Faking keyword arguments to functions in C++(nibblestew.blogspot.com)
29 pointsby ibobev3 days ago8 comments
  • aleksiy12312 hours 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.

    • delta_p_delta_x9 hours ago
      > I pretty much always prefer using an options struct

      This is essentially what Vulkan does; there's a CreateInfo struct for every object creation or command function. And even then they managed to sort of mess it up, because they also have functions and objects suffixed with a '2', and the pNext extension mechanism.

  • seeknotfind13 hours 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.
  • DaiPlusPlus13 hours 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.

    • rileymat212 hours ago
      Python calls them keyword arguments.
      • tonyarkles11 hours ago
        They do, but they're also not strictly required to be named explicitly:

        They can be:

            >>> def foo(bar=None):
    ...   print(bar)
    ...
    >>> foo()
    None
    >>> foo(bar="baz")
    baz
    >>> foo(bar="baz", baz="azp")
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: foo() got an unexpected keyword argument 'baz'
        But you can also use them generically:

            >>> def bar(**kwargs):
    ...   print(kwargs)
    ...
    >>> bar()
    {}
    >>> bar(site="HN", user="tonyarkles")
    {'site': 'HN', 'user': 'tonyarkles'}
      • cenamus12 hours ago
        And I suppose Lisp started that with :key args
      • antonvs12 hours ago
        Python’s perversity is fractal.
    • akoboldfrying12 hours 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...

  • asalahli10 hours ago
    There was a similar CppCon talk[0] in 2018. Highly recommended (by me, fwiw), as is any other talk by Richard Powell

    0. https://youtu.be/Grveezn0zhU

  • p0w3n3d11 hours ago
    I did this in my C project 7 years ago, as this is standard in C and gave a lot of readability, in fact more I guess... but a lot of preprocessor code too
  • kccqzy13 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.
    • rwmj13 hours ago
      Don't C++ designated initializers require you to initialize in struct order? That makes them kind of annoying to use.
      • ozgrakkurtan hour ago
        You can disable the lint for this if you don’t use constructors/destructors in your project or if you are sure you won’t do funky stuff in the struct initializer afaik
      • greenbit7 hours ago
        IMO it renders them kind of pointless, as you can then just leave out the field names (keywords, in pythonese) and get the same effect. It's very disappointing that they're naught but comments without the punctuation.
    • manwe15013 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
  • ranger_danger3 hours ago
    Wouldn't that also mean you now need to define a new struct for every method that you want to do this with?
  • akoboldfrying13 hours 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!