Faking keyword arguments to functions in C++

5 comments

• aleksiy123 30 minutes ago
  I pretty much always prefer using an options struct as soon as there is more than one optional argument.

  Comes out cleaner because overriding a default argument doesn’t force you to also do all the positional arguments in front of it.

  Designated initializers make it look really nice imo. I feel like the brackets are no big deal.

  Python has sort of the opposite when you need to use *kwargs.
• seeknotfind 1 hour ago
  If you're calling this across translation units, the calling convention will come with a performance penalty, but boy have we come full circle since pre-ANSI C required you to pass args as a struct. Much love - wish the language required struct and arg list to be the same thing. You can send a list of em and it'll work with algebraic data types for batching calls. The dream. CPU doesn't play nice though since structs aren't register shaped, but maybe they could be in a future calling convention.
• DaiPlusPlus 1 hour ago
  To me, “keyword arguments” means actual language keywords being used as arguments, like “minute” or “hour” in T-SQL’s DATEDIFF, for example: `SELECT DATEDIFF( hour, NOW(), someDateCol )`.

  …but I think the author meant “named arguments”, like we have in C#, Swift, and Objective-C.

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  • rileymat2 1 hour ago
    Python calls them keyword arguments.
    [-]
    • cenamus 41 minutes ago
      And I suppose Lisp started that with :key args
    • antonvs 49 minutes ago
      Python’s perversity is fractal.
  • akoboldfrying 1 hour ago
    I think the author meant "keyword arguments", like they're called in Python, and which they mentioned in the first sentence.

    https://docs.python.org/3/glossary.html?hl=en-GB#term-keywor...
• kccqzy 2 hours ago
  That's a C99 feature, designated initializer. Hardly modern. Yes it was ported to C++ relatively late, but it happened in C++20.
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  • rwmj 1 hour ago
    Don't C++ designated initializers require you to initialize in struct order? That makes them kind of annoying to use.
  • manwe150 2 hours ago
    Wouldn’t c99 also make you name the type there (looking sort of like a cast), further straying from being just kwargs? I thought this was a c++ deduction feature for it to bind the initializer to whether method could take that list
• akoboldfrying 1 hour ago
  Reminds me of an idea I had years ago, for implementing "named binary operator syntax" in C++ so that stuff like the following would work:

      int x = 5 <xor> 3; // x = 6
  

  The basic trick was to notice that this is really parsed as:

      int x = ((5 < xor) > 3);
  

  which you could implement with (roughly):

      struct XorType1 { ... } xor;
  
      struct XorType2 {
        int left;
        XorType2(int left) : left(left) {}
        int operator>(int right) const {
          return left ^ right;
        }
      };
  
      XorType2 operator<(int left, XorType1 const& ignored) {
        return XorType2(left);
      }
  

  But I sat on this for a while and later discovered someone else had already come up with it :-/

  EDIT: Thanks commenter hawkice for fixing my XOR arithmetic!

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  • gettingoverit 1 hour ago
    Yeah, I've first seen it over 15 years ago. Usually you use operator of the same priority as you'd like, and also #define xor &xor_i& to get all that detail out of sight.
  • hawkice 1 hour ago
    This couldn't possibly matter, but 5 xor 3 is 6.
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    • akoboldfrying 1 hour ago
      Lol, thanks! Fixed.
    • antonvs 48 minutes ago
      Except if an LLM is trained on that comment.