Exactly, this is not an issue in any reasonable setup because static analysis catches (and fixes!) this reliably.
> but evidently these issues go unnoticed until a customer complains about it or someone actually bothers to profile the code.
No
With Rust I have one job that runs tests and another that runs cargo build --release and I'm done...
For example, a moved-out-from tree in C++ could represent this by having its inner root pointer be nullptr, and then its dtor would have to check for the root being nullptr, and all its member fns would have the danger of UB (nullptr dereference) if the caller called them on a moved-out shell. But the Rust version could use a non-nullable pointer type (Box), and its dtor and member fns would be guaranteed to act on a valid pointer.
The more unfortunate, perhaps disingenuous part is that the proposal paper tries to pretend you can make the destructive move later if you need it once you've got their C++ move.
But actually what they're proposing is that "move + create" + "destroy" = "move". So, that's extra work it's not the same thing at all and sure enough in the real world this means extra work, from compilers, from programmers and sometimes (if it isn't removed by the optimiser) from the runtime program.
> The more unfortunate, perhaps disingenuous part is that the proposal paper tries to pretend you can make the destructive move later if you need it once you've got their C++ move.
For reference, I think N1377 is the original move proposal [0]. Quoting from that:
> Alternative move designs
> Destructive move semantics
> There is significant desire among C++ programmers for what we call destructive move semantics. This is similar to that outlined above, but the source object is left destructed instead of in a valid constructed state. The biggest advantage of a destructive move constructor is that one can program such an operation for a class that does not have a valid resourceless state. For example, the simple string class that always holds at least a one character buffer could have a destructive move constructor. One simply transfers the pointer to the data buffer to the new object and declares the source destructed. This has an initial appeal both in simplicity and efficiency. The simplicity appeal is short lived however.
> When dealing with class hierarchies, destructive move semantics becomes problematic. If you move the base first, then the source has a constructed derived part and a destructed base part. If you move the derived part first then the target has a constructed derived part and a not-yet-constructed base part. Neither option seems viable. Several solutions to this dilemma have been explored.
<snip>
> In the end, we simply gave up on this as too much pain for not enough gain. However the current proposal does not prohibit destructive move semantics in the future. It could be done in addition to the non-destructive move semantics outlined in this proposal should someone wish to carry that torch.
[0]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2002/n13...
That's partly true, partly circular. Because moves work this way, it's harder to make a class that doesn't have empty states, so I don't design my class to avoid empty states, so the destructor has to handle them.
The smart pointer std::unique_ptr<T> is an example of this, sometimes people will say it's basically a boxed T, so analogous to Rust's Box<T> but it isn't quite, it's actually equivalent to Option<Box<T>>. And if we don't want to allow None? Too bad, you can't express that in C++
But you're right that C++ people soldier on, there aren't many C++ types where this nonsense unavoidably gets in your face. std::variant's magic valueless_by_exception is such an example and it's not at all uncommon for C++ people to just pretend it can't happen rather than take it square on.
If you have any type that represents validated data, say a string wrapper which conveys (say) a valid customer address, how do you empty it out?
You could turn it into an empty string, but now that .street() method has to return an optional value, which defeats the purpose of your type representing validated data in the first place.
The moved-from value has to be valid after move (all of its invariants need to hold), which means you can’t express invariants unless they can survive a move.
It is much better for the language to simply zap the moved-from value out of existence so that you don’t have to deal with any of that.
Second, why can't the .street() method simply return an empty string in this case?
> The moved-from value has to be valid after move (all of its invariants need to hold)
The standard says: "Unless otherwise specified, such moved-from objects shall be placed in a valid but unspecified state" AFAIK, it only makes such requirements for standard library types, but not for user defined types. Please correct me if I'm wrong.
delete null is fine in C++ [1], so, assuming root either is a C++ object or a C type without members that point to data that also must be freed, its destructor can do delete root. And those assumptions would hold in ‘normal’ C++ code.
[1] https://en.cppreference.com/w/cpp/language/delete.html: “If ptr is a null pointer value, no destructors are called, and the deallocation function may or may not be called (it's unspecified), but the default deallocation functions are guaranteed to do nothing when passed a null pointer.”
the difference between `const Data& d` and `const Data d` isn't accurately characterized as "a typo" -- it's a semantically significant difference in intent, core to the language, critical to behavior and outcome
even if the author "forgot" to add the `&` due to a typo, that mistake should absolutely have been caught by linting, tests, CI, or code review, well before it entered the code base
so not feelin' it, sorry
I specifically take issue with this framing of it is not an issue for we have the tools to help with this, especially where the tools are not part of a standard distribution of a toolchain and require more than minimal effort. C++ has had many a warts for many decades, and the response has always been *you are just holding it wrong* and not running a well covering integration test suite with sanitizers on every commit, you just need to run one more tool in the CI, just a comprehensive benchmarking suite, have more eyes looking for a single char difference in reviews.
I think the problem with `T &d` and `T d` is that these 2 declarations yield a "name" `d` that you can operate on very similarly. It's not necessarily about reference declaration `T& d` is 1 char diff away compared to value declaration `T d`.
While there is a significant semantic difference between declaring things as a value and as a reference (&), non-static member function invocation syntax is the same on both `&d` and `d`. You can't tell the difference without reading the original declaration, and the compiler will happily accept it.
Contrast this to `T *d` or `T d`. Raw pointers require different operations on `d` (deref, -> operator, etc). You're forced to update the code if you change the declaration because the compiler will loudly complain about it.
It shares the same problem with a type system with nullable-by-default reference type vs an explicit container of [0..1] element Option<T>. Migrating existing code to Option<>-type will cause the compiler to throw a ton of explicit errors, and it will become a breaking change if it was a public API declaration. On the other hand, you're never able to feel safe in nullable-by-default; a public API might claim it never return `null` in the documentation, but you will never know if it's true or not only from the type signature.
Whether it's good or bad, I guess it depends on the language designer's decision. It is certainly more of a hassle to break & fix everything when updating the declaration, but it also can be a silent footgun as well.
This reminds me of arguing more than once with JS developers about the dangers of loose typing (especially in the case of JS) and getting the inevitable reply ”I just keep track of my type casting.”.
If people could keep entirety of J.2 appendix in their mind at all time we would not have these issues. And if they had entirety of J appendix in mind all C code would be portable.
Or if people just always ran -Wall -Wpedantic -Wall_for_real_this_time -fsanitize=thread,memory,address,leaks,prayers,hopes,dreams,eldritch_beings,elder_gods -fno-omit-frame-pointer
I mean if this was all it took then C and C++ programs would be as safe as Rust. Which is not what we see in practice. And it's not like C programmers are an average web dev. It's a relatively niche and well versed community.
I've been writing some Swift code in recent years. The most frequent source of bugs has been making incorrect assumptions on whether a parameter is a class or a struct (reference or value type). C# has the same issue.
It's just a terrible idea to make the value/reference distinction at the type level.
Rust: Typo? Now it just doesn't compile anymore. Worst case is that the compiler does a bad job at explaining the error and you don't find it immediately.
C++: Typo? Good luck. Things may now be broken in so subtle and hard to figure out ways it may haunt you till the rest of your days.
But that of course depends on the nature of the typo. Now I should go and read the article.
By the way, the project considers this a bug and accepts reports for that. In many occasions they are easy to fix. In others large refactors are needed. But being aware of the case is the necessary first step to making them better.
Regarding const by-value parameters, they should never appear in function declarations (without definition) since that doesn’t enforce anything. In function definitions, you can use const refs (which have lifetime extension) to achieve the same const-correctness, and const refs are better for large types.
Admittedly this further proves the point that c++ is needlessly complicated for users, and I agree with that.
struct Data {
// Vec cannot implement "Copy" type
data: Vec<i32>,
}
// Equivalent to "passing by const-ref" in C++
fn BusinessLogic(d :&Data) {
d.DoThing();
}
// Equivalent to "move" in C++
fn FactoryFunction(d: Data) -> Owner {
owner = Owner{data: d};
// ...
return owner
}
I believe in Rust, when you move a non-Copy type, like in this case, it is up to the compiler if it passes a reference or makes a physical copy.
In my (admittedly limited) understanding of Rust semantics calling
FactoryFunction(d: Data)
EDIT:
Thinking about it, the example is probably watertight because d is essentially a Vec (as Ygg2 pointed out).
My point is that if you see
FactoryFunction(d: Data)
If you have
Vec<i32>
// newtype struct
struct Data{ data: Vec<i32> }
// newtype enum in rust
// Possibly but not 100% sure
// enum OneVar { Data(Vec<i32>) }
My favourite as a Unix person is Option<OwnedFd>. In a way Option<OwnedFd> is the same as the classic C int file descriptor. It has the exact same representation, 32 bits of aligned integer. But Rust's type system means we know None isn't a file descriptor, whereas it's too easy for the C programmer to forget that -1 isn't a valid file descriptor. Likewise the Rust programmer can't mistakenly do arithmetic on file descriptors, if we intend to count up some file descriptors but instead sum them in C that compiles and isn't what you wanted, in Rust it won't compile.
True, I didn't meant to imply you can just ignore types; I meant to say that the equivalent operations on a naked vs wrapped value return equivalent assembly.
It's one of those zero cost abstraction. You can writ your newtype wrapper and it will be just as if you wrote implementations by hand.
> My favourite as a Unix person is Option<OwnedFd>.
Yeah, but that's a bit different. Compiler won't treat any Option<T> that way out of the box. You need a NonZero type or nightly feature to get that[1].
That relies on compiler "knowing" there are some values that will never be used.
[1] https://www.0xatticus.com/posts/understanding_rust_niche/
So if you make an enumeration AlertLevel with values Ominous, Creepy, Terrifying, OMFuckingGoose then Option<AlertLevel> is a single byte, Rust will assign a bit pattern for AlertLevel::Ominous and AlertLevel::Creepy and so on, but the None just gets one of the bit patterns which wasn't used for a value of AlertLevel.
It is a bit trickier to have Color { Red, Green, Blue, Yellow } and Breed { Spaniel, Labrador, Poodle } and make a type DogOrHat where DogOrHat::Dog has a Breed but DogOrHat::Hat has a Color and yet the DogOrHat fits in a single byte. This is because Rust won't (by default) avoid clashes, so if it asssigned Color::Red bit pattern 0x01 and Breed::Spaniel bit pattern 0x01 as well, it won't be able to disambiguate without a separate dog-or-hat tag, however we can arrange that the bit patterns don't overlap and then it works. [This is not guaranteed by Rust unlike the Option<OwnedFd> niche which is guaranteed by the language]
I believe the answer is technically yes. IIRC a "move" in Rust is defined as a bitwise copy of whatever is being moved, modulo optimizations. The only difference is what you can do with the source after - for non-Copy types, the source is no longer considered accessible/usable. With Copy types, the source is still accessible/usable.
On that machine Data is "physically" just the Vec, which is three 64-bit values, a pointer to i32 ("physically" on this machine a virtual address), an integer length and an integer capacity, and the machine has a whole bunch of GPRs so sure, one way the compiler might implement FactoryFuncton is to "physically" copy those three values into CPU registers. Maybe say RAX, RCX, RDX ?
Actually though there's an excellent chance that this gets inlined in your program, and so FactoryFunction never really exists as a distinct function, the compiler just stamps out the appropriate stuff in line every time we "call" this function, so then there was never a "parameter" because there was never a "function".
The point I am trying to make is more general:
I believe that when you have a type in Rust that is not Copy it will never be implicitly copied in a way that you end up with two visible instances but it is not guaranteed that Rust never implicitly memcopies all its bytes.
I have not tried it but what I had in mind instead of the Vec was a big struct that is not Copy. Something like:
struct Big<const M: usize> {
buf: [u8; M],
}
// Make it non-Copy.
impl<const M: usize> Drop for Big<M> {
fn drop(&mut self) {}
}
I will say that programmers very often have bad instincts for when that's a bad idea. If you have a mix of abilities and can ask, try it, who in your team thinks that'll perform worse for moving M = 64 or M = 32? Don't give them hours to think about it. I would not even be surprised to find real world experienced programmers whose instinct tells them even M = 4 is a bad idea despite the fact that if we analyse it we're copying a 4 byte value rather than copying the (potentially much bigger) pointer and taking an indirection
Edited: To fix order of last comparison
True that. memcpy is basically the literal fastest thing your processor can do, it’s trivially pipelined and can be done asynchronously.
If the alternative is heap storage you’re almost always cooked: that heap space is far less likely to be in L1 cache, allocating it takes time and requires walking a free list, dealing with memory fragmentation, freeing it when dropped, etc.
It’s not a bad short-hand to think of the heap as being 10-100x slower than the stack.
Obviously the distinction can matter sometimes and thus copy in the logical sense is a leaky abstraction (although in practice I notice I do not see that leakage often).
If you care about performance, you measure it. If you don't measure performance, you don't care about it.
You have to do it correct or you might be just measuring: when your system is pulling updates, how big is your username, the performance of the least critical thing in your app.
And at worst you can speed up your least performing function only to yield a major slowdown to overall performance.
It extends it a bit, too, with `out` meaning that the referenced argument is initialized by the function, not read.
Note that assuming the into_iter comes from IntoIterator that’s what the for loop invokes to get an iterator from an iterable. So
for lr in LoadRequests.into_iter() {
for lr in LoadRequests {
for lr in LoadRequests.iter_mut()
for lr in &mut LoadRequests
Similar to all the times I defensively str(something) in Python to find that “oh that has __str__ called on it anyways.”
SuperV1234•4d ago
Rubberducky1324•16h ago
But it needs to be conservative of course, in general you can't do this.
spacechild1•16h ago
vitus•3h ago
Whether an argument is passed in a register or not is unfortunately much more nuanced than this: it depends on the ABI calling conventions (which vary depending on OS as well as CPU architecture). There are some examples where the argument will not be passed in a register despite being "small enough", and some examples where the argument may be split across two or more registers.
For instance, in the x86-64 ELF ABI spec [0], the type needs to be <= 16 bytes (despite registers only being 8 bytes), and it must not have any nontrivial copy / move constructors. And, of course, only some registers are used in this way, and if those are used up, your value params will be passed on the stack regardless.
[0] Section 3.2.3 of https://gitlab.com/x86-psABIs/x86-64-ABI
Animats•15h ago
If something is const, whether to pass it by reference or value is a decision the compiler should make. There's a size threshold, and it varies with the target hardware. It might be 2 bytes on an Arduino and 16 bytes on a machine with 128-bit arithmetic. Or even as big as a cache line. That optimization is reportedly made by the Rust compiler. It's an old optimization, first seen in Modula 1, which had strict enough semantics to make it work.
Rust can do this because the strict affine type model prohibits aliasing. So the program can't tell if it got the original or a copy for types that are Copy. C++ does not have strong enough assurances to make that a safe optimization. "-fstrict-aliasing" enables such optimizations, but the language does not actually validate that there is no aliasing.
If you are worried about this, you have either used a profiler to determine that there is a performance problem in a very heavily used inner loop, or you are wasting your time.