C++20 Is Feature Complete; Here’s What Changes Are Coming

https://hackaday.com/2019/07/30/c20-is-feature-complete-heres-what-changes-are-coming/

by jandeboevrie • 7 years ago

78 64 7 years ago

hackaday.com

ilaksh• 7 years ago

Sounds amazing. Anyone have code examples that integrate a lot of the newer stuff?

Dumb question. Why isn't omitting 'return' always an error in gcc rather than just a warning?

plorkyeran2• 7 years ago

int foo() { bar(); } is a perfectly well-formed function as long as bar() never returns. Allowing this without requiring bar() to be annotated as noreturn is generally considered a mistake, but correcting it would break existing valid programs.

ComputerGuru2• 7 years ago

C++17 already made backward compatibility a non-goal (within reason, obviously) and broke previously valid programs.

temac• 7 years ago

but correcting it would break existing valid programs

That's very often the case when enhancing static safety guarantees. And that's often worth it.

ahartmetz• 7 years ago

In any case, just add -Werror=return-type to your compiler flags... After I had forgotten a return once, that option saved me several times.

Gibbon1• 7 years ago

Likely answer is, doesn't really cause problems for anyone in practice. Experienced C++ programmers fix warnings.

Also the ABI specifies how to call and get the return value of a function. The long and the short if it is the execution thread doesn't go off the rails if there is a mismatch in expected behavior. Your data may be garbage tho.

DerekL• 7 years ago

It would be nice if the program would abort if a non-void function returns without a value. The only cost would be a slightly larger executable.

Ididntdothis2• 7 years ago

Looks pretty good. But I don’t understand why they don’t do string interpolation like C# and others do.

“I= {I}”

Is much better and less error prone than

“I= {}”, I

Or am I missing something?

_hardwaregeek• 7 years ago

Probably some syntactical reason. If you allow arbitrary expressions inside string interpolation, you'd need to have some form of state in your lexer to figure out when to lex the characters into tokens or just keep them as raw characters. It's not so easy as looking for another brace, as you can have expressions that have braces (lambdas). Plus, string interpolation can then be nested, which you may not want.

royjacobs• 7 years ago

I don't think you'd be able to evaluate your expression without some kind of runtime reflection. Specifically, how is a standard library method supposed to interpolate {l}? It doesn't have any information about which variables are present in some kind of format string.

Ididntdothis2• 7 years ago

I am pretty sure C# does the interpolation at compile time but it probably helps that it has a string type as first class citizen to the compiler has more information.

plorkyeran2• 7 years ago

The format library is a pre-existing library which is being standardized. String interpolation cannot be implemented in a library, and requires language extensions. Perhaps they should have done that instead, but that's a significantly more complicated task than merely adopting something that's already well tested and proven (and even that was far from trivial).

hawski2• 7 years ago

I am not necessarily opposing string interpolation as a language feature, but I have an idea how could it work as a library. Implement string suffix and overload comma operator or overload reminder operator. It would look like this:

  "Hello {} {}!"i, 2, "worlds"

Or with reminder operator as in Python:

  "Hello {} {}!"istr % istr(2, "worlds")

But yeah, enough of this.

bsder• 7 years ago

C++ (and C) have a fairly odd characteristic in that it is really hard to definitively figure out what is an identifier.

It's one of the reasons C++ takes so long to compile.

It's why the "modern" languages all seem to be converging on having an explicit token that signals "This thing is an identifier : this thing is a declaration".

gigatexal4• 7 years ago

I’m not a C++ dev. I took one class of it in college and even then it was 90% C and only hit pointers and classes on the final unit.

But holy smokes this is not your grandmother’s C++, is it just me or is the language trying to be all things to all people? It’s getting too big, no? I mean how long do you have to be writing C++ code to fully master the language? Or can you be a high level dev without ever touching the new features? I’m assuming production codebases are likely still stuck on older versions of the language and might never move on.

I hope golang doesn’t grow this large over time.

sp332• 7 years ago

Oh yeah, C++ has been huge for decades. You pick a subset and program in that. The changes in C++11 had structural implications that took years to work through which is why it was late. The working name was C++0x because they expected to release it in '08 or maybe '09. http://www.stroustrup.com/C++11FAQ.html

noobermin3• 7 years ago

This is what people say, how will C++ then not turn out like lisp then? Lisp became so idiomatic that people couldn't share code.

DoofusOfDeath• 7 years ago

Oh yeah, C++ has been huge for decades. You pick a subset and program in that.

My impression is that the C++ standards committee sees the standard like a pile of papers on a professor's desk, where most programmers only need to understand what's currently visible on the top. And so they keep on adding papers to the top, they're getting the least-bad combo of innovation and backwards-compatibility.

My experience has been different. These days I mostly deal with codebases that are about 50% original C++ code, and 50% third-party code pulled in via CMake "super builds".

Even within the first 50% that's ostensibly under the control of a single organization, I find a large variety of C++ coding styles in effect. Continuing with the papers-on-desk metaphor, I generally have to deal with code that draws from the entire stack of papers.

alexhutcheson4• 7 years ago

Yes, it's very complex and hard to stay on top of current best practices.

However, every one of the major features listed in this post replace an existing language feature with something that is easier/safer/less verbose. For new code you shouldn't have to use the old features, and you can just rely on the new ones. Existing code is trickier, which is already a problem for large C++ codebases. You'll find dramatically different styles depending on when the code was written.

All in all, though, the recent changes have made C++ much more pleasant to program in. They're mostly less complex for an application developer than the things they are replacing, and the really complex bits (concepts, etc.) are only needed for writing complicated template libraries.

eikenberry• 7 years ago

Does anyone know of a mind-map or info-graphic that sort of lays out the new features and which older features they replace. Something to help someone who hasn't looked at C++ in years get a feel for how to map things?

MadWombat3• 7 years ago

"every one of the major features listed in this post replace an existing language feature"

That is the part that annoys the hell out of me in regards to C++. Nothing ever gets replaced, things just get added on top of everything. And to add to that there is generally very little indication which is the "new, safe and cool way" and which is "old, shoot yourself in the foot" way of doing things. Like std::string is cool and generally you want to use it everywhere... except you can use []char everywhere and be completely unaware that you are doing anything wrong (well, until you segfault on memory access). Or unique_ptr and shared_ptr and auto_ptr. Cool things, but you can use a regular pointer in their place and again, never realize something is wrong (again, until you segfault). And every C++ book I ever saw happily discusses raw pointers and memory allocation and []char strings without ever giving you any warning that under most normal circumstances you should not use any of this.

pizza234• 7 years ago

every one of the major features listed in this post replace an existing language feature with something that is easier/safer/less verbose

I'd argue that the each new feature adds on top of an existing one. 30+ years of existing knowledge won't vanish, and will cause decades of unintended Frankenstein codebases (followed by decades of even more Frankenstein ones).

gigatexal4• 7 years ago

That makes sense. So you could take a C++11 codebase and incrementally change things to C++20 (using a complaint compiler of course). I wonder if there will be a huge market for people who truly understand all versions of the language who will be consultants hired to upgrade large codebases

saagarjha• 7 years ago

I mean how long do you have to be writing C++ code to fully master the language?

To be honest I don’t think you can ever fully master the language.

wheels• 7 years ago

You could, at one point. C++98 and C++03 were minor updates, meaning C++ had 22 years where learning it wasn't a fast moving target (between C++89 and C++11). I would happily call myself an expert in that dialect of C++, but getting there took 5-10 years.

I can't call myself a C++ expert anymore, and, in fact, find it hard for anyone to, because with a language so complex, idioms and best practices aren't evolving at the same rate that the standard is changing, especially in cases where one has to deal with a mix of modern C++ and legacy (C++03 and earlier) C++.

tejohnso• 7 years ago

I recently started trying to learn c++ and I'd say I've got the basics down.

Sometimes I'll watch a talk by Scott Meyers or Jason Turner and realize how little I know, and how deeply complicated things can get. Then I go back to my code base and continue being productive writing useful code. I know it's probably not perfect, but it's still satisfying.

As for reaching perfection or mastery, that's an enjoyable journey, even knowing that the final destination is likely unreachable.

asauce• 7 years ago

One of the worst interview questions I have had was "Rate your C++ knowledge from 1 - 10".

I had just finished my second year of college, and although I felt fairly confident in my C++ abilities I had no idea how to answer. I thought it would be best to be honest and said "4" based on the subset of the huge language I knew (honestly this rating was probably too high at the time!).

One of the interviewers was obviously unhappy with the answer, and was unpleasant the rest of the interview. I have always wondered what they expected from such an vague question.

meheleventyone• 7 years ago

It’s just a trap to quiz people on language errata when they say a high number the guy would have been just as unpleasant in a different way.

leetcrew• 7 years ago

I agree this is a bad question, but when people ask, they almost always mean relative to your peers. they're not trying to decide whether to hire a summer intern or a senior dev. they just wanted to hear that you were better than most of your classmates.

noobermin3• 7 years ago

I guarantee you his/er knowledge was probably a 3 or lower. If this were reddit I would ironically reply they clearly were just jealous of you or didn't believe you had such a well-developed grasp of C++ fresh out of college.

ksec• 7 years ago

One of the worst interview questions I have had was "Rate your C++ knowledge from 1 - 10".
One of the interviewers was obviously unhappy with the answer, and was unpleasant the rest of the interview. I have always wondered what they expected from such an vague question.

I knew enough of C++ to know my knowledge on the subject is approximately close to Zero.

Basically no one on earth knows C++, they mostly know a subset of it.

*C++ here referring to Modern C++, Pre 2000 era C++ was still manageable.

Edit: On that thought it seems to me every PL in 90s were all manageable. Somewhere along the line things got ugly.

tcbawo2• 7 years ago

The safe answer to this horrible question is 7. If they are expecting you to be an expert as an undergraduate, your mentoring possibilities are probably nil. They also probably associate unfamiliarity with inability. You probably wouldn't want to work there.

agumonkey• 7 years ago

I don't write cpp but it seemed c++11 and later were almost shrinking.

gumby• 7 years ago

is it just me or is the language trying to be all things to all people?

Well yes, in the sense that it's a systems programming language designed not to be a single-paradigm language (you can write procedural, or somewhat functional, or classic OOP, or generic functions-based OOP) and to be very efficient at all.

Some of the features are seemingly arcane in order to preserve the zero-cost abstraction approach as much as possible.

Or can you be a high level dev without ever touching the new features?

So some features are arcane as I wrote above but some are deliberately high level, designed to make it easier to write safe, efficient high-level code without getting into the weeds. Being able to type

    for (auto& a_frob : from_manager) ...

and have it be fast, efficient and not leak memory is a feature, right?

I hope golang doesn’t grow this large over time.

Go's explicit design goals are the opposite: have only a few constructs, a specific "opinion" as to how things should be done, and to deliberately limit expressiveness in favor of other objectives. So I doubt it will come close.

Edit: I don't agree with go's design criteria, but I respect them.

empyrical• 7 years ago

Despite the slight differences they have from the C99 version of them, I'm pretty happy to see designated initializers in C++20

ncmncm2• 7 years ago

This article might seem confusing.

That's because it is. There is hardly a paragraph in it that is wholly correct, where they actually mean anything.

C++ itself is much easier to understand than the article.

The real purpose for all the constexpr-related apparatus, as well as much of Concepts, is to make it much rarer that anybody needs to do "template metaprogramming". Instead of writing Template Metaprograms to run at compile time (a very strange functional pattern-matching language practically nothing like C++) you can usually write ordinary C++ code.

Coroutines are a new flow control primitive showing up in lots of languages lately. The C++ version is the same as the others.

The main benefit you will see from Concepts is overwhelmingly better error messages when you misuse a library component, whether Standard or your own.

Modules is a big step toward enabling a modern build infrastructure, as already seen in lots of newer languages that got benefit of hindsight.

All the old stuff is still there, because billions of lines of old code still use it. You don't have to use it in new code, but it all still works.

The biggest difference between old and new C++ code is that good new code uses value types more, and reference types less, making everything safer.

Coding modern C++ is overwhelmingly more fun than coding C++98 was, but it is just as fast as ever, and better than ever for writing fast, powerful, nice-to-use, no-compromise libraries.

hackandtrip• 7 years ago

I got had to fight with template metaprogramming lately, and I can't be happier than reading about this. Do you have any sources when one can learn in a non-confusing way about this new stuff, especially Concepts?

favorited• 7 years ago

If you're looking for a high-level example, the Wikipedia article one where a function is constrained to only accept an argument which implements operators "==" and "!=".

https://en.wikipedia.org/wiki/Concepts_(C%2B%2B)

hawski2• 7 years ago

I am C++ developer by career and I am pretty much bashing the language all the time. To preserve my sanity I like to use C in after-hours. However looking up C++20 and the direction whole language is going I see there are things I would really like to see in C, while preserving C simplicity. Standardized attributes like [[fallthrough]] and others for functions seem nice. Anonymous functions are nice. They look a bit ugly, but function pointers already look even worse. It would not hurt to have modules and consteval functions in C, but then one still needs templates to really ditch the preprocessor and that's a tough one to solve.

Having said all that C++ is really in a need of a clean cut between legacy features and the blessed way. Backwards compatibility is a noble cause, but there is also the whole weight it pulls around. Maybe something like [[deprecated]] attribute could be extended to handle more cases. Maybe C++ needs it's own "unsafe" block, something like

  [[unsafe]] {
    *((void*)0) = 0xDEAD;
  }

Essentially having an ability to clearly mark all those different styles of different versions of C++ really. So that compiler could help you to control it and in case of a green field project totally guide you to use only the most modern idioms with an escape hatch. Of course the problem is that to help with the complexity more complexity would have to be added.

I am a bit on the crossroads. From one side I start to see, that there is some hope for C++. But on the other I feel like running away, especially since I'm looking for a job. In no particular order: C, Rust, Java, Kotlin, Scheme, Julia, Go, OCaml, Free Pascal, Swift, TypeScript, Lua/Terra, Zig.

I had a funny thing with C++. One project I worked on was used in the debug version all the time, to be able to drop to debugger when needed. At that point I started to wonder why not just use Java already, instead of trying to do a manual Java and fail at it.

lousken• 7 years ago

I've had C++98 at school, but how would I learn C++17? Scott Meyers wrote C++14 but he retired from C++ development so what are the best alternatives?

antisemiotic• 7 years ago

Changes from c++14 to c++17 are nowhere near as dramatic as c++98 to c++11, so you can go with Scott Meyers and fill in the gaps from cppreference.com, for example.

C++20 Is Feature Complete; Here’s What Changes Are Coming

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