64 lines
2.4 KiB
Plaintext
64 lines
2.4 KiB
Plaintext
Per [RFC 401][rfc401], if you have a function declaration `foo`:
|
|
|
|
```
|
|
// For the purposes of this explanation, all of these
|
|
// different kinds of `fn` declarations are equivalent:
|
|
struct S;
|
|
fn foo(x: S) { /* ... */ }
|
|
extern "C" { fn foo(x: S); }
|
|
impl S { fn foo(self) { /* ... */ } }
|
|
```
|
|
|
|
the type of `foo` is **not** `fn(S)`, as one might expect.
|
|
Rather, it is a unique, zero-sized marker type written here as `typeof(foo)`.
|
|
However, `typeof(foo)` can be _coerced_ to a function pointer `fn(S)`,
|
|
so you rarely notice this:
|
|
|
|
```
|
|
let x: fn(S) = foo; // OK, coerces
|
|
```
|
|
|
|
The reason that this matter is that the type `fn(S)` is not specific to
|
|
any particular function: it's a function _pointer_. So calling `x()` results
|
|
in a virtual call, whereas `foo()` is statically dispatched, because the type
|
|
of `foo` tells us precisely what function is being called.
|
|
|
|
As noted above, coercions mean that most code doesn't have to be
|
|
concerned with this distinction. However, you can tell the difference
|
|
when using **transmute** to convert a fn item into a fn pointer.
|
|
|
|
This is sometimes done as part of an FFI:
|
|
|
|
```
|
|
extern "C" fn foo(userdata: Box<i32>) {
|
|
/* ... */
|
|
}
|
|
|
|
let f: extern "C" fn(*mut i32) = transmute(foo);
|
|
callback(f);
|
|
```
|
|
|
|
Here, transmute is being used to convert the types of the fn arguments.
|
|
This pattern is incorrect because, because the type of `foo` is a function
|
|
**item** (`typeof(foo)`), which is zero-sized, and the target type (`fn()`)
|
|
is a function pointer, which is not zero-sized.
|
|
This pattern should be rewritten. There are a few possible ways to do this:
|
|
|
|
- change the original fn declaration to match the expected signature,
|
|
and do the cast in the fn body (the preferred option)
|
|
- cast the fn item fo a fn pointer before calling transmute, as shown here:
|
|
|
|
```
|
|
let f: extern "C" fn(*mut i32) = transmute(foo as extern "C" fn(_));
|
|
let f: extern "C" fn(*mut i32) = transmute(foo as usize); // works too
|
|
```
|
|
|
|
The same applies to transmutes to `*mut fn()`, which were observedin practice.
|
|
Note though that use of this type is generally incorrect.
|
|
The intention is typically to describe a function pointer, but just `fn()`
|
|
alone suffices for that. `*mut fn()` is a pointer to a fn pointer.
|
|
(Since these values are typically just passed to C code, however, this rarely
|
|
makes a difference in practice.)
|
|
|
|
[rfc401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md
|