(impl)方法的适用性方面错过了什么？

下面是Cargo.toml条:

[package]
name = "FutureExperiment"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[dependencies]
random-number = "0.1.8"


[lib]
name="function_pointers_experiment"
path="src/lib.rs"