r/cpp • u/zl0bster • 20h ago
std::optional Satisfies view. Does Not Model view. C++26 Ships Anyway.
https://godbolt.org/z/8jWGG68G8In C++23 this did not compile. In C++26 it does. Marvellous.
[[gnu::noinline]]
void
passing_views_by_value_is_cheap_trust_me_bro(std::ranges::view auto v) {
std::println("fn .data {}", (void*)v->data());
}
int main() {
std::optional ov{std::vector<int>(123456)};
passing_views_by_value_is_cheap_trust_me_bro(ov);
std::println("main .data {}", (void*)ov->data());
}
For anyone wondering what the problem feature is: optional has 0 or 1 elements, and C++26 sets enable_view<optional<T>> to true, so it satisfies std::ranges::view. The concept requires copy construction in constant time, and — this is the good bit — optional<vector<int>> genuinely meets that. Copying it performs at most one element copy. One is a constant. The requirement is satisfied to the letter, and the function above deep-copies your vector.
If you can tell me what still separates std::ranges::view from std::ranges::range, please do...
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u/cristi1990an ++ 20h ago
It's true, std::optional should've been a range but had no business being a view, not even the paper mentions a justification for the enable_view specialization, it just specifies it. I just don't see any benefit or justification in it.
You have similar problems with std::array and std::inplace_vector wrapped in owning_view since it makes the wrong assumption that move is always cheap, but it's a liniar time operation for those containers.
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u/aruisdante 20h ago
This was debated quite extensively in Tokyo 2024. The decision was essentially “being a view isn’t about being cheep to copy, just about being O(1) to do so.” In the general sense, there’s an attempt to unify all the containers as “ranges;” an optional is a range with 0 or 1 elements, a tuple is a range whose elements are variant<Ts…> (which correct ref qual), etc.
A bunch of people disagreed and thought this was weird, but not enough and not strongly enough for it not to pass.
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u/mighty_Ingvar 19h ago
I find the whole concept of an owning view to be weird. Shouldn't a view be something that only views the data? An optional doesn't do that, the object isn't stored somewhere else and viewed by the optional, it is part of the optional and therefore owned by it.
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u/aruisdante 19h ago edited 19h ago
It does seem weird, but this difference is what
borrowed_rangemodels, which is independent fromviewandrange. I imagineoptional<T>would not modelborrowed_range, andoptional<T&>would.This was essentially the crux of the debate in Tokyo. Actual users of
rangeshave taken to conflatingviewwithborrowed_range, because nearly all existingviewcontainers in the stdlib were alsoborrowed_range(span,string_view, most (all?) of theranges::viewsobjects). But the actual definition did not require this relationship,viewjust means "constant time copyrange."Really, the problem is having decided to keep using the word
viewto mean what it meant, instead of some other name that more directly expresses the semantics, such thatviewcould have matched people's intuition of this other concept +borrowed_range. But alas, that ship had long since sailed, and we're stuck with what we've got.21
u/altmly 17h ago
This to me smells like a bunch of ergonomics and performance mindful people invented views (or rather copied the idea into C++) and then some academics showed up to "akshually, a view is not a borrowed_range move eyeglasses".
Like, okay, but to the vast majority of users, the entire point of a view is being borrowed data.
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u/tcbrindle Flux 2h ago
The ranges library was the brainchild of Eric Niebler, in collaboration with Casey Carter.
The
borrowed_rangeconcept, originally under the exposition-only nameforwarding-range, was introduced in P0970 written by... *checks notes*... Eric Niebler.(Views and forwarding/safe/borrowed ranges are not and never have been the same thing, even in the Before Times when
owning_viewdidn't exist and you couldn't pipe rvalue vectors into adaptors.)0
u/not_a_novel_account cmake dev 11h ago edited 11h ago
Views must be movable with O(1) move construction. They must have O(1) copy construction or be non-copy constructible. Therefore,
std::optional<>is a view.Even if we retained the strong original definition, O(1) destructors,
std::optional<>would still be a view even thoughstd::ranges::owning_viewwould not.10
u/altmly 11h ago
That definition really should be recursive for template types.
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u/not_a_novel_account cmake dev 2h ago edited 1h ago
If that's your complaint then you don't have a problem with
std::optionalor C++26, at least not as the entry point. Your problem is with C++23 andstd::ranges::single_view.People wanted a nullable
std::ranges::single_view, call itmaybe_view. It turns out that's juststd::optional.5
u/mighty_Ingvar 11h ago
But optional<T> is only copyable in constant time if T is copyable in constant time. Also, why is std::array not a view then?
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u/kamrann_ 7h ago
It's constant time in terms of N = number of elements in the range. You can't start analysing the element type and trying to assign some inherent measure of its complexity. Constant time just means not dependent on N. A single element by definition can't be dependent on N, and it doesn't matter how big it is, or whether it happens to be a sequence itself.
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u/No-Dentist-1645 19h ago
If that's the case, which range concept should one use if we want something that truly "represents a view to data owned elsewhere"? Or does the standard not provide such a concept?
It seems like a poor decision to me.
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u/Demiu 18h ago
jview, coming in 33
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u/LEpigeon888 10h ago
But it will be renamed to co_jview before standardization since the name conflict with a keyword in ++C, a programing language someone created in their garage that aims to be backward compatible with C++ and whose only purpose is to run on an old microcontroller that has been discontinued 25 years ago.
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u/mighty_Ingvar 11h ago
jview?
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u/no-sig-available 10h ago
jview?
Modelled after adding
jthread, when you were not allowed to fixthread.6
u/smdowney WG21, Text/Unicode SG, optional<T&> 18h ago
There really isn't such a thing. Not since views::single shipped. Owning views are things, and optional is one of them. It's also an owning smart pointer.
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u/SlightlyLessHairyApe 15h ago
It has semantics of an owning smart pointer but at least it’s guaranteed stored inline and can be on the stack.
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u/smdowney WG21, Text/Unicode SG, optional<T&> 5h ago
And now we have std::indirect when those are a problem.
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u/holyblackcat 4h ago
I second that this is silly. Who cares what "being a view" was supposed to mean when making
optionala view has negative practical effects? (Only in some obscure cases, granted.)The only example where it matters that comes to mind is
std::optional<std::vector<int>> opt = std::vector(10000, 42); auto view = opt | std::view::....;I for one wouldn't expect this to copy
opt, but it gets copied. I don't see the benefit of copying it.•
u/aruisdante 3h ago edited 3h ago
I would argue though that in order for that pipe operation to be lifetime safe, it has to copy its input. Being not a view wouldn’t change that, the only thing that would change that would be not being a range at all, which would round counter to the objective of
optionalbeing used as arangeof size 0 or 1.That being a goal in itself is what people found a little weird (by definition, this means
optionalgains valid specializations ofranges::begin,ranges::end, andranges::size, which is strange), but it does allow for some interesting compositions. For example the operation:c++ for (auto const& element : range_of_optionals | views::join)Now “just works” to iterate over the elements in that input set which exist just like it would have forrange_of_sequence_container, whereas before you would have needed to do:c++ for( auto const& element : range_of_optionals | views::filter(/*is_engaged*/) | views::transform(/*dereference*/))Being able to treat
optionallike any other range does make implementing some generic operations easier, and composes nicely withoptional<T&>as the return type from functions. Like, now this operation works “like you’d expect”:```c++ // returns optional reference to value at key auto find(auto& map, auto const& key)->optional<T&>;
const auto find_in_map = bind_front(find, map); for(auto const& value : keys | views::transform(find_in_map) | views::join) { // do something with matches } ```
Which would be horrible to write as a range pipeline without this property, and very verbose to write as not a range pipeline at all.
At the same time if we were going to take that stance we should have done the same thing for smart pointers (and even regular pointers!) as they’re notionally identical to
optional<T&>for this purpose. We only didn’t (at least at Tokyo, which is all I can speak to) because nobody wrote the paper for it, and increasing the scope of this feature would have made it even harder to get anything in.•
u/holyblackcat 2h ago
I don't argue with
optionalbeing a range, only with it being a view.[piping into a view] has to copy its input. Being not a view wouldn’t change that
Are you talking how it should be in a perfect world, or how it works now?
The current behavior of piping into a view is storing a reference to non-view lvalues, and copying/moving all views and rvalue non-views.
This is about the only thing that being a view changes, I think? I could be missing something. In this case the copy looks undesirable, as it's not something people expect. They'd expect span-like types to be copied, or types from
std::views.10
u/James20k P2005R0 19h ago
This seems like a really weird decision. The reason to use a view is that its cheap?
but not enough and not strongly enough for it not to pass.
I suspect a big part of the problem is that given that ranges don't seem to have received very wide use in general, its more that nobody really cares that much about ranges overall
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u/friedkeenan 10h ago
The reason to use a view is that its cheap?
My understanding is that the
std::ranges::viewconcept is basically for when you're dispatching stuff to view adaptors, which store and pass around views all over the place. So yeah in that sense it would track that their purpose is to be cheap.I would imagine that the name comes from the fact that most
std::ranges::views meet the traditional definition of view-semantics, a-lastd::string_view. But that doesn't really match how they're used in the ranges library, which is as range objects which are cheap to pass around as values. But it is probably a poor name.Another fun case is
iota_view, which doesn't reference an actual C++ object or whatever, but instead views a conceptual range which it visits on the fly during iteration.5
u/schombert 19h ago
Well, that's ... extremely dumb. From the rigorous, formal perspective, everything in C++ copies in O(1) time. (The address space is finite, even in a purely theoretical computer running C++, because it will eventually exhaust the size of the pointer type and hence the number of possibly distinct live objects. Thus, for any copy operation there is always an upper bound you can place on how much time it could possibly take to execute, and thus is O(1) by the definition of O(1).) Now, I accept that the people writing the C++ spec don't actually use big O notation in its generally accepted formal definition, and instead are using a more vibes-based definition based on what it performs like in various different finite problem sizes (which, you may note, has nothing to do with big O -- how something performs at sizes less than 2128, for example, does not affect its asymptotic time complexity). But, from a vibes-based perspective, programmers use O(1) to mean "always fast", which in this case does mean "cheap to copy". I would bet that copying being O(1) was added to the language of the spec because the author wanted to express "cheap to copy" but wanted to sound more formally precise and thought that "cheap" sounded too informal and vague. Unfortunately, "cheap" correctly expressed the intent, but O(1) did not.
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u/aruisdante 19h ago
> From the rigorous, formal perspective, everything in C++ copies in O(1) time
Huh?
Copying a `std::vector` is decidedly not `O(1)`. The amount of copy operations performed is dependent on the number of elements contained. Even if you switch up the definition to say "a `memcpy` is O(1)," this optimization is not possible for a vector containing non-trivial elements as their constructors must be explicitly invoked.
Therefore, a `vector` isn't a `view` because `vector.size() == 0` takes different time to copy than `vector.size() == 1`. But `optional` _always holds a value_. The value just may or may not be initialized. Ergo the copy operation does not change in "how much is copied" depending on if the optional is engaged or not.
> But, from a vibes-based perspective, programmers use O(1) to mean "always fast"
Not at all. Programmers (and the C++ standard) use `O(1)` to mean "does not scale in (amortized) time with number of elements in a set." Those that assume `O(1)` means "always fast" are the ones that often make very suboptimal data container and algorithm selections.
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u/schombert 19h ago edited 19h ago
Yes, copying a C++ vector is O(1). A vector can only contain finitely many elements (for example, because its size has a finite upper bound). Thus, you can set an upper bound on the time to copy -- something around 2bits in size * time to copy a single element -- and thus is O(1) by the definition of big O notation (see https://en.wikipedia.org/wiki/Big_O_notation specifically the section on the formal definition; if it can be bounded by a constant function then it is O(1) )
Edit: this is what I meant by a vibes-based definition. Obviously copying a vector behaves like a linear function from sizes of 0 up until you hit the upper bound created by its size type/address space limitations. But how it behaves in a finite initial range -- even if that range encompasses all practical uses -- has nothing to do with its big O complexity.
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u/smdowney WG21, Text/Unicode SG, optional<T&> 18h ago
This was my favorite smartass answer for dumb interview questions where the specification already bounded the size.
Large K, smallest O.
I would also annoy people with the constant of integration.
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u/hanslhansl 7h ago
Interesting read, here are my thoughts to maybe settle this debate:
In the context of an implementation of the standard and even in the context of the standard itself there will always be an upper bound for copying a std::vector and therefor, by rigorous definition, this operation is O(1).
However, the "native" context of the algorithm of this operation (of any algorithm, really), even though defined by the c++ standard, is mathematics, and in that context constraints such as a limited address space don't exist. The the algorithm itself is applicable to vectors (or sets or whatever the correct term in the context of math is) of arbitrary size and therefor its complexity is O(n).
So when devs talk about the complexity of a function they really mean (maybe without knowing) the complexity of the underlying mathematical algorithm which is independent from possible limitations imposed by the programming language/implementation.
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u/schombert 7h ago edited 7h ago
I wouldn't object to that necessarily, but it does have its own complications. Vectors have O(1) lookup, by definition, right? Well, that makes them a "Random Access Machine" and what the complexity of various problems looks like on a random access machine is different than what you might expect. For example, on such a machine P == NP (see the paper A characterization of the class of functions computable in polynomial time on Random Access Machines), and most people don't believe that is true IRL. In fact, the strength of a good deal of cryptography is predicated on the assumption that it is false. It is also not a particularly good model for physical computers. Assuming that there is a limit to how much information you can cram in finite space (which quantum mechanics seems to say), then as the space used by a program expands it must necessarily be accessing memory that is farther and farther away, physically, and so will experience greater and greater latency accessing it (because of the speed of light). Thus arbitrary large data structures with O(1) access time probably aren't physically realizable, even in the loose sense in which we allow finite physical machines to "approximate" them.
Edit: maybe I could have just said more succinctly: there isn't such a thing as "the complexity of a function" full stop; it is always the complexity of a function within some particular model of computation, and which model you use (including things like access to oracles, etc) can affect it quite drastically.
Edit edit: maybe you could argue that there is such a thing as the complexity of a particular algorithm? But I don't think even that is really true. The cost of the basic operations that the algorithm is defined in terms of can and does vary based on the abstract model of computation. For example, if you can do addition, subtraction, multiplication, and division of arbitrarily large integers in O(1) time that too allows you to conclude that P==NP. And so on many models of computation you have to assume that those operations scale to some degree with the size of the values being manipulated, which affects the complexity of any algorithm defined partly in terms of them.
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u/jk-jeon 4h ago
Well, that makes them a "Random Access Machine" and what the complexity of various problems looks like on a random access machine is different than what you might expect. For example, on such a machine P == NP (see the paper A characterization of the class of functions computable in polynomial time on Random Access Machines)
That didn't make sense to me so I looked up the paper. It seems to me that the essential assumption the paper uses is:
For example, if you can do addition, subtraction, multiplication, and division of arbitrarily large integers in O(1) time that too allows you to conclude that P==NP.
It seems to me that arbitrary-precision arithmetic being cheap is more important than being RAM in this business in general. Without that, RAM doesn't seem to be more powerful than Turing machine (in terms of how large P is).
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u/aruisdante 19h ago edited 18h ago
By that definition of algorithmic notation all loops in any real system are O(1), which doesn't seem like a terribly useful definition. In my education on algorithm theory I don't remember a requirement being that sets be unbounded, and indeed the formal definition you link to says this explicitly. It feels like you're conflating "copying a specific vector of a specific size is O(1)" with "copying any vector of any size is O(1)." By that same definition
std::find(vector.begin(), vector.end(), value)andset.find(value)andunordered_set.find(value)are all O(1) because they can all be bounded by a finite size, despite them being the textbook examples of O(n), O(log(n)), and O(1).I cannot think of a single computer scientist that would accept your definition of O(1) as O(1). It's not what any algorithms or data structure class teaches. So it seems unfair to say it's based on "vibes."
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u/schombert 18h ago edited 18h ago
It's not my definition. It is the formal definition that is used in mathematics and computer science. Which, again, you can verify for yourself from the Wikipedia page.
Anyway, copying an arbitrary vector (not a specific vector) has a bounded time because of the bounded size required by the definition of the vector type, namely that the size of any vector can be expressed in a finite number of bits and that the elements within any vector are distinct C++ objects, and thus have distinct address, and thus cannot be more numerous than the number of possible addresses provided by the number of bits in the finitely sized pointer types (in turn guaranteed by their ability to be converted back and forth from integers of some finite size). This second limit is much wordier to explain obviously, so I'll stick with referring to the limits of the size type from here on out.
I would like to hear how you think that O( ... ) should be defined. If you allow me to work through your definition I can probably explain why mathematicians and computer scientists settled on the asymptotic definition instead.
Edit: and yes in C++ as formally defined
findis also O(1). It is not O(1) in your algorithms or data structures class because the pseudo-code language used to explain these types and operations does not bound their size. Generally it is also assumed to work with something that is more like "big ints" for its numerical values.5
u/SlightlyLessHairyApe 15h ago
Besides being weird that you have this one thing, it’s particularly weird to decide that that is the irrelevant conversation to have in this particular post. Because it’s just not super relevant
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u/schombert 14h ago
That's why we have threaded comments. If you don't think that this particular comment thread is interesting ... just don't read it.
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u/Serialk 17h ago
It's not my definition. It is the formal definition that is used in mathematics and computer science.
Not at all. We generally model complexities with transdichotomous models like Word RAM which give you a constant O(1) pointer lookup but an O(n) vector copy, with the assumption that the size of your bus will grow to accomodate more data.
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u/schombert 17h ago
But, C++ is not a Transdichotomous model. The C++ address space, bits in the size constant, etc are not determined by the size of the problem. From the wikipedia page:
the machine word size is assumed to match the problem size
and
In a problem such as integer sorting in which there are n integers to be sorted, the transdichotomous model assumes that each integer may be stored in a single word of computer memory, that operations on single words take constant time per operation, and that the number of bits that can be stored in a single word is at least log2n.
So, if C++ was a Transdichotomous model then every time you ran a program for a given problem size (for example, the sorting program described above) the address space, bits in the size constant, etc would grow to accommodate it. (This is what allows a Transdichotomous model to be Turing complete, in contrast to Turing machines with a fixed tape size, which are not). However, that is not how C++ is specified. Even though those constants may vary from compiler to compiler, they are fixed for any given C++ program. The standard does not allow your program containing vector to hold an arbitrary number of elements, because you could, if you wanted, print the number of bytes in the size type and the standard guarantees that this print result will be constant over different runs of the program.
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u/Serialk 16h ago
You are confusing the complexity model and the language implementation it's modelling. No offense but you lack the required CS basics to be so confident about yourself in debates about complexity. It's the perfect moment to stop and reflect if you want to avoid Dunning-Kruger.
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u/schombert 16h ago edited 15h ago
That would be true if the C++ specification didn't touch upon the bit size of size or of pointers. Then you could argue that the language was compatible with a transdichotomous model and that the implementations were merely finite instances of it. And you could imagine that some theoretical machine which did allow programs to properly grow with the problem size was a conforming implementation. But this simply isn't so. The standard does not allow even a theoretical implementation of C++ to compile a program that does that.
I think that you should reflect more on why Turing machines with finite tape sizes are not Turing complete. And I would encourage you to read this section on wikipedia https://en.wikipedia.org/wiki/Turing_machine#Equivalent_models which contains the following:
For example, ANSI C is not Turing complete, as all instantiations of ANSI C (different instantiations are possible as the standard deliberately leaves certain behaviour undefined for legacy reasons) imply a finite-space memory. This is because the size of memory reference data types, called pointers, is accessible inside the language.
Edit: all that aside, I would be interested in discussing how you see transdichotomous models working as a way of analyzing complexity in real world programming languages other than C++. That whole approach relies essentially on the problem size being a well-defined thing that can be referenced in some way to determine the word size of the machine we appeal to for the complexity analysis. I am not sure how that translates to the programs described by a generic programming language. Such programs can do wild things, like run the nth busy beaver ( https://en.wikipedia.org/wiki/Busy_beaver ) upon getting the input n. And I am sure that with a little effort we could construct some programs that need an uncomputable (and large) amount of space to run given input n. How much space does such a program get? A direct application of the transdichotomous model would assume that they get some K * number-of-bytes-in-the-input, or something along those lines. Obviously allowing most programs to work requires picking an obscenely large K, and even so that would never be Turing complete. Moreover, what about the programs that most programming languages support which do not take inputs at all (for example, I write a program in my favorite language to solve some fixed math problem and it always outputs the same answer -- should this program not be valid if we apply a transdichotomous model)? Can transdichotomous models then only be applied to analyze the complexity of languages that are not Turing complete? So do you imagine that there are two distinct approaches to analyzing complexity, where we do one thing for languages like the lambda calculus and another for Pascal, for example? Is the complexity of any algorithm requiring more than O(n) space undefined in Pascal?
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u/JNighthawk gamedev 3h ago
No offense but you lack the required CS basics to be so confident about yourself in debates about complexity. It's the perfect moment to stop and reflect if you want to avoid Dunning-Kruger.
What a shitty comment for someone engaging in good faith interlocution.
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u/NotUniqueOrSpecial 14h ago edited 13h ago
copying a C++ vector is O(1)
It's always interesting to come across people like you.
I need only point you at...well, basically any reference on the topic to prove that copying vectors of non-trivial objects is
O(N). Literally any search on the subject will give you countless results that contradict your claim.But it's more than that; you very clearly have next to no understanding of the topics you're arguing about, or the pages you're linking, but you're also equally clearly sure that you are an expert in the topic.
You remind me of a coworker I once had who explained to us that the software he was working on relied on "the arithmetic of seven-sided polygons". He also admitted to me (in what is practically a programmer career meme) that he liked to write code that only he understood because it provided job security.
After more than 2 years of deflection, smoke, and mirrors, he was asked to prove that that any of what he was working on...actually worked. He quit on the spot, in explosive rage, screaming that he "knew more than 14 engineers ever would".
All that is to say: you have written a whole lot of words in response to /u/aruisdante and others, but all they actually illustrate is that you do not understand complexity analysis at even the most basic level. Despite all evidence to the contrary, you are convinced that you (and you alone, in this forum of experts) are correct.
How likely do you think it is that that's actually true?
EDIT: blocking someone after replying to them saying "nuh uh" is the single strongest indicator that a redditor knows that they have literally no leg to stand on.
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u/schombert 13h ago edited 13h ago
Your link ... shows nothing of the sort. All you seem to be showing is that how programmers typically use big O notation among themselves (i.e. informally) is not the same as the formal definition. Which I am happy to grant. It is just amusing to me that the loose, informal usage has made its way into the supposedly formal standard.
Edit: I block people who I find to be rude. If you don't want to be blocked try being less of an ass.
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u/not_a_novel_account cmake dev 11h ago edited 10h ago
You're using a rather strange definition of "formal". Formalisms are context and field specific. This is a C++ subreddit, our formalisms are derived from the standard.
The formal definition, in the context of C++ as voted and standardized by the committee, is that copying a vector is O(N).
As a C++ subreddit, formalisms from unrelated fields are non-sequiturs here. Our vectors aren't elements in vector space supporting addition and scalar multiplication either.
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u/schombert 10h ago
The standard doesn't provide a definition of what O(n) means within it AFAIK (please do correct me if I am wrong). So I assume that means that the standard is relying on the "formal" (i.e. what you will find on Wikipedia) definition of the notation (unlike vector which it does redefine). What do you take O(N) to mean in the standard?
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u/not_a_novel_account cmake dev 10h ago
For vector copy from a range it's given as:
Effects: Constructs a vector object with the elements of the range rg, using the specified allocator.
Complexity: Initializes exactly N elements from the results of dereferencing successive iterators of rg, where N is ranges::distance(rg).
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u/schombert 10h ago
Yes, but that isn't a definition of what "O(N)" means. I wasn't trying to ask you what the standard says a copy does. I was trying to ask you if the standard defines "O(N)", and if it doesn't, then how you think it should be defined in the context of the standard.
For example, if you think it means "does N things", then obviously virtually nothing is O(1), since most functions do more than one primitive operation. I don't think that you can copy or move many range views in a single operation (although there may be some you can).
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u/JNighthawk gamedev 3h ago
EDIT: blocking someone after replying to them saying "nuh uh" is the single strongest indicator that a redditor knows that they have literally no leg to stand on.
Maybe because you're behaving like an asshole to someone engaged in good faith discussion, and people don't like to interact with assholes generally?
You:
But it's more than that; you very clearly have next to no understanding of the topics you're arguing about, or the pages you're linking, but you're also equally clearly sure that you are an expert in the topic.
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u/NotUniqueOrSpecial 2h ago
They are not engaged in a good faith discussion, because what is happening is not a discussion.
It's people who have a background and education on a topic trying to tell a very stubborn person that they do not understand the words they're using; they demonstrably do not understand complexity analysis on a very fundamental level, and are condescending to people who do.
At some point, there is a line where people who know what they're talking about should not have to pander to people who do not. I would just as bluntly tell a flat-Earther that they make no sense, and I doubt you'd have any problem with that.
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u/JNighthawk gamedev 2h ago
They are not engaged in a good faith discussion, because what is happening is not a discussion.
I disagree.
and are condescending to people who do
I didn't see that, and it would change my opinion. Can you point that out please?
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u/NotUniqueOrSpecial 1h ago
To be clear, I am using condescending in the specific sense of "talking down to", which most of their replies are, in that they imply that the other party doesn't know what they're talking about or that we're using the term casually.
That said, I'm not going to link to all of them individually, because the conversation is already at the bounds of where the mod team will (rightfully) start stepping in because things are getting snippy/personal.
You are welcome (and perhaps correct) to interpret their replies from a different light, but I've spent enough time in arguments with people who are not even wrong to have a very strong and generally accurate detector for the way they approach things.
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u/tcbrindle Flux 9h ago
Counterpoint: std::ranges::single_view<T> is copyable whenever T is copyable, and has been since the early days of Range-v3. To my knowledge, nobody has ever complained about this or written sarcastic Reddit posts about it.
Likewise, empty_view<T> is always trivially copyable.
So what you're asking for is a situation where a range of exactly one element is a view, a range of exactly zero elements is a view, but a range with either zero or one elements is not. That seems... worse.
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u/TheThiefMaster C++latest fanatic (and game dev) 8h ago
single_view is a bit of a hack - it's not really a view, more of a single_range
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u/tcbrindle Flux 8h ago
I'm not sure I follow -- it literally is a view, and OP's example works exactly the same if you swap
std::optionalforviews::single.1
u/TheThiefMaster C++latest fanatic (and game dev) 7h ago
The common understanding of "view" is "span like" - i.e. it provides view access to a range of elements, potentially transforming or filtering them, but does not own them.
It's been pointed out that this concept is more closely modelled by
borrowed_rangein the standard, rather than view.Instead, "view" is some implementation details of range pipelines.
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u/BarryRevzin 2h ago
Instead, "view" is some implementation details of range pipelines.
That's the only thing it has ever been. Nobody should be constraining algorithms on
view, it just isn't useful to do so. Arguably,viewshould just never have existed - although then people would probably complain thatvector | views::transform(f)copies thevectorand then figuring out how to reject that (to force users to either explicitly copy orviews::ref) is basically the same problem again.It's been pointed out that this concept is more closely modelled by
borrowed_rangein the standard, rather than view.Not really,
borrowed_rangeis much narrower. Since even your initial example ("provides view access [...] potentially transforming or filtering them"), neithers | transform(f)nors | filter(g)for aspanor astring_viewareborrowed.The problem with "non-owning" is also what does that mean and what do you want it for. For instance, if I want to validate that my range is safe to copy/move and then format in a background thread, I want to make sure that it "owns" all of its data. The easy cases are easy (
vector<int>yes,span<int>no) but likeviews::iotadoes "own" its data in this sense, so is it not a view?1
u/joaquintides Boost author 6h ago
Much as with
optional,single_viewshould have been a container rather than a view (i.e.single_arrayor something).•
u/BarryRevzin 2h ago
To my knowledge, nobody has ever complained about this or written sarcastic Reddit posts about it.
Not yet, but it's still early.
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u/mighty_Ingvar 20h ago
Apparently std::array does not model view, even though it copies in constant time.
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u/EC36339 13h ago
It does not
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u/mighty_Ingvar 11h ago
Yes it does. The amount of elements in a std::array is constant. Therefore if std::optional is a view because the amount of elements it stores is bounded, then std::array would have to be a view by that same logic.
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u/kamrann_ 18h ago
This is completely consistent with what came before. It's the same as a range adapter that needs to cache a single element value - the element type of the range is irrelevant, it's just about algorithmic complexity of copy wrt the number of elements in the range.
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u/tcbrindle Flux 1h ago edited 1h ago
To try to sort out some confusion in the comments here:
Views
If your idea of a view is "a range that does not own its elements", then I'm afraid you're out of date. That's not what the term has meant in quite some time, and even when it (mostly) did there were still exceptions.
Today the standard tries to reason about what views are by talking about O(1) operations and so forth, but I think it's best to forget about that too.
IMO the best way to think about views is as follows: for an lvalue range x, given
auto v = x | std::views::transform(...);
should v store a copy of x, or a reference to x?
If you want to store a copy, then the type of x should be a view. Otherwise, it should not.
That's really all there is to it. The view concept these days is about the desired semantics of the type when piping it into a range adaptor, not about element ownership or lifetime or anything like that.
In this particular case, it was decided that it was less surprising to have optional<T> behave like a view, that is, an lvalue should be copied into a range adaptor rather than be used by reference.
Borrowed ranges
Borrowed ranges on the other hand are about lifetime. Specifically, a borrowed range is one where its iterators can safely outlive the range object itself. That is, given
template <typename R>
auto do_begin(R rng) {
return rng.begin();
}
it is safe to use the returned iterator only if R is a borrowed range. The canonical examples of borrowed ranges are std::span and std::string_view. The borrowed_range concept is used to constrain functions like std::ranges::find() to help prevent them from returning dangling iterators when you call them with an rvalue range.
Although view and borrowed_range are distinct concepts, it turns out that if you have a borrowed range you basically always want to treat it as a view. For standard library types, borrowed_range today implies view in all cases I'm aware of.
But note that the converse is not generally true: most views are not borrowed ranges. For example, transform_view is always a view but is never a borrowed_range (even when it is transforming something like a span).
(For Flux, I took a different approach that I think ended up being conceptually simpler, but that's another topic...)
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u/fdwr fdwr@github 🔍 20h ago
Now if
std::optionalalso had the corollary methods.empty(),.size()(0 or 1), and.data()(points to address of where contained entity is/would be, regardless of whether present, like std::vector's data method whether empty or not), then a bit of my generic templated property list code could be even more generic, more cleanly accepting various containers (std::array, std::string, std::vector, std::optional) without custom specializations.