我在锈WinAPI的工作，有需要回调的某些功能（如EnumWindows()）。回调通常接受一个额外的参数，其可用于某些自定义数据传递给回调（类型LPARAM其是用于i64的别名）。
我已经派Vec<T>对象作为LPARAM到WinAPI的回调和它工作得很好。例如“拆包”的lparam值到Vec<RECT>看起来像这样在我的情况：
unsafe extern "system" fn enumerate_callback(hwnd: HWND, lparam: LPARAM) -> BOOL {
let rects = lparam as *mut Vec<RECT>;
}
而是通过一个载体，我现在必须通过关闭。我不能用一个函数指针作为我的封盖具有捕捉到一些变量，如果我用一个函数，将不可访问。在C ++中，我会用std::function<>我特定的任务，我认为，在锈相应的抽象是一个封闭。
我对拆包代码如下所示：
unsafe extern "system" fn enumerate_callback(hwnd: HWND, lparam: LPARAM) -> BOOL {
let cb: &mut FnMut(HWND) -> bool = &mut *(lparam as *mut c_void as *mut FnMut(HWND) -> bool);
// ...
}
SSCCE：
use std::os::raw::c_void;
fn enum_wnd_proc(some_value: i32, lparam: i32) {
let closure: &mut FnMut(i32) -> bool =
unsafe { (&mut *(lparam as *mut c_void as *mut FnMut(i32) -> bool)) };
println!("predicate() executed and returned: {}", closure(some_value));
}
fn main() {
let sum = 0;
let mut closure = |some_value: i32| -> bool {
sum += some_value;
sum >= 100
};
let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
enum_wnd_proc(20, lparam);
}
（Qazxswpoi）
我得到这些错误：
Playground
我想知道：
error[E0277]: expected a `std::ops::FnMut<(i32,)>` closure, found `std::ffi::c_void`
--> src/main.rs:5:26
|
5 | unsafe { (&mut *(lparam as *mut c_void as *mut FnMut(i32) -> bool)) };
| ^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut<(i32,)>` closure, found `std::ffi::c_void`
|
= help: the trait `std::ops::FnMut<(i32,)>` is not implemented for `std::ffi::c_void`
= note: required for the cast to the object type `dyn std::ops::FnMut(i32) -> bool`
error[E0606]: casting `&mut [closure@src/main.rs:12:23: 15:6 sum:_]` as `*mut std::ffi::c_void` is invalid
--> src/main.rs:17:19
|
17 | let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
error[E0606]: casting `*mut dyn std::ops::FnMut(i32) -> bool` as `i32` is invalid
--> src/main.rs:17:18
|
17 | let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: cast through a thin pointer first
error[E0277]: expected a `std::ops::FnMut<(i32,)>` closure, found `std::ffi::c_void`
--> src/main.rs:17:19
|
17 | let lparam = (&mut closure as *mut c_void as *mut FnMut(i32) -> bool) as i32;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected an `FnMut<(i32,)>` closure, found `std::ffi::c_void`
|
= help: the trait `std::ops::FnMut<(i32,)>` is not implemented for `std::ffi::c_void`
= note: required for the cast to the object type `dyn std::ops::FnMut(i32) -> bool`
值将它传递给该回调的正确方法？我使用防锈的稳定版本。
首先，一些逻辑错误的代码：
i64在许多平台上（如64位）。指针可以使用所有这些位。截断指针，然后调用在被截断的地址的功能会导致非常糟糕的事情。通常你想用一台机器尺寸整数（i32或usize）。isize值必须是可变的。这个问题的肉是闭包是占用大小未知的程序员的具体类型，但是编译器知道的。 C函数被限制为采取机尺寸的整数。
由于封落实sum特征之一，我们可以采取封闭的实现，特性的参考来生成一个特质对象。以一参一特质导致了包含两个指针大小值的脂肪指针。在这种情况下，它包含一个指向被关闭，在数据和一个指向虚函数表，实现了性状的具体方法。
通常，Fn*的任何引用或Box将要产生一个脂肪指针。
在64位机，脂肪指针将总共128位，和流延到机器大小的指针将再次截断数据，导致非常糟糕的事情发生。
该解决方案一样，一切都在计算机科学别的，就是增加一些抽象层以上：
dynamically-sized type type
我们采取一个第二参考脂肪指针，它创建了一个薄的指针。这个指针是只有一台机器整数的大小。
也许图将有助于（或伤害）？
use std::os::raw::c_void;
fn enum_wnd_proc(some_value: i32, lparam: usize) {
let trait_obj_ref: &mut &mut FnMut(i32) -> bool = unsafe {
let closure_pointer_pointer = lparam as *mut c_void;
&mut *(closure_pointer_pointer as *mut _)
};
println!(
"predicate() executed and returned: {}",
trait_obj_ref(some_value)
);
}
fn main() {
let mut sum = 0;
let mut closure = |some_value: i32| -> bool {
println!("I'm summing {} + {}", sum, some_value);
sum += some_value;
sum >= 100
};
let mut trait_obj: &mut FnMut(i32) -> bool = &mut closure;
let trait_obj_ref = &mut trait_obj;
let closure_pointer_pointer = trait_obj_ref as *mut _ as *mut c_void;
let lparam = closure_pointer_pointer as usize;
enum_wnd_proc(20, lparam);
}
因为我们正在使用原始的指针，它现在是程序员的责任，以确保封闭会超越它是用来！如果Reference -> Trait object -> Concrete closure
8 bytes 16 bytes ?? bytes
某处存储指针，你必须非常小心，不要使用它关闭删除后。
作为一个侧面说明，使用enum_wnd_proc铸造性状对象时：
mem::transmute产生更好的错误信息：
use std::mem;
let closure_pointer_pointer: *mut c_void = unsafe { mem::transmute(trait_obj) };
error[E0512]: transmute called with types of different sizes
--> src/main.rs:26:57
|
26 | let closure_pointer_pointer: *mut c_void = unsafe { mem::transmute(trait_obj) };
| ^^^^^^^^^^^^^^
|
= note: source type: &mut dyn std::ops::FnMut(i32) -> bool (128 bits)
= note: target type: *mut std::ffi::c_void (64 bits)
。
也可以看看