fn项和fn指针之间的实际区别是什么? [英] What's the practical difference between fn item and fn pointer?

问题描述

fn func(_: i64) -> bool {
    true
}

fn func_of_func(callback: &fn(i64) -> bool, arg: i64) -> bool {
    (*callback)(arg)
}

fn main() {
    let is_positive = &func;
    println!("{}", func_of_func(is_positive, 8));
    println!("{}", func_of_func(is_positive, 8));
}

无法编译:

error[E0308]: mismatched types
 --> src/main.rs:9:33
  |
9 |     println!("{}", func_of_func(is_positive, 8));
  |                                 ^^^^^^^^^^^ expected fn pointer, found fn item
  |
  = note: expected reference `&fn(i64) -> bool`
             found reference `&fn(i64) -> bool {func}`

为什么我传递了一个指针而不是fn时会发生此错误?我想知道使用fn和指向fn的指针之间的实际区别.

Why does this error occur while I have passed a pointer, not fn? I want to know the practical difference between using fn and pointer to fn.

推荐答案

fn(i64) -> bool是已经有函数指针，因此&fn(i64) -> bool是引用指向函数指针.由于函数指针是Copy，因此您永远没有任何理由要编写此指针.

fn(i64) -> bool is already a function pointer, so &fn(i64) -> bool is a reference to a function pointer. Since function pointers are Copy, you should never have any reason to write this.

如果您要编写一个函数，将类似函数的参数用作参数，则通常应使用泛型(或impl Fn，如

If you're writing a function that takes something function-like as an argument, you should usually use generics (or impl Fn, as in Mike Graham's answer, which means the same thing):

fn func_of_func<F: FnOnce(i64) -> bool>(callback: F, arg: i64) -> bool {
    callback(arg)
}

这意味着，当使用诸如func之类的函数项调用func_of_func时，callback将被编译为直接函数调用而不是函数指针，这对于编译器来说更容易优化.

This means that when you call func_of_func with a function item such as func, callback will be compiled to a direct function call instead of a function pointer, which is easier for the compiler to optimize.

如果函数不能通用(也许是因为它是对象安全特征的成员)，则通常应使用特征对象，从而允许调用者传递闭包:

If the function cannot be made generic (perhaps because it's a member of an object safe trait), you should usually use trait objects instead, which allows the caller to pass a closure:

fn func_of_func(callback: &dyn Fn(i64) -> bool, arg: i64) -> bool { ... }
fn func_of_func(callback: &mut dyn FnMut(i64) -> bool, arg: i64) -> bool { ... }
// using `FnOnce` requires boxing
fn func_of_func(callback: Box<dyn FnOnce(i64) -> bool>, arg: i64) -> bool { ... }

仅当函数绝对不能捕获任何内容时，才应使用函数指针.它们对于使用C的FFI以及在通用结构中作为PhantomData的类型参数最有用.

Function pointers should only be used when the function definitely cannot capture anything. They are mostly useful for FFI with C, and as type parameters to PhantomData in generic structs.

• 在Rust中，什么是`fn()-> ()`?

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