C++ 20: The Core Language

I&#x27;m glad this hasn&#x27;t turned into (so far) the usual &quot;c++ is dumb&quot; flame fest.<p>I&#x27;ve really enjoyed programming in c++17 for the last three years (I had the luxury of starting from an empty buffer) and find the language pretty expressive. If you are able to use it as a &quot;new&quot; language, ignoring its C roots and features that exist pretty much just for back compatibility, it&#x27;s really quite expressive, powerful, and orthogonal.<p>I&#x27;m actually pretty glad the c++ committee has been willing to acknowledge and deprecate mistakes (e.g. auto_ptr), to squeeze out special cases (e.g. comma in subscripting) and to attempt to maintain generality. Conservatism on things like the graphics standard is reducing the chance of the auto_ptr or std::map mistakes.

In commercial settings, I encounter several barriers to using C++ versions newer than 2011:<p>(1) Most C++ code <i>I</i> encounter is C++11, and I deal with lots of projects. It&#x27;s rarely sensible to change the language version of a large code base without a very good reason.<p>(2) Many developers are well-familiar with C++11.<p>(3) There&#x27;s no <i>widespread</i> perception that C++14 or later bring compelling language improvements.<p>(4) Some (most?) C++11 developers are already uneasy with the complexity of C++11, and aren&#x27;t eager to incur the learning curve and (perhaps) new set of pitfalls associated with newer C++ versions.<p>(5) Some C++11 developers look at the language trajectory, especially in terms of complexity, and have decided that their next language(s) will be e.g. Rust or Julia instead of C++14, C++17, etc.<p>I suspect these factors all contribute to the momentum that C++11 seems to have.

Right now my biggest things I like from C++20 are:<p>- Concepts (Now I can use static polymorphism without all those SFINAE template hacks)<p>- Designated Initializers (Finally!!! Using POD structs became way more convenient now)<p>- Coroutines (Would be pretty nice for writing games &#x2F; interactive applications)<p>The things I don&#x27;t have any interest in (but don&#x27;t care if it&#x27;s in or not)<p>- Ranges (Too much complexity for something you could do with plain old for&#x2F;if loops...)<p>The things I&#x27;m worried about:<p>- Modules (Theoretically really good, but in practice the whole thing is starting to become a mess, especially when it&#x27;s interacting with the preprocessor and external libraries, and when trying to preserve backward compatibility.)

Features I wish C++20 had:<p>* An opinionated, modern packaging and dependency story (like go modules, Rust crates)<p>* built-in library support for logging, http, zip, gzip, json, yaml, template rendering, RFC3339 datetime reading&#x2F;writing<p>* the dream: compliant compilers must be able to compile down to static binaries, cross compiling built-in.<p>Features C++20 actually has:<p>* new fancy spaceship operator that I&#x27;ll now have to learn and never use...

On thing I appreciate is mdspan -- multidimensional projections onto a regular linear vector. I&#x27;ve always had to spin these myself which means they don&#x27;t fully participate as ordinary containers (who&#x27;s going to go tho all that work for their own application code).<p>I&#x27;m hoping I can map them into GPU space -- looks like the standard has adequate control for this.

I&#x27;ve been anxiously awaiting modules and concepts. Both are incredibly important to reduce developer burden and improve the development experience.<p>Modules: While it is important to be able to fine-tune linking and compilation in some settings, in most, and especially for beginners, this should be handled by the compiler. Especially when compared to other modern languages, C++ is much more complex to understand the toolchain and what&#x27;s going on under the covers with the dichotomy of compilation and linking. Header files were a hack that have been around for too long, and there needs to be less separation between the interface and actual binary code when compiling. This is a headache both for novices and the experienced.<p>Concepts: The missing link for templates. Besides removing some of the roundabout-ism of SFINAE by providing compile-time verifiable interfaces, I think the biggest benefit of concepts will be the error messages. Right now, the state of errors in heavily templated code is abysmal, and this translates to a lot of wasted time for developers. Concepts should allow the errors to indicate exactly what&#x27;s wrong.<p>I can&#x27;t wait to be able to use these in public code. Some compilers support concepts with a flag already (and GCC 10 has it without the special flag), though none support modules yet...

Could someone describes the keyword &quot;unlikely&quot; and &quot;likely&quot; in a bit more details? It seem to be a very niche thing to add to the language. Is it expected that those hints will have an impact important enough regarding what the optimizer can do?

Been away from C++ a long time. If picking it up again, is it better to just start with C++17 or 20 or stick to C++11 which seems to be the defacto production version?

I haven’t done C++ a while but I really like where this is going. It’s a complex language for sure but I always liked about it that whatever needed to be done the language would not get in the way. There is something to be said for working in the language your OS and a lot of other libraries are written with. You know that you have high probability of achieving what you need to do. It may need a lot of work though.

With consteval (and its cousins), std::source_location and modules you could pretty much compile C++ without the preprocessor. Hooray!

Hopefully between consteval and string literal template parameters, we&#x27;ll be able to detect literal const char* in templates.<p>Would be nice for my threaded logger to know if it can copy the pointer instead of the whole string, knowing it was initialized statically.

Is there a good &quot;tutorial for modern c++&quot; resource out there? The last c++ I used heavily was c03 and a sprinkle of c11. I find reading modern c++ to be somewhere between frustrating and an exercise in hieroglyphics thanks to how much it&#x27;s changed. Something along the lines of &quot;basics of why to use the different std pointers&#x2F;views instead of raw pointers&quot;, lambdas, all the new const-like bits, and whatever else have become &quot;core&quot; features.

I&#x27;m not sure I grok the `&lt;=&gt;` operator and the example they show seems to just show that `a == b`. Can someone explain what&#x27;s going on with that?

This book is worth checking out if you don&#x27;t know c++17:<p><a href="https:&#x2F;&#x2F;www.bfilipek.com&#x2F;2018&#x2F;08&#x2F;cpp17indetail.html?m=1" rel="nofollow">https:&#x2F;&#x2F;www.bfilipek.com&#x2F;2018&#x2F;08&#x2F;cpp17indetail.html?m=1</a><p>That guy has a great blog where he writes at length about each feature, and the book is a compilation of that.

Some parts of C++20 are really good, like most of what is stated in the article.<p>But C++ would not be C++ if it had no shenanigans coming. Unless they have changed things within some months the new calendar&#x2F;datetime library is hilarious.<p>It has features like being able to write a date like: 1&#x2F;2&#x2F;2000y. ooh fancy operatoe overloading. Which one is month? The first one ofcourse as it they didn’t standardize on any of the existing ISO standards, or allow you to choose, but selected ”Customary US way” as the Only True C++ Way(tm).<p>Also do not forget the abuse of operator overloading like in the good 90’s when misusing it was the recommended thing.

Are there any quality tutorials for learning C++17 for basic tasks? Much &quot;how do I do X in C++&quot; is very outdated from that perspective.

Did modules make it?

Why &#x27;basic_fixed_string&#x27;? Is it really <i>so basic</i> that it has to be mentioned in the name? Is it just in case there might be a &#x27;fixed_string&#x27; that isn&#x27;t quite as basic, in the future?

C++ should adopt Rust-like approach on deprecating things faster and a tools like `cargo fix`&#x2F;`cargo clippy --autofix` to automatically fix&#x2F;suggest more modern alternative to the deprecated language constructions.

What are people using C++ for in 2019? Last time I used it was for a Qt desktop application of all things. I would imagine its main usage is in high performance or hardware constrained environments these days, but I&#x27;m curious what people are doing with it.<p>I do a lot of node.js programming, which uses C++ to write extensions, so here&#x27;s hoping I rememeber it if&#x2F;when I need to.