How can I store function pointers in an array? [du

问题:

This question already has an answer here:

• Accepting named functions vs. unboxed closures : distinct types issues 1 answer

How do you stick functions (or function pointers) into an array for testing purposes?

fn foo() -> isize { 1 }
fn bar() -> isize { 2 }

fn main() {
    let functions = vec![foo, bar];
    println!("foo() = {}, bar() = {}", functions[0](), functions[1]());
}

This code in the Rust playground

This is the error code I get:

error: mismatched types:
 expected `fn() -> isize {foo}`,
    found `fn() -> isize {bar}`
(expected fn item,
    found a different fn item) [E0308]

    let functions = vec![foo, bar];
                              ^~~

Rust is treating my functions (values) as different types despite having the same signatures, which I find surprising.

回答1:

At some point recently, each function was given its own, distinct type for... reasons that I don't recall. Upshot is that you need to give the compiler a hint (note the type on functions):

fn foo() -> isize {
    1
}
fn bar() -> isize {
    2
}
fn main() {
    let functions: Vec<fn() -> isize> = vec![foo, bar];
    println!("foo() = {}, bar() = {}", functions[0](), functions[1]());
}

You can also do this like so:

let functions = vec![foo as fn() -> isize, bar];