You can't Rust that
lucumr.pocoo.org
lucumr.pocoo.org
I'm not really sure I understand what he's getting at with that description.
I'm getting at that an object in C++ is generally immovable. If it does move (which is new in C++11 i think?) a move ctor is invoked. Rust does not have a move ctor, it does not let you do things that would prevent moving an object in the first place (at least currently).
That statement is confusing as the spec very specifically notes that the moved-from object remains valid, so saying that it's invalidated is odd.
https://www.boost.org/doc/libs/1_54_0/doc/html/boost_asio/re...
I'm not confusing anything, and the spec very explicitly notes that a moved-from object is in a "valid but unspecified state", not in an invalid state.
> The standard guarantees that the object is still around.
The standard's guarantees are significantly stronger than that, and furthermore
> But because its data got stolen, its internal state must now be something invalid or empty depending on the semantics of the object.
That is absolutely not a hard rule, a trivial move constructor is a copy constructor and does not affect the moved-from object in any way.
That's mostly a detail because you can't rely on that to do anything interesting. Basically you can assign a new value to the object, or just destruct it. Having a "valid" but unknown value is completely useless.
What is sad is that the same mechanism has been overloaded for at least 2 different things: simple perf optim (when you fall into this situation of no guarantee), and movable unique ownership (like unique_ptr) where you actually can read the resulting object after the move an expect a predictable (in a portable and standardized way ) outcome.
In c++ you get copy semantics by default, requiring std::mov to get move semantics. In rust it is the opposite.
And even then:
1. std::move only creates an rvalue reference, it doesn't necessarily move anything (not only does that depend on the presence of a move ctor & the receiver's arguments, there's still no requirement that anything actually moves)
2. a move in the C++ sense "empties" the object, the object is still there and accessible in "some valid but otherwise indeterminate state", objects can even fully specify the exact state they're in when moved-from (unique_ptr does)
This is one area where Rust really makes life much easier.
well, take for instance this function :
my_struct eat_pointer(std::unique_ptr<int>&& x) {
if(x && *x > 0)
return my_struct{std::move(x)};
return my_struct{};
}
auto my_pointer = std::make_unique<int>(-456);
auto res = eat_pointer(std::move(my_pointer));
// at this point my_pointer hasn't moved a bit
I don't think so, actually. As far as I'm aware moves are only done when explicitly asked for, because any implicit move would have the consequence of breaking references and being unsafe in general if you're not careful.
Certainly not. Move are attempted (in the sense that the move constructor/operator= will be called if it exists) from rvalue references. Those are for example all temporaries. That has let accelerate all C++ programs using std objects by simply recompiling them (with the new versions of the objects supporting the move while it did not exist in the old versions)
no, every time you have a function returning a temporary you can have a move, e.g.
std::vector<int> myfun();
struct myclass { myclass(std::vector<int> v); };
myclass c{myfun()};
In C++ an object can be passed by reference, by copy (implicitly invoking the copy constructor), by move (implicitly invoking the move constructor), and additionally the compiler may choose to elide the copy/move. Whereas in Rust, you can only pass by reference (borrowing) or by bitwise copy (moving). There are no implicit copy or move constructors, and AFAIK at least currently the compiler doesn't hoist objects directly to the callee's stack frame.
Yes, it does. This is the "retptr" optimization.
Stepping into the specifics of the options, you've classified things a little inconsistently: C++ allows arguments to be references or values, and the caller can choose for the latter to be by copy or by move. This the same as Rust, with references and values and the caller choosing by copy (with an explicit .clone()) or by move (default).
You can argue that Clone isn't part of the language, or that being explicit doesn't count, but I don't think that's so interesting (certainly by move in C++ is often equally explicit: std::move).
And, without move constructors, hoisting things into caller's stack frames is just a run-of-the-mill optimisations for returned values following the standard "as-if" rule, no need for explicit enabling in the standard. C++ needs it because part of the semantic model is running user-defined copy/move constructors on 'return', so the standard needs to make it okay to not do this in some cases, but without the user-defined code, it isn't necessary.
I think the main way Rust reduces cache churn, is that it makes it safer to pass references/pointers around instead of making copies. For example, if I have f(char*) in C, I might be worried about what f is doing with that pointer. It might save it somewhere, or try to free it, or who knows what. In C++, I might prefer to write f(std::string) to avoid those worries, but that comes at the cost of unnecessarily copying the string. (You could pass it by reference to avoid the copy, but then you still have to worry about f saving the reference.) In Rust, I can safely write f(&str), and I know f can't do anything dirty with that reference.
FWIW my assumption always is that it's "a borrow". I.e., the called function should not make any assumptions how the memory was allocated, what is its lifetime, or even try to free it or keep it in an associated datastructure that persists the lifetime of the function. The only thing it should normally assume is that it points to valid memory.
In my experience the instances where ownership moves across function boundaries are far and far between, and can easily be documented, often simply by giving it an appropriate name.
It needs to at least make the assumption that the memory will not be invalidated while the function did not return. In the presence of multi threading that in itself might already be surprisingly hard to guarantee sometimes. Also typically functions make assumptions about the immutability of things in such cases which are often just not true.
An architecture with global data tables instead of OOP helps here, because it has much lower need for pointer arguments of unclear origin. I also agree very much with your approach "handles instead of pointers" (if by handles you mean integer array indices). Indices for example trivially survive reallocation of a dynamically growable array.
But for how long? Forget about multithreading for a moment, but can my function store the pointer for later use? This is incredibly common in OOP, and dangling pointers/references are one of the (if not the) top sources of bugs in C and C++. And unlike other sources of bugs, there’s no good fundamental remedy against it.
As I wrote, "lifetime of the function". Maybe that was poorly worded - of course I mean "until the function call returns". (I think "borrow" is universally understood in this way).
That means, again, don't assume you are allowed to keep the pointer argument in an associated datastructure. In general we don't know where the data is coming from, how it was allocated, how or when it will be freed, etc. It's just that: data.
And as I wrote in the other comment, don't do OOP. It's wrong. I think it has had a good twenty years of hype to prove any merits, and failed to do so. Yes, OOP without a garbage collector cannot be done reliably because the pointers will kill you. But that's not an indication that pointer management is impossible. It's an indication that we shouldn't do OOP. Even in a GCed language it's still a pain to maintain all these redundant links, and to write algorithms that collect the data that was littered in a thousand places and is accessible only through a hard to understand object interface.
There are much simpler ways to organize a program. Pointer management can be pretty easy when the project structure is right, and we focus on the data instead an overly bureaucratic object graph of funky objects that do almost nothing in the name of "reusability".
Other modules use integer handles to access data. They might convert the integer handle to a pointer and iterate over the data with the help of the module, but these pointers are not stored. The module's interface has very clear and simple rules which commands from its API invalidate those pointers (because they might trigger reallocation). But of course only few commands do invalidate pointers, and they are typically not interleaved with the other commands. And typically getting pointers is not even needed, because the relevant functionality is implemented in the module that owns the data.
There are generally no pointers inside the data (perhaps a few const char pointers to static or quasi-static data) and most intra-module links are realized using integer indexes, just like it's done in relational databases.
So in a way that is actually "object-oriented", but not in the usual sense of creating a million instances of some class that are linked in a complicated way. Every module is a kind of object. So the number of these objects is typically static. All the linking between these objects is simply done by: the linker :-)
The distinction I guess is that in rust you can move things assuming you have no references to them, while in c++ you can't really (I mean, move constructors, but that's really a fancy copy where the initial thing still exists in some form). And moving things is pretty normal - as is taking pointers to them - just not at the same time.
This is something I've struggled a bit with when playing with Rust. I've had a hard time understanding whether I'm telling the compiler to move or copy. In other words I need to understand better what I'm telling it to do (I probably just need to buckle down and study the new book in this area better). It wasn't immediately obvious to me when I last played with it.
You're not telling it anything, in Rust copy v move is a type-level concern: either the type is "move" (the default) or it's "copy".
There is no difference at the machine-level either (it's semantically doing a memcpy either way, Rust has no move ctor v copy ctor and its move semantics are somewhat different than C++), the difference is whether the compiler will let you use the source object afterwards.
Incidentally, I believe you're not really "telling the compiler to move or copy" in C++ either, you're only suggesting that it can move and whether it actually does so depends on the type's ctors and IIRC the receiver's overloads: std::move on a type which can't be moved or with a target which doesn't care does not do anything.
Return values that fit into a register will be returned in a register, and his example is an 8 byte struct, so that returns in a register. Return values larger than a register will add an implicit first argument that is a pointer to memory where the return value should be written to. In that sense, it is very similar to C++ in that you are initializing into an allocated buffer.
As for "Refcounts are not Dirty", I would greatly disagree. Using refcounts to get around an overly aggressive borrow checker seems to be an ugly developing pattern in rust, and I feel they are giving away the performance many are fighting for by adding all these little inefficiencies to idiomatic rust. Add some refcounting here, add a Box or other indirection there, a chained monadic interface that can't short circuit and has to continually do error/null checks, etc... Soon it is death by a thousand papercuts. People fight hard for that extra 5% in performance only to have it taken away from them in interface and language issues.
Edit: Forgot about handles. Ugh. Completely unacceptable when you want to grow your tree data structure and you have to do a realloc and basically copy every node. If your tree is complete, then you are copying the whole tree every time you start a new level. The conclusions sound more like ugly hacks, than what you would properly design.
About the refcounts: since the counting is explicit (calls to clone()) they at least in my experience don't really show up. Most of the refcounted objects I deal with bump the refcounts once when some task spawns and decrements it when it ends. I have yet to see refcounts to change in hot code paths.
//EDIT for your edit:
> Edit: Forgot about handles. Ugh. Completely unacceptable when you want to grow your tree data structure and you have to do a realloc and basically copy every node.
Sure, but that's not my point anyways. At any point you can fall down to writing unsafe code and building a safe abstraction on top of it. This is to help developers not run into walls. I don't think that handles are the best thing invented but I don't think "well you can't do that in Rust until we some time in the future" and not provide an alternative is a particularly good suggestion.
Atomic ref-counts can actually have a surprising amount of overhead in many multithreaded scenarios, so much so that going back to raw pointers over shared pointers in C++ for some use cases of passing pointers around can provide a huge win.
let data = Arc::new(Mutex::new(Data::new()));
lazy_static! {
// 0 -> None
// 1 -> Circles
// 2 -> Bezier
static ref DRAW_ON_TOUCH: Arc<Mutex<u32>> = Arc::new(Mutex::new(0));
}
Yes. It errors at compile time (`std::rc::Rc<i32>` cannot be sent between threads safely)
Of course. Rust is memory safe. The compiler will prevent you from sharing non-thread-safe reference counted objects between threads.
This is a significant advantage over C++, where the compiler offers no such protection and so the typical solution is just to use atomic reference counting (shared_ptr) everywhere.
Separately emotionally I feel different about Arc<T> because all the counting is explicit and I see it. Understanding where shared_ptr<T> in C++ counts however can be quite a challenge, even if you consider yourself proficient with C++.
They could also happen if you used non-thread-safe reference counting on a thread-safe object. Now this seems to be rare in practice, because non-thread-safe reference counting is unusual. If it became more prevalent, however, I could see it happening.
As beneficial to your argument as it would be for that change to be the definition of "software", I don't think that's likely.
I am well aware of the pressure on developers to get requested feature done quickly. But the vast majority of bugs that get shipped would be easily prevented by more thorough processes. All those projects writing highly reliable software demonstrate this. But the price tag has a few zeroes appended as a result.
Rust’s type system is arguably one of them.
Type systems are tools that simply the process of checking that you do not mangle data by applying the wrong instructions to them. They didn't come out of nowhere.
It also shows in the cost of building the software. Most software companies aren't willing to pay the cost of that process. Like it or not, you will almost certainly interact with software written by these companies, either directly or indirectly.
Here's Niko Mastakis talking about a case where that happened to him (I think): https://youtu.be/lO1z-7cuRYI?t=4m
"... trusting the programmer is fundamentally wrong, no matter the programming language"
To learn more about this, look here: https://doc.rust-lang.org/beta/nomicon/send-and-sync.html
Also, the Arc isn't needed in lazy_static: just a mutex is enough for mutability, the sharing (which is what Arc controls) comes from being a static.
No, not unless the object is actually shared among multiple threads.
> Using refcounts to get around an overly aggressive borrow checker seems to be an ugly developing pattern in rust, and I feel they are giving away the performance many are fighting for by adding all these little inefficiencies to idiomatic rust. Add some refcounting here, add a Box or other indirection there, a chained monadic interface that can't short circuit and has to continually do error/null checks, etc... Soon it is death by a thousand papercuts. People fight hard for that extra 5% in performance only to have it taken away from them in interface and language issues.
Do you have any suggestions?
And then comes a point where you realize that you need to write _some_ low level code that is unsafe. You read a lot about unsafe Rust and it almost allows you to almost work as if you are working with C. You can transmute a reference to native pointer type and then transmute it back as a mutable reference with a static lifetime if you want. But at that point you have lost the safeties and now you have to know exactly how to craft your transmute trickery such that you know there will be little to no overhead.
Perhaps it all comes down to the borrow checker being less strict about things. I can't tell exactly in which ways, perhaps about the lifetimes of references. Something doesn't feel as natural to reason about working in Rust as it does in C but I can't put my finger on it.
One thing I can say though is I wish I didn't need to use lazy static for non-primitive static globals.
But overall I really like the general direction that the language is heading. Love the way dependencies are managed although TOML wasn't the best format choice in my mind. crates.io, docs.rs and the entire community and the ecosystem are great and unique too.
I don't like it's API at the moment, but perhaps it could be turned into something nicer?
The performance hit isn't as it bad as it seems, because you only adjust the reference counts when you actually take an extra reference. This is different from shared_ptr in C++, because C++ automatically calls the copy constructor and so it's easy to end up with lots of reference count traffic.
Observe that, if we assume that the time spent in malloc() and free() dominates the time spent to adjust one reference count (which is a safe assumption), then the additional time overhead of Rc with a single owner is effectively zero.
> add a Box or other indirection there
Why do you ever need to Box unnecessarily in Rust? This is more of an issue with C++, where shared_ptr has an extra indirection and interior pointers and "new" encourage heap allocation.
I actually think that Rust in practice has the opposite problem: people are afraid to Box when they shouldn't be, causing unnecessary memcpy traffic.
> a chained monadic interface that can't short circuit and has to continually do error/null checks
How is this more overhead than in C?
> Edit: Forgot about handles. Ugh. Completely unacceptable when you want to grow your tree data structure and you have to do a realloc and basically copy every node. If your tree is complete, then you are copying the whole tree every time you start a new level. The conclusions sound more like ugly hacks, than what you would properly design.
Yeah, I would like to make a crate with an interface similar to petgraph but that actually mallocs every node separately. This should be readily doable.
I think the reason why nobody has made this crate yet is that copying the nodes on growth isn't as big of a concern in practice as it might initially seem, because the time spent in the growth case is amortized and made up for by extremely fast allocation of new nodes.
Well only if you pass shared_ptr by value, which isn’t really idiomatic. But yeah the default is dumb in C++ and smart in Rust.
Every single large, "modern", C++ codebase I've worked with uses way more refcounts than is typical in Rust.
(and it's worse because C++ refcounting patterns involve more refcount churn due to how they're designed on the copy constructor, and shared_ptr uses atomics even when unnecessary. Though these large codebases tend to use a slightly different design that lets them decide atomicity per-type)
const a = {x: 1, y: 2};
console.log(a.x); // 1
a.x = 2;
console.log(a.x); // 2
Pointers point at bits of memory (its literally an address to a physical piece of memory or virtual memory allocated by something else - an OS for example) which store things like the 64bit floats '1' and '2' you are referring to in Javascript, and that is something someone writing C needs to think about pretty explicitly and is a source of errors. In Javascript you don't have to think about it very often in explicitly those terms as the runtime takes care of thinking about that for you.
You create all of these string, numbers, objects, etc... that the runtime has to keep track of for you using pointers, and once it thinks you are done with that memory it frees it up.
https://en.wikipedia.org/wiki/Garbage_collection_(computer_s...
However, as I and other have said, while someone writing JavaScript doesn't have to think much about memory, you still have to think about references and values. Its also the case that the Garbage Collector isn't perfect, and sometimes as a JavaScript programmer you can accidentally create memory problems of your own:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memo...
I would quickly disabuse yourself of the notion that all values are immutable in JavaScript, as otherwise you will cause yourself and colleagues a lot of pain in future. As someone that has to write or maintain a lot of JavaScript, saying that I don't have to think about data changing over the course of a program doesn't strike me as true at all (I wish it was).
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data...
As linked, only some of the primitive values in javascript are immutable. So while you can be confident in this:
const x = 'a perfectly well formed string';
// lots of code in between
console.log(x) // will always print 'a perfectly well formed string'
You cannot be confident in this at all:
const x = {
a: 'a perfectly well formed string within an object'
// lots of code in between
console.log(x.a) // absolutely no guarantee that x.a will point at the same string as at the time x was initialized as an object, or that the key will even exist (will fallback to the type 'undefined' if you attempt to read it).
JavaScript has references and values like most languages, but you aren't dealing with memory as explicitly as Rust or C. The reason is in large part because memory is garbage collected in Javascript.
// suppose initially x == 0x123456 and x has type char *
x++
// x == 0x123457
Or if you say:
*x = 'a'
The fact that the pointer itself, as in the address to memory (as opposed to the variable holding that address or the contents of the memory being referred to) is immutable is of approximately no value to anyone.
And frankly, it makes no difference (for these examples) whether this is a language with pointers or references. You can still set variables holding references to new values (if the language allows mutable variables), and you can still mutate the memory that references address (if the language allows mutation of the objects referred to by references), and that's plenty of rope to hang yourself on even without being able to do pointer arithmetic on the address values.
The semantics of a programming language is what it is, not what you want it to be, unless you explicitly choose a language that allows you to express exactly what you mean.
The original statement was: "(I program in JavaScript where all values are immutable)" Its meaning depends on the semantics of 'values' and 'immutable'. If by 'values' you only mean primitive values (excluding objects and arrays) or by immutable you only mean immutable identity (excluding mutable state), you're comparing apples to oranges instead of Rust to JavaScript.
Immutable.js and PureScript exists for a reason: in JavaScript, not all values are immutable.
Object states are mutable, of course. But they are not values. You can't bind object states to variables. If you use objects as proxies for values that exist in your head but not in in JavaScript's semantics, that's fine, so long as you keep in mind the distinction between what you wish you were using and what you are actually using.
Start at the `Data` struct. It contains a Copy on Write (`CoW`) reference to a vector of bytes (`u8`) with a lifetime labeled `'a`. This is the Handle for the data. You get one by calling `Data::new` and passing in something that can be converted to the CoW.
The example is hard coded to work with a vector of u32s (driven by the `Slice<u32>` in `Header`). To use it, you'd call `get_target` with an index and get a u32 back. The other methods on data are doing the pointer math (offset) and casting (`transmute`, `from_raw_parts`) the byte array into a slice of u32s in a safe way.
I don't see anything verifying that the byte array passed in is, in fact, a bunch of u32s so I assume that's a given.
https://github.com/getsentry/symbolic/tree/master/symcache/s...
Is this swap/deref pattern something that can or should be wrapped up into a create?
Unfortunately, Rust currently requires breaking some conventions and using unsafe quite a bit to do this without overflowing the stack, but it's just an extra keyword or two compared to C, and the safety guarantees outside of the unsafe code make up for it.