I'm getting the follow compile error:
static optionsRegex: regex::Regex
= match regex::Regex::new(r###"$(~?[\w-]+(?:=[^,]*)?(?:,~?[\w-]+(?:=[^,]*)?)*)$"###) {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ statics cannot evaluate destructors
Ok(r) => r,
Default => panic!("Invalid optionsRegex")
};
More details: I need to access a compiled regexp to be used by struct when creating. Any Rust documentation links or explanation appreciated.
P.S. I think I understand that Rust needs to know when to destruct it but I have no idea how to make it other than just avoid making it static and pass some struct with all the regexps every time it's needed when creating the struct.
Lazily initializing and safely re-using a static variable such as a regular expression is one of the primary use-cases of the once_cell crate. Here's an example of a validation regex that is only compiled once and re-used in a struct constructor function:
use once_cell::sync::OnceCell;
use regex::Regex;
struct Struct;
impl Struct {
fn new(options: &str) -> Result<Self, &str> {
static OPTIONS_REGEX: OnceCell<Regex> = OnceCell::new();
let options_regex = OPTIONS_REGEX.get_or_init(|| {
Regex::new(r###"$(~?[\w-]+(?:=[^,]*)?(?:,~?[\w-]+(?:=[^,]*)?)*)$"###).unwrap()
});
if options_regex.is_match(options) {
Ok(Struct)
} else {
Err("invalid options")
}
}
}
playground
Related
I want to create some references to a str with Rc, without cloning str:
fn main() {
let s = Rc::<str>::from("foo");
let t = Rc::clone(&s); // Creating a new pointer to the same address is easy
let u = Rc::clone(&s[1..2]); // But how can I create a new pointer to a part of `s`?
let w = Rc::<str>::from(&s[0..2]); // This seems to clone str
assert_ne!(&w as *const _, &s as *const _);
}
playground
How can I do this?
While it's possible in principle, the standard library's Rc does not support the case you're trying to create: a counted reference to a part of reference-counted memory.
However, we can get the effect for strings using a fairly straightforward wrapper around Rc which remembers the substring range:
use std::ops::{Deref, Range};
use std::rc::Rc;
#[derive(Clone, Debug, Eq, Hash, PartialEq)]
pub struct RcSubstr {
string: Rc<str>,
span: Range<usize>,
}
impl RcSubstr {
fn new(string: Rc<str>) -> Self {
let span = 0..string.len();
Self { string, span }
}
fn substr(&self, span: Range<usize>) -> Self {
// A full implementation would also have bounds checks to ensure
// the requested range is not larger than the current substring
Self {
string: Rc::clone(&self.string),
span: (self.span.start + span.start)..(self.span.start + span.end)
}
}
}
impl Deref for RcSubstr {
type Target = str;
fn deref(&self) -> &str {
&self.string[self.span.clone()]
}
}
fn main() {
let s = RcSubstr::new(Rc::<str>::from("foo"));
let u = s.substr(1..2);
// We need to deref to print the string rather than the wrapper struct.
// A full implementation would `impl Debug` and `impl Display` to produce
// the expected substring.
println!("{}", &*u);
}
There are a lot of conveniences missing here, such as suitable implementations of Display, Debug, AsRef, Borrow, From, and Into — I've provided only enough code to illustrate how it can work. Once supplemented with the appropriate trait implementations, this should be just as usable as Rc<str> (with the one edge case that it can't be passed to a library type that wants to store Rc<str> in particular).
The crate arcstr claims to offer a finished version of this basic idea, but I haven't used or studied it and so can't guarantee its quality.
The crate owning_ref provides a way to hold references to parts of an Rc or other smart pointer, but there are concerns about its soundness and I don't fully understand which circumstances that applies to (issue search which currently has 3 open issues).
How do I get over something like this:
struct Test {
foo: Option<fn()>
}
impl Test {
fn new(&mut self) {
self.foo = Option::Some(self.a);
}
fn a(&self) { /* can use Test */ }
}
I get this error:
error: attempted to take value of method `a` on type `&mut Test`
--> src/main.rs:7:36
|
7 | self.foo = Option::Some(self.a);
| ^
|
= help: maybe a `()` to call it is missing? If not, try an anonymous function
How do I pass a function pointer from a trait? Similar to what would happen in this case:
impl Test {
fn new(&mut self) {
self.foo = Option::Some(a);
}
}
fn a() { /* can't use Test */ }
What you're trying to do here is get a function pointer from a (to use Python terminology here, since Rust doesn't have a word for this) bound method. You can't.
Firstly, because Rust doesn't have a concept of "bound" methods; that is, you can't refer to a method with the invocant (the thing on the left of the .) already bound in place. If you want to construct a callable which approximates this, you'd use a closure; i.e. || self.a().
However, this still wouldn't work because closures aren't function pointers. There is no "base type" for callable things like in some other languages. Function pointers are a single, specific kind of callable; closures are completely different. Instead, there are traits which (when implemented) make a type callable. They are Fn, FnMut, and FnOnce. Because they are traits, you can't use them as types, and must instead use them from behind some layer of indirection, such as Box<FnOnce()> or &mut FnMut(i32) -> String.
Now, you could change Test to store an Option<Box<Fn()>> instead, but that still wouldn't help. That's because of the other, other problem: you're trying to store a reference to the struct inside of itself. This is not going to work well. If you manage to do this, you effectively render the Test value permanently unusable. More likely is that the compiler just won't let you get that far.
Aside: you can do it, but not without resorting to reference counting and dynamic borrow checking, which is out of scope here.
So the answer to your question as-asked is: you don't.
Let's change the question: instead of trying to crowbar a self-referential closure in, we can instead store a callable that doesn't attempt to capture the invocant at all.
struct Test {
foo: Option<Box<Fn(&Test)>>,
}
impl Test {
fn new() -> Test {
Test {
foo: Option::Some(Box::new(Self::a)),
}
}
fn a(&self) { /* can use Test */ }
fn invoke(&self) {
if let Some(f) = self.foo.as_ref() {
f(self);
}
}
}
fn main() {
let t = Test::new();
t.invoke();
}
The callable being stored is now a function that takes the invocant explicitly, side-stepping the issues with cyclic references. We can use this to store Test::a directly, by referring to it as a free function. Also note that because Test is the implementation type, I can also refer to it as Self.
Aside: I've also corrected your Test::new function. Rust doesn't have constructors, just functions that return values like any other.
If you're confident you will never want to store a closure in foo, you can replace Box<Fn(&Test)> with fn(&Test) instead. This limits you to function pointers, but avoids the extra allocation.
If you haven't already, I strongly urge you to read the Rust Book.
There are few mistakes with your code. new function (by the convention) should not take self reference, since it is expected to create Self type.
But the real issue is, Test::foo expecting a function type fn(), but Test::a's type is fn(&Test) == fn a(&self) if you change the type of foo to fn(&Test) it will work. Also you need to use function name with the trait name instead of self. Instead of assigning to self.a you should assign Test::a.
Here is the working version:
extern crate chrono;
struct Test {
foo: Option<fn(&Test)>
}
impl Test {
fn new() -> Test {
Test {
foo: Some(Test::a)
}
}
fn a(&self) {
println!("a run!");
}
}
fn main() {
let test = Test::new();
test.foo.unwrap()(&test);
}
Also if you gonna assign a field in new() function, and the value must always set, then there is no need to use Option instead it can be like that:
extern crate chrono;
struct Test {
foo: fn(&Test)
}
impl Test {
fn new() -> Test {
Test {
foo: Test::a
}
}
fn a(&self) {
println!("a run!");
}
}
fn main() {
let test = Test::new();
(test.foo)(&test); // Make sure the paranthesis are there
}
When writing callbacks for generic interfaces, it can be useful for them to define their own local data which they are responsible for creating and accessing.
In C I would just use a void pointer, C-like example:
struct SomeTool {
int type;
void *custom_data;
};
void invoke(SomeTool *tool) {
StructOnlyForThisTool *data = malloc(sizeof(*data));
/* ... fill in the data ... */
tool.custom_data = custom_data;
}
void execute(SomeTool *tool) {
StructOnlyForThisTool *data = tool.custom_data;
if (data.foo_bar) { /* do something */ }
}
When writing something similar in Rust, replacing void * with Option<Box<Any>>, however I'm finding that accessing the data is unreasonably verbose, eg:
struct SomeTool {
type: i32,
custom_data: Option<Box<Any>>,
};
fn invoke(tool: &mut SomeTool) {
let data = StructOnlyForThisTool { /* my custom data */ }
/* ... fill in the data ... */
tool.custom_data = Some(Box::new(custom_data));
}
fn execute(tool: &mut SomeTool) {
let data = tool.custom_data.as_ref().unwrap().downcast_ref::<StructOnlyForThisTool>().unwrap();
if data.foo_bar { /* do something */ }
}
There is one line here which I'd like to be able to write in a more compact way:
tool.custom_data.as_ref().unwrap().downcast_ref::<StructOnlyForThisTool>().unwrap()
tool.custom_data.as_ref().unwrap().downcast_mut::<StructOnlyForThisTool>().unwrap()
While each method makes sense on its own, in practice it's not something I'd want to write throughout a code-base, and not something I'm going to want to type out often or remember easily.
By convention, the uses of unwrap here aren't dangerous because:
While only some tools define custom data, the ones that do always define it.
When the data is set, by convention the tool only ever sets its own data. So there is no chance of having the wrong data.
Any time these conventions aren't followed, its a bug and should panic.
Given these conventions, and assuming accessing custom-data from a tool is something that's done often - what would be a good way to simplify this expression?
Some possible options:
Remove the Option, just use Box<Any> with Box::new(()) representing None so access can be simplified a little.
Use a macro or function to hide verbosity - passing in the Option<Box<Any>>: will work of course, but prefer not - would use as a last resort.
Add a trait to Option<Box<Any>> which exposes a method such as tool.custom_data.unwrap_box::<StructOnlyForThisTool>() with matching unwrap_box_mut.
Update 1): since asking this question a point I didn't include seems relevant.
There may be multiple callback functions like execute which must all be able to access the custom_data. At the time I didn't think this was important to point out.
Update 2): Wrapping this in a function which takes tool isn't practical, since the borrow checker then prevents further access to members of tool until the cast variable goes out of scope, I found the only reliable way to do this was to write a macro.
If the implementation really only has a single method with a name like execute, that is a strong indication to consider using a closure to capture the implementation data. SomeTool can incorporate an arbitrary callable in a type-erased manner using a boxed FnMut, as shown in this answer. execute() then boils down to invoking the closure stored in the struct field implementation closure using (self.impl_)(). For a more general approach, that will also work when you have more methods on the implementation, read on.
An idiomatic and type-safe equivalent of the type+dataptr C pattern is to store the implementation type and pointer to data together as a trait object. The SomeTool struct can contain a single field, a boxed SomeToolImpl trait object, where the trait specifies tool-specific methods such as execute. This has the following characteristics:
You no longer need an explicit type field because the run-time type information is incorporated in the trait object.
Each tool's implementation of the trait methods can access its own data in a type-safe manner without casts or unwraps. This is because the trait object's vtable automatically invokes the correct function for the correct trait implementation, and it is a compile-time error to try to invoke a different one.
The "fat pointer" representation of the trait object has the same performance characteristics as the type+dataptr pair - for example, the size of SomeTool will be two pointers, and accessing the implementation data will still involve a single pointer dereference.
Here is an example implementation:
struct SomeTool {
impl_: Box<SomeToolImpl>,
}
impl SomeTool {
fn execute(&mut self) {
self.impl_.execute();
}
}
trait SomeToolImpl {
fn execute(&mut self);
}
struct SpecificTool1 {
foo_bar: bool
}
impl SpecificTool1 {
pub fn new(foo_bar: bool) -> SomeTool {
let my_data = SpecificTool1 { foo_bar: foo_bar };
SomeTool { impl_: Box::new(my_data) }
}
}
impl SomeToolImpl for SpecificTool1 {
fn execute(&mut self) {
println!("I am {}", self.foo_bar);
}
}
struct SpecificTool2 {
num: u64
}
impl SpecificTool2 {
pub fn new(num: u64) -> SomeTool {
let my_data = SpecificTool2 { num: num };
SomeTool { impl_: Box::new(my_data) }
}
}
impl SomeToolImpl for SpecificTool2 {
fn execute(&mut self) {
println!("I am {}", self.num);
}
}
pub fn main() {
let mut tool1: SomeTool = SpecificTool1::new(true);
let mut tool2: SomeTool = SpecificTool2::new(42);
tool1.execute();
tool2.execute();
}
Note that, in this design, it doesn't make sense to make implementation an Option because we always associate the tool type with the implementation. While it is perfectly valid to have an implementation without data, it must always have a type associated with it.
I'm coming from mostly OOP languages, so getting this concept to work in Rust kinda seems hard. I want to implement a basic counter that keeps count of how many "instances" I've made of that type, and keep them in a vector for later use.
I've tried many different things, first was making a static vector variable, but that cant be done due to it not allowing static stuff that have destructors.
This was my first try:
struct Entity {
name: String,
}
struct EntityCounter {
count: i64,
}
impl Entity {
pub fn init() {
let counter = EntityCounter { count: 0 };
}
pub fn new(name: String) {
println!("Entity named {} was made.", name);
counter += 1; // counter variable unaccessable (is there a way to make it global to the struct (?..idek))
}
}
fn main() {
Entity::init();
Entity::new("Hello".to_string());
}
Second:
struct Entity {
name: String,
counter: i32,
}
impl Entity {
pub fn new(self) {
println!("Entity named {} was made.", self.name);
self.counter = self.counter + 1;
}
}
fn main() {
Entity::new(Entity { name: "Test".to_string() });
}
None of those work, I was just trying out some concepts on how I could be able to implement such a feature.
Your problems appear to be somewhat more fundamental than what you describe. You're kind of throwing code at the wall to see what sticks, and that's simply not going to get you anywhere. I'd recommend reading the Rust Book completely before continuing. If you don't understand something in it, ask about it. As it stands, you're demonstrating you don't understand variable scoping, return types, how instance construction works, how statics work, and how parameters are passed. That's a really shaky base to try and build any understanding on.
In this particular case, you're asking for something that's deliberately not straightforward. You say you want a counter and a vector of instances. The counter is simple enough, but a vector of instances? Rust doesn't allow easy sharing like other languages, so how you go about doing that depends heavily on what it is you're actually intending to use this for.
What follows is a very rough guess at something that's maybe vaguely similar to what you want.
/*!
Because we need the `lazy_static` crate, you need to add the following to your
`Cargo.toml` file:
```cargo
[dependencies]
lazy_static = "0.2.1"
```
*/
#[macro_use] extern crate lazy_static;
mod entity {
use std::sync::{Arc, Weak, Mutex};
use std::sync::atomic;
pub struct Entity {
pub name: String,
}
impl Entity {
pub fn new(name: String) -> Arc<Self> {
println!("Entity named {} was made.", name);
let ent = Arc::new(Entity {
name: name,
});
bump_counter();
remember_instance(ent.clone());
ent
}
}
/*
The counter is simple enough, though I'm not clear on *why* you even want
it in the first place. You don't appear to be using it for anything...
*/
static COUNTER: atomic::AtomicUsize = atomic::ATOMIC_USIZE_INIT;
fn bump_counter() {
// Add one using the most conservative ordering.
COUNTER.fetch_add(1, atomic::Ordering::SeqCst);
}
pub fn get_counter() -> usize {
COUNTER.load(atomic::Ordering::SeqCst)
}
/*
There are *multiple* ways of doing this part, and you simply haven't given
enough information on what it is you're trying to do. This is, at best,
a *very* rough guess.
`Mutex` lets us safely mutate the vector from any thread, and `Weak`
prevents `INSTANCES` from keeping every instance alive *forever*. I mean,
maybe you *want* that, but you didn't specify.
Note that I haven't written a "cleanup" function here to remove dead weak
references.
*/
lazy_static! {
static ref INSTANCES: Mutex<Vec<Weak<Entity>>> = Mutex::new(vec![]);
}
fn remember_instance(entity: Arc<Entity>) {
// Downgrade to a weak reference. Type constraint is just for clarity.
let entity: Weak<Entity> = Arc::downgrade(&entity);
INSTANCES
// Lock mutex
.lock().expect("INSTANCES mutex was poisoned")
// Push entity
.push(entity);
}
pub fn get_instances() -> Vec<Arc<Entity>> {
/*
This is about as inefficient as I could write this, but again, without
knowing your access patterns, I can't really do any better.
*/
INSTANCES
// Lock mutex
.lock().expect("INSTANCES mutex was poisoned")
// Get a borrowing iterator from the Vec.
.iter()
/*
Convert each `&Weak<Entity>` into a fresh `Arc<Entity>`. If we
couldn't (because the weak ref is dead), just drop that element.
*/
.filter_map(|weak_entity| weak_entity.upgrade())
// Collect into a new `Vec`.
.collect()
}
}
fn main() {
use entity::Entity;
let e0 = Entity::new("Entity 0".to_string());
println!("e0: {}", e0.name);
{
let e1 = Entity::new("Entity 1".to_string());
println!("e1: {}", e1.name);
/*
`e1` is dropped here, which should cause the underlying `Entity` to
stop existing, since there are no other "strong" references to it.
*/
}
let e2 = Entity::new("Entity 2".to_string());
println!("e2: {}", e2.name);
println!("Counter: {}", entity::get_counter());
println!("Instances:");
for ent in entity::get_instances() {
println!("- {}", ent.name);
}
}
I am writing a wrapper/FFI for a C library that requires a global initialization call in the main thread as well as one for destruction.
Here is how I am currently handling it:
struct App;
impl App {
fn init() -> Self {
unsafe { ffi::InitializeMyCLib(); }
App
}
}
impl Drop for App {
fn drop(&mut self) {
unsafe { ffi::DestroyMyCLib(); }
}
}
which can be used like:
fn main() {
let _init_ = App::init();
// ...
}
This works fine, but it feels like a hack, tying these calls to the lifetime of an unnecessary struct. Having the destructor in a finally (Java) or at_exit (Ruby) block seems theoretically more appropriate.
Is there some more graceful way to do this in Rust?
EDIT
Would it be possible/safe to use this setup like so (using the lazy_static crate), instead of my second block above:
lazy_static! {
static ref APP: App = App::new();
}
Would this reference be guaranteed to be initialized before any other code and destroyed on exit? Is it bad practice to use lazy_static in a library?
This would also make it easier to facilitate access to the FFI through this one struct, since I wouldn't have to bother passing around the reference to the instantiated struct (called _init_ in my original example).
This would also make it safer in some ways, since I could make the App struct default constructor private.
I know of no way of enforcing that a method be called in the main thread beyond strongly-worded documentation. So, ignoring that requirement... :-)
Generally, I'd use std::sync::Once, which seems basically designed for this case:
A synchronization primitive which can be used to run a one-time global
initialization. Useful for one-time initialization for FFI or related
functionality. This type can only be constructed with the ONCE_INIT
value.
Note that there's no provision for any cleanup; many times you just have to leak whatever the library has done. Usually if a library has a dedicated cleanup path, it has also been structured to store all that initialized data in a type that is then passed into subsequent functions as some kind of context or environment. This would map nicely to Rust types.
Warning
Your current code is not as protective as you hope it is. Since your App is an empty struct, an end-user can construct it without calling your method:
let _init_ = App;
We will use a zero-sized argument to prevent this. See also What's the Rust idiom to define a field pointing to a C opaque pointer? for the proper way to construct opaque types for FFI.
Altogether, I'd use something like this:
use std::sync::Once;
mod ffi {
extern "C" {
pub fn InitializeMyCLib();
pub fn CoolMethod(arg: u8);
}
}
static C_LIB_INITIALIZED: Once = Once::new();
#[derive(Copy, Clone)]
struct TheLibrary(());
impl TheLibrary {
fn new() -> Self {
C_LIB_INITIALIZED.call_once(|| unsafe {
ffi::InitializeMyCLib();
});
TheLibrary(())
}
fn cool_method(&self, arg: u8) {
unsafe { ffi::CoolMethod(arg) }
}
}
fn main() {
let lib = TheLibrary::new();
lib.cool_method(42);
}
I did some digging around to see how other FFI libs handle this situation. Here is what I am currently using (similar to #Shepmaster's answer and based loosely on the initialization routine of curl-rust):
fn initialize() {
static INIT: Once = ONCE_INIT;
INIT.call_once(|| unsafe {
ffi::InitializeMyCLib();
assert_eq!(libc::atexit(cleanup), 0);
});
extern fn cleanup() {
unsafe { ffi::DestroyMyCLib(); }
}
}
I then call this function inside the public constructors for my public structs.