Too true, and this is too good. Start with part 1 (and the comments) if you haven’t.
- https://news.ycombinator.com/item?id=43468976
- https://www.reddit.com/r/programming/comments/1jjluxe/writin...
1. hilarious because it itself is an example of this behavior, we aren't discussing the article, but instead a tangent from a few words from it
2. an instance of Parkinson's Law of Triviality: it's just easier to respond to a comment than to read an entire article. Plus, many people read comments first to try and determine if an article is worth reading. So you end up with engagement in spinoff discussion, especially when the original is harder to read or understand to a general audience.
I’ve implemented something like this before, without macros. It’s a little ugly, but not that bad IMO.
If you write a native range-based for loop:
for (auto foo : obj) { /* do something */ }
auto it = foo.begin();
auto end = foo.end();
for (; it != end; ++it) {
auto foo = *it;
/* do something */
}
The main drawback is that you need to temporarily store each item in the begin object before it’s moved into the iteration variable. This is because you have to already know whether a next item exists at the point of `it != end`, but then the item isn’t actually retrieved until `*it`. The extra move has a slight cost, but the compiler can often optimize it away to nothing. You can also avoid this if the for loop uses a reference type (`for (auto& foo : obj)`).
The technique you so well describe in your comment does not work with the original range for :(
Not that it is an issue anymore, but I'd forgive anyone who tried to write code like this when range for was added from not trying anymore, because they remember the original semantics.
The page: https://cppreference.com/w/cpp/language/range-for.html
reads like a testimony in a negligence court case.
In C++ this is equivalent to an InputIterator.
// erase
map.erase(2);
// iterate
for (auto &[k,v]: map) {
// do stuff
}
In reality, very few real-life containers can support this pattern, which is why this is a headline case for Rust, because it statically prevents this bug.
But yes, for removal the correct thing is always to use `std::erase_if` (C++) or `retain()` (Rust). For insertions, the only real solution is to build up a separate collection while iterating and then merging it into the original container when done. Yucky, but won't crash.
For unordered_map (and every hash table in the known universe) erasing anything invalidates all iterators, so you can't iterate while erasing. For std::map, you can if you're very, very careful (erasing invalidates the iterator you're currently on, but if you cache the next iterator, THEN erase the current one, it'll probably work, but be very fiddly). Most languages forbid this entirely: e.g. Rust's ownership model doesn't allow it, Python throws an exception, etc. It's just a very bad idea in general.
for(decltype(cont)::const_iterator it=cont.begin();it!=cont.end();++it){
if(Keep(it.first)){
++it;
}else{
it=cont.erase(it);
}
}
EDIT: just saw your example and checked cppreference, it says the return value "Iterator following the last removed element" for std::unordered_map. So i think you need to add an `it--` after your erase, otherwise it will "skip over" the next element. Right?
Also just read this little nugget on cppreference for unordered_map::erase:
> Removes specified elements from the container.The order of the remaining elements is preserved. (This makes it possible to erase individual elements while iterating through the container.)
This seems like a crazy guarantee to put in the standard, it must really limit the kinds of hash tables you can make that matches the unordered_map interface.
It’s a great and useful guarantee.
> it must really limit the kinds of hash tables you can make that matches the unordered_map interface.
Many libraries treat containers as “abstract” with many possible implementations. STL explicitly does not. It’s a specific data structure from a computer science class.
It feels natural to assume that the implementers and long time WG21 members must understand the language, but this is not true. C++ spiralled well beyond the capability of a person to actually understand it years ago.
Maybe that's unfair, but it's unusual. This is a constructed language, its syntax and semantics matter greatly and yet in practice our grasp of its meaning is more akin to that for say, English, than for Perl or Rust.
There are contradictory style guides, people with well meant rules of thumb contradicted by a world of real usage, numerous dialects, not to mention people who strongly believe other users of the language are "doing it wrong" and yet what they're doing works fine.
https://devblogs.microsoft.com/directx/omm/
Compare the C++ code samples with the Modern C++ usually talked about C++ conferences.
This is why I keep saying I only see such high bar code on conference slides, and my hobby projects, those DirectX code samples are much more closer to daily C++ written in big corporations, ironically even those that are C++ compiler vendors with seats at WG21.
On the other hand, there are many industries where I don't see Rust taking anything meaningful away from either C or C++, regardless of how much we complain about them, or how much better Rust happens to be over them.
I think open-source software like https://github.com/microsoft/WSL is probably more representative of what modern C++ companies look like. Plenty of files that just interact with OS C APIs, but no shortage of modern C++ features in use.
In what way does interact with C APIs become an argument to use alloca() with raw pointers?
Just recently I also submitted an issue on Azure C++ SDK regarding use of C style strings and arrays, what is the excuse there?
CreateFileA API thing is only available for compatibility with prehistoric software written for Win9x from the nineties. Also, the SDK comes with CAtlFile class which wraps CreateFileW / ReadFile / CloseHandle and guarantees closing handle in the destructor.
They are using a raw pointer in OMMSet structure which leaks memory. I would replace the raw pointer with std::unique_ptr<D3D12_RAYTRACING_OPACITY_MICROMAP_HISTOGRAM_ENTRY[]>
As for the _alloca, I think it’s OK however I usually assert() the size being allocated is reasonable, like under 256kb.
About non-Unicode WinAPI functions, I don’t use them at all in the software I’m developing, nor the TCHAR conditional typedef. VC++ compiler defines wchar_t as a built-in type for the UTF16 strings used in WinAPI functions.
Any project which only targets Windows, or at least it can only be used in Windows-specific code. Even if you currently only target Windows, baking platform-specific types into your core is foolish.
jandrewrogers•1d ago
Yes, I understand, compatibility. At some point, clean new code bases should not be burdened with that albatross.
thenewwazoo•1d ago
https://abseil.io
jandrewrogers•1d ago
There are many good libraries out there, or fragments of libraries, but I’ve never found one that really scratches this itch.
hoten•1d ago
krapht•1d ago
https://github.com/martinus/unordered_dense provides better replacements for unordered_map/set.
The STL is missing B-trees and B-heaps, as well as d-heaps.
STL is also missing a radix sort, which is even more sorely missed now that we have std::executor::par_unseq to play with.
maattdd•1d ago
jll29•1d ago
monkeyelite•1d ago
112233•1d ago
monkeyelite•1d ago
112233•17h ago
The idea to base everything on iterators instead of ranges, blowing up the code (and amount of errors) — it would have made sense for pure algorithms, but stl insists on having ownership of object placed in the containers.
No sane way to work with binary data. Inconsistent allocation and exception ideology through the years. Mysteriously missing functionality.
What little ruby, python, php I have written — nothing has felt as clunky as STL.
E.g. vector has no "sort" member function. What great benefit not adding it for convenience has brought?
platinumrad•1d ago
Pretty much everything else (e.g. iostreams) is horrible.
TuxSH•1d ago
Heck, even std::atomic was designed with only x64 in mind (it clearly shows), and is unusable outside it. One is incentivized to write their own "atomic" class until P3330R0 is approved for RMW-centric platforms ISAs like Aarch32 and Aarch64.
And of course, Rust already has "fetch_update"...
spacechild1•1d ago
It certainly wasn't.
> and is unusable outside it
Total hyperbole. It's perfectly usable on ARM and other platforms.
P3330R0 looks a nice addition, though.
TuxSH•1d ago
The idiomatic way to do RMW (outside simple stuff like fetch-increment) with std::atomic maps 1:1 with x64 assembly and since fetch_update isn't provided, it's the only way to do it. It's way too close for comfort. See [1] for a comparison
> Total hyperbole. It's perfectly usable on ARM and other platforms.
It's not hyperbole. std::atomic is portable, but that's all it is.
std::atomic is about 30% to 40% (with outlined atomics on, which is the default) slower than handrolled asm (or custom reimplementations that provide fetch_update -- same thing). See [2] for a benchmark.
[1] https://godbolt.org/z/EasxahTMP
[2] https://godbolt.org/z/Y9jvWbbWf
spacechild1•1d ago
> std::atomic is about 30% to 40% (with outlined atomics on, which is the default) slower than handrolled asm
Only for certain CAS operations. A 30% or 40% performance penalty doesn't sound too dramatic and certainly makes it "usable" in my book.
I appreciate your insight, but it could have been delivered with less hyperbole.
TuxSH•1d ago
> they explicitly support weak memory models.
Sure, but memory ordering is orthogonal to LL/SC vs CAS.
To me, fetch_update not being present from std::atomic's inception is major design oversight as CAS can be emulated via LL/SC but not the other way round.
Furthermore, fetch_update code is easy to read and less awkward to write than CAS loops (which currently are the only way std::atomic offers, and this is what I'm complaining about)
> Only for certain CAS operations. A 30% or 40% performance penalty doesn't sound too dramatic and certainly makes it "usable" in my book.
I disagree. Atomic variables (atomic instructions) are usually used to implement synchronization primitives, and are thus often meant to be used in very hot paths. 30% perf drops are actually quite bad, in that regard.
Of course if one is restricting themselves to using only member methods (fetch_add, fetch_or, etc.), then all is fine because these methods are optimized.
All in all, C++'s stdlib (the parts that aren't just __builtin wrappers, to be precise) is actually quite fine for most use-cases, like PC applications. Indeed, it is when one has latency constraints and/or severe memory constraints (e.g. < 512 KiB) that the stdlib feels like a hindrance.
spacechild1•1d ago
> Sure, but memory ordering is orthogonal to LL/SC vs CAS.
Sure, but your original claim was that std::atomic has been designed with only x64 in mind. That's what I meant to argue against.
I agree that the omission of something like fetch_update() has been an oversight and I hope that it will make it into the C++ standard!
As a side note, here's what the Rust docs say about fetch_update():
> This method is not magic; it is not provided by the hardware. It is implemented in terms of AtomicUsize::compare_exchange_weak, and suffers from the same drawbacks.
https://doc.rust-lang.org/std/sync/atomic/struct.AtomicUsize...
So Rust's std::sync::atomic is equally "useless"? :)
TuxSH•1d ago
Looks like their (Rust) main motivator was readability. Whereas P3330R0 has that + performance on non-CAS hardware in mind. In any case, Rust's function could be optimized in the future, if they decide on it.
monkeyelite•1d ago
platinumrad•1d ago
tialaramex•13h ago
colejohnson66•1d ago
Const-me•1d ago
Mostly agree about the algorithms. Another good thing in C++ are low-level mathematic parts of the standard library in <cmath> and <complex>.
Containers are OK, but neither usability nor performance are IMO great. Node-based containers like red-black trees and hash maps come with a contract which says pointers are stable, by default this means one malloc() per element, this is slow.
However, there’re large areas where C++ standard library is lacking.
File I/O is questionable to say the least. Networking support is missing. Unicode support is missing, <codecvt> deprecated in C++17 because they found out it no longer implements current Unicode standard and instead of fixing the standard library they dropped the support. Date and calendars support only arrived in C++/20. No built-in way to represent money amount, e.g. C# standard library has fixed-size 16 bytes type for decimal floating-point numbers, Java standard library has arbitrary-precision integers and decimals.
account42•1d ago
Fixed point types would be nice but can be implemented on your own and integers representing sufficiently small denominations (cents or fractions thereof) work in a pinch to deal with monetary amounts. And for the interface between libraries you will need to deal with things like currencies anyway and that goes well past the scope of a standard library.
Networking is also not something that is all that stable on the OS level beyond the basic socks API and you can just use that from C++ if you want to. There is no benefit from cloning the API into the C++ standard.
Same for filesystems - operating systems are different enough here that applications are better off handling the differences directly as can be seen in the unsatisfying attempt to abstract them in std::filesystem.
Pushing every functionality you can think of into the standard library is a mistake IMO. It should be reserved for truly ossified OS interfaces, basic vocabulary types and generic algorithms. Everything else is bloat that will be obsolete anyway sooner rather than later.
Const-me•1d ago
Standard libraries of Java, JavaScript, and C# are counter-examples.
> you can just use that from C++ if you want to
Technically, C++ standard could feature an abstract interface for a stream of bytes. Would be useful not only for TCP sockets, also for files and pipes.
BTW I’ve been programming C++ for living for decades now, and I never saw production code to use C++ <iostream> header. Instead, C++ developers I worked with have been using either <cstdio> or OS-specific APIs.
> applications are better off handling the differences directly
Many other languages have managed to design high-level platform agnostic abstractions over these things, and implemented them in the standard libraries.
> reserved for truly ossified OS interfaces
By now this applies to files, file systems, pipes, and TCP sockets. While there’re some features hard to abstract away (examples include controlling file permissions, and probably asynchronous I/O), many real-world applications don’t need them. They just need basic stuff like read and write bytes from binary streams, concatenate paths, create/open/delete files, listen and accept TCP sockets.
account42•14h ago
You mean because of the 16-bit character type baked into the language even though Unicode has moved past that?
> Technically, C++ standard could feature an abstract interface for a stream of bytes.
That's what iostreams are. Turns out such abstractions come with overhead and other limitations and you need to use OS-specific APIs for even slightly advanced features anyway.
> Many other languages have managed to design high-level platform agnostic abstractions over these things, and implemented them in the standard libraries.
And these lowest common denominator "abstraction" result in developers making software that doesn't work like users of the OS expect.
> By now this applies to files, file systems, pipes, and TCP sockets.
Not at all. Async interfaces are all the rage these days. Meanwhile browsers have moved from TCP to QUIC (which is much more than a stream of bytes so would need a completely different abstraction) and it's not unlikely that other applications will want to move to it too. You can make a basic bitch abstraction for these but if everyone that cares about performance needs to fall back to OS-specific interfaces then that doesn't help that much.
Const-me•9h ago
That type is not for characters anymore; all these languages treat these values as UTF-16 units. Unlike C++, their standard libraries provide functions to convert strings between UTF-8 and UTF-16, apply Unicode normalization, decode strings into a sequence of code points, etc.
> That's what iostreams are
iostreams tried to implement binary streams, text streams, and object formatting with the same API. Predictably failed all these tasks, the features are too different. A minimalistic C++ API for a stream of bytes might look something like that:
> result in developers making software that doesn't work like users of the OS expectCan you elaborate? When I write string content = File.ReadAllText( path ) in C#, the standard library does exactly the same thing as some C++ library could do: open file for read access, read it to the end, and because C# strings are UTF-16 convert the bytes from UTF-8 to UTF-16.
> Async interfaces are all the rage these days. Meanwhile browsers have moved from TCP to QUIC
I think both points are exotic stuff. Not sure I would want to see them in C++ standard library. Also going to be hard to design a useful platform-agnostic abstractions. OTOH, Unicode strings, file systems, and streams of bytes are literally everywhere in software.
simonask•1d ago
Every serious C++ project worth its salt includes additional dependencies to patch it up, and it looks like that will be the case in perpetuity, because these problems are unfixable in the holy name of ABI stability.
Don't get me wrong, ABI stability is a worthy goal. The committee should just have realized that the current approach to it is untenable. The result is a very half-baked situation where ABI stability is not technically guaranteed, but nothing can be fixed because of it.
What a mess.
Rust takes a much, much more cautious approach (because of C++'s history), including explicitly not supporting Rust-native ABI stability and flat out discouraging dynamic linking. Also not very great, but it's sensible as long as there are no clearly superior solutions.
account42•1d ago
Those are the result of a constant stream of people complaining that the C++ standard library is bad because it doesn't contain their pet feature.
Needing additional dependencies beyond the standard library is not problem but how things should work. Because requirements differ and one persons useful dependency is another persons vestigial bloat.
simonask•1d ago
The problem here isn’t that it’s bloated (I don’t particularly think it is), but that the things it provides are often very far from best in class.
account42•14h ago
prydt•1d ago
It has proper container classes based on B-trees and its also got an async runtime.
[1] https://github.com/capnproto/capnproto/blob/v2/kjdoc/index.m...
jeroenhd•1d ago
SerenityOS' original "make no use of external libraries" approach made one of the more complete re-imaginings of the C++ application landscape I've seen. It really changed my perspective on what C++ could be.
tyrellj•1d ago
https://github.com/SerenityOS/serenity/blob/master/AK/ https://github.com/SerenityOS/serenity/blob/master/AK/Result... https://github.com/SerenityOS/serenity/tree/master/Userland/...
kgeist•1d ago
>The reason it caused the division at community is that the ones that you could not use both on the same project, this meant that for a while you had two "D" languages that were separate, libraries made with one did not worked with the other...