C++20 Improved the For-Loop Syntax

(lzon.ca)

25 points | by jpmitchell 3 days ago

13 comments

  • UncleOxidant 13 minutes ago
    Reading this, I really can't make much sense of it:

         for (int i=0; auto&& it: vec)
             cout << (++i) << ": " << it << endl;
    
    It's certainly not obvious what's going on there at a glance.

    This is at least a bit more pythonic:

         for (auto [i, it] : std::views::enumerate(vec)) {
            std::cout << i << ": " << it << "\n";
         }
  • galkk 37 minutes ago
    Only people who saw nothing bad in passing pairs .begin(), .end() in tons of places in c++ for like 30 years can say that thing like ‘auto&&’ improves anything.
  • kazinator 11 minutes ago

      1> (mapdo (op put-line `@1: @2`) '#"how now brown cow" 0)
      how: 0
      now: 1
      brown: 2
      cow: 3
      nil
    
    mapdo: map for side effects of calling the function, not calculating a result, like map.

    op: produce a lambda function out of an expression in which @1, @2, ... explicitly indicate the insertion of positional arguments, which are then implicitly bestowed onto the generated lambda.

    `...`: quasistring syntax: supports @ notations for interpolating. @1, @2 of op do not require a double @@ inside a quasistring.

    put-line: ordinary function to put a string to a stream (standard output by default) followed by newline.

    #"...": string list literal: contents are broken on whitespace and denote a list of strings #"foo bar" -> ("foo" "bar"). Requires ' quote in front to be quoted literally, and not evaluated as a compound expression applying the argument "bar" to the operator "foo". Yes, there is a #`...` quasi string list for templating over this.

    0: ordinary integer zero. But endowed with the power of being iterable. Where an iterable thing is required, 0 denotes the whole numbers 0, 1, 2, ...

    These are some of the ingredients in my one-member research programme into nicer Lisp coding.

  • WalterBright 23 minutes ago
    C++ should copy D's elegance:

        import std.stdio;
    
        string[9] vec = [ 
          "the", "quick", "brown", "fox", 
          "jumped", "over", "the", "lazy", "dog" 
        ];
    
        void main()
        {
          foreach (i, s; vec)
            writeln(i, ": ", s);
        }
    • WalterBright 11 minutes ago
      Although D is a strongly typed language, it is very good at type inference. The `s` is inferred as `string`, and `i` as `size_t`.
  • HarHarVeryFunny 3 days ago
    C++20 also has an enumerate() generator, so if you like the python syntax you can just do:

    for (auto [i,v] : std::views::enumerate(vec)) std::cout << i << ": " << v << std::endl;

    FWIW C++23 also has a python-like print and println:

    std::println("{}: {}", i, v);

    • arikrahman 12 minutes ago
      This is the way I've been doing it and with less hiccups.
    • Maxatar 9 minutes ago
      std::views::enumerate is a C++23 feature, not C++20.
    • BigTTYGothGF 22 minutes ago
      The enumerate is a better solution than the one in the blog post.
  • adityamwagh 47 minutes ago
    > It seems to me that the C++ Standards Committee is doing a decent job maintaining the language, and introducing useful features when it makes sense to do so.

    This can’t be further from truth. C++ is essentially Frankenstein’s monster.

    • gregdaniels421 18 minutes ago
      An articulate creature that hounds its creator for abandoning it?

      If anything it is more of a Chimera

    • saghm 15 minutes ago
      And the real monster was the one who created it all along!
  • WCSTombs 3 days ago
    The C++20 version is still clearly inferior to the Python and Lua examples because you still have to manually increment the counter in the loop body. IMO the sibling comment by HarHarVeryFunny has a much better C++ equivalent for this idiom, even if it's slightly more verbose.
    • Gualdrapo 1 hour ago
      > you still have to manually increment the counter in the loop body

      It doesn't look like that to me, the ++i thing seems to be just to start printing the array from 1 (I don't know how things are in Python nowadays but I know in Lua arrays start at 1, so there's no need for something like this in there), the value of i is still increasing without telling it explicitly to do so

      • sheept 1 hour ago
        Python is 0-indexed. In OP's example, the start parameter makes i start at 1.

        Their C++17 example prints starting from 0. Probably a mistake.

        If you look at the linked page for C++20[0], other types can be put in the initializer statement, so it's unlikely the loop auto-increments.

        [0]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p06...

        • kbenson 58 minutes ago
          So, it's just a while loop in a for loop's clothing? That's not going to be confusing for people at all...
          • comex 39 minutes ago
            No, it's a for loop that happens to include an unrelated variable declaration.
      • moron4hire 1 hour ago
        No, the increment is necessary to keep the counter going. The choice of pre-increment versus post-increment handles the "start at 1" issue you mentioned, but it isn't auto incrementing.
  • drnick1 9 minutes ago
    Give me the explicit C syntax any day over this monstrosity. I refuse to write anything other than C++98, the last version of C++ that built on C without trying to turn it into a completely different language.
  • HeavyStorm 22 minutes ago
    Given both Lua and python use a enumerator, the C++ example isn't really the correct equivalent.

      for (auto [i, v] : std::views::enumerate(vec)) {
          std::cout << i << ": " << v << '\n';
      }
    
    This is how you'd do it.
  • hackthemack 4 minutes ago
    I guess I am officially old and no longer know what the cool "it" is any more. The C++17 example seems much more clear to me. It is more typing, but so what? It is not that much more typing, and it looks like code that people have written for 40 years.
  • Gualdrapo 1 hour ago
    The way this website shows the programming languages is odd. They're blue and slanted and if you hover your mouse cursor over them they have a color transition, so you'd think they are links - and yet you click on them and nothing happens
  • dvt 35 minutes ago
    I am so happy I haven't written a line of C++ in like 15 years. Absolutely disgusting language. Every time I look up one of their new standards, I'm like how does anyone keep all of this in their head (usually on top of stuff like boost, etc.)? No wonder LLMs are a thing.
    • lioeters 11 minutes ago
      C++ is a decades-long mistake that spawned other misguided directions in the entire software industry. We're still trying to undo many of the bad ideas, but I'm afraid even Rust has too much of C++ in it, not only in terms of syntax but mentality and culture. LLMs are making things worse by automatically generating code in such bloated languages, where people have less need to even look at the horrific spaghetti inside. The whole thing needs to be reconsidered from the ground up, from first principles, by actual creative thinking human beings with lessons of hindsight.

      > Gall's Law: A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.

  • fourseventy 1 hour ago
    Just what C++ needs, more bloat lol.