Show HN: C++11 inline object definitions

by matt42 • 12 years ago

70 34 12 years ago

Deregibus• 12 years ago

Very cool. Anonymous types can be really handy in C#, especially when dealing with LINQ to objects code. This looks like it could be a similar fit.

TillE• 12 years ago

That's surprisingly concise and not too hackish, especially compared to some other techniques I've seen for adding basic reflection to your C++ classes.

matt428• 12 years ago

This is thanks to the C++11 variadic templates that symplifies (a lot) the writting of such abstractions. The object model is only 100 lines of C++11: https://github.com/matt-42/iod/blob/master/iod.hh

zedpm4• 12 years ago

This is interesting, but I can't help but wonder at the wisdom of writing C++ in this fashion. What's the use case for this? I perhaps naively think that the problem domains for C++ and Javascript are very different, and that this sort of thing may make sense in the domains where C++ doesn't make sense.

I think I'd be frustrated if I was working on someone's C++ code and I found objects springing up out of nowhere like that.

wwwwwwwwww• 12 years ago

What's the use case for this?

job security and returning more than 1 object from a function without return pointers in the args

klmr• 12 years ago

returning more than 1 object from a function without return pointers in the args

Actually that’s solved by returning `std::tuple` and `std::tie` – or using custom classes.

matt428• 12 years ago

Yes, the goal is not to replace C++ classes. But I believe that there is cases when passing an object without having to declare a class can save time without impacting the readability of the code.

The other big advantage of this construct it that it provides introspection on objects at compile time, thus allowing to generate super fast serializers and deserializers without having to annotate the class.

zedpm4• 12 years ago

I probably should have started out my initial comment by saying thanks for building something and sharing it with the world. I didn't intend for my comments to sound overly critical or like an attack, though at least one individual seems to have gone off the deep end because of them.

Thanks also for taking a bit more time to explain how you see it being used. Between your comments and some of the other constructive replies, it makes more sense to me now.

hoilogoi• 12 years ago

For me, testing comes to mind.

chillingeffect3• 12 years ago

That is a laughably narrow worldview. It's even sadder that instead of reasoning out why some domains don't make sense, you direct a burden on the community to persuade you. After matt-42 took the time to create this, present it and share it, you're scoffing at him and challenging everyone else to open up your closed mind. I'm content to leave you confused in the corner, but you've sprayed your little pee on the beautiful garden where I walk each day so I'll give you a tip merely so it smells better to me and the rest my colleagues.

My tip isn't about iod. It's this: Try re-approaching your evaluation of iod with the worldview that IS useful and sketch out some scenarios. If you can show that it doesn't make sense, you'll engage someone intelligent in a conversation. If you won't invest that time in yourself, don't expect anyone else to invest their time in you.

I think I'd be frustrated if I was working on someone's C++ code and I found objects springing up out of nowhere like that.

That sounds like a personal problem having to do with self-control. You should get a blog.

Speaking of which, the website listed in your profile, c10n.net, is down.

zedpm4• 12 years ago

I find your vitriol unwelcome and inappropriate for a discussion that could be and ought to be constructive. I could spend some time explaining my position and defending my previous comment to you, but you haven't earned that.

I will note, however, that questions like the one I posed are in general useful to open up discussion about something new like this. Instead of viewing it as an attack, see it as initiating some exposition on specific cases where this would be useful.

Pacabel• 12 years ago

There's absolutely nothing wrong with questioning something that somebody has worked on. In fact, such questioning should be encouraged.

Good ideas and creators will be able to withstand such scrutiny without any problems at all. Bad ones can use it as an opportunity to improve.

_pmf_• 12 years ago

I perhaps naively think that the problem domains for C++ and Javascript are very different

That's just, like, your opinion, man.

dbpokorny• 12 years ago

I'm a Python / Scheme / JavaScript programmer, and I've touched C++ once, to implement a generic list, and I spent a grand old afternoon chasing template errors. I think I actually got the thing to work, but it was more or less a Monkey-vs-Keyboard type situation. It was at that point that I decided that C++ was too much for my brain.

Frankly I find this hilarious. It looks ideal for hard-coded config information and test data, but the reality is that this wasn't all that complex to begin with...

So, one of the fun applications that I can think of for this is essentially a simple, short Scheme VM along the lines of chapter 4 of Abelson and Sussman.

But yeah, I would take a long, hard look at the situation before using this for real work. But then again, I wouldn't touch C++ for real work in the first place.

matt428• 12 years ago

A nice usecase is named parameters, usefull when a function is taking lots of parameters, or when the order of parameters of the same type can be switched by error :

  fun(iod(arg1_ = 1, arg2_ = 4)

yyyymmdd2• 12 years ago

actually: auto obj = [ x=1, y="foo" ]{};

reubenmorais• 12 years ago

This just declares a lambda with two parameters with default values. You can't do obj.x, you can't declare "objects" inside of obj, you can't return obj in the parent scope.

Why would you ever do:

  [x=1, y="foo"]{
    // use x, y
  }();

When you can just do:

  auto x = 1;
  auto y = "foo";
  // use x, y

?

matt428• 12 years ago

I have no idea if it is in the standard, but this actually compile with gcc 4.9 -std=c++1y

auto obj = [ x=1, y="foo" ]{}; obj.x; obj.y;

matt428• 12 years ago

It is in the C++14 standard:

For every init-capture a non-static data member named by the identifier of the init-capture is declared in the closure type. This member is not a bit-field and not mutable. http://isocpp.org/files/papers/N3648.html

matt428• 12 years ago

Amazing :) I did not know that you could access lambda capture expressions. This can indeed reproduce parts of iod's features. However, this syntax does not provides static introspection, and does not support nesting properly :

auto obj = [ name = 1, a = [name = 1]{} ]{}; obj.a.name;

test.cc: In function ‘int main()’: test.cc:5:38: warning: already captured ‘name’ in lambda expression auto obj = [ name = 1, a = [name = 1]{} ]{}; ^ test.cc:7:9: error: ‘struct main()::<lambda()>’ has no member named ‘name’ obj.a.name;

yyyymmdd2• 12 years ago

that's not syntax not supporting nesting, that's gcc having bugs with nested captures. clang++ does not error on this. (strange, but this does not compile with gcc and clang versions I have currently at hand. It did compile a few weeks ago when I tried it...)

greggman• 12 years ago

This is awesome! I was just thinking of porting some JavaScript based stuff to C++ and not looking forward to dealing with all my adhoc JSON stuff and having to write classes for everyone of them.

JoachimSchipper• 12 years ago

Note that something like this is actually part of C99 (but not of C++ - the C++ grammar was apparently too tortured to support this):

    struct example_t {
        int foo, bar, baz;
    } example = {
        .foo = 1,
        .bar = 42,
    };
    /* unnamed members are implicitly set to 0 / NULL */
    assert(example.baz == 0);

combined with compound literals, you can do stuff like

   example = (struct example_t){
      .foo = 2,
      .baz = 1
   };
   assert(example_cmp(&example,
      &(struct example_t){ .foo = 2 }) == 0);

It's one of C99's neater features, I think. I can imagine C++ programmers liking something like iod.

Show HN: C++11 inline object definitions

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