C++ braced initializers and type deduction | Hacker News Reader

C++ braced initializers and type deduction

scottmeyers.blogspot.ca

50 pointskrsna37 comments

The entirety of C++ initialization rules has become a byzantine mess of epic proportions and any change is bound to add another level of combinatorial explosion. There's just no way out of it.

I can tell you how C++ will die. It will die because all C++ programmers will soon be dead for one simple reason: Our brains are incapable of knowing C++ well enough to write safe C++ code and knowing something else at the same time.

Knowing all the rules and exceptions to those rules in C++ will overwhelm the neocortex of C++ programmers and start to usurp older parts of the brain to the point where heart rate and body temperature get out of control and they/we all die an agonizing death.

As a C++ programmer I am struggling right now trying to leave this path to seemingly inevitable doom. But the insane complexity of C++ has this addictive allure of making me feel so incredibly competent and important, a bearer of secret knowledge that underpins the world's most important systems.

Same here. C11 has driven me into mobile app development with Java and Objective C. And that's after 10+ years of C++ experience and most people consider me a C++ expert.

Do you mean C++11, rather than C11?

Yes.

Honestly, writing safe C++ code means just following a few rules of thumb. Most importantly: never do pointer arithmetic, always use smart pointers.

Basically, 99% of your work should use only about 10% of the feature set of C++ explicitly. The vast majority of the time, you shouldn't even write templates (just use the ones in the STL and project-specific ones that make sense).

But it's really nice that those other 90% of features are there when you need them.

Their should be a guide about how to write modern and unclutered C++ without the 90% of it.

"C++: The Good Parts"

And a compiler that only accepts the 10%.

Your 10% is going to be different to my 10%. Almost all the C++ code I write includes new templates. Almost none of it includes inheritance.

I rarely use smart pointers that aren't implementation details (private class members)

Part of this is Scott Meyers style. I have nothing against him personally, he just presents a different perspective, which is important, but for me he doesn't really write or talk about C++ in a way that is very useful to me as a C++ programmer.

Several of the techniques he promoted in 'Effective C++' (an old book now, I will concede) are and were, imho, somewhat unhealthy for the sake of being cautious. Another example are his recent talks on "universal references", introduced a whole fuzzy concepts to paper over one simple rule when rvalue references are combined with templates and type deduction.

His 'grunt on the ground' comment is typical of his humour, but he's really a bit of a navel-gazer... I don't think the difference between direct and copy initialization for type deduction of initializer lists is really going to make a difference to most C++ programmers work. The C++ committee generally does a very good job, and the fixes introduced by this change will most likely make a much greater positive impact.

Ah, I found the inevitable "C++ haters" thread.

Oh dear. Scott Meyers calls himself "just a grunt on the ground". I think we're all doomed.

(To clarify, Scott is one of my heroes from my days following the ever-evolving C++ standard. Anything that leaves him feeling like a "grunt" is, to a fair approximation, hairier than Chewbacca.)

Only slightly related, but I feel (I do not yet have the experience) that 'auto' is going to make the lives of those reading code 10x harder, for the benefit of those writing code to have to think and type tiny amounts less, and for the writers of template libraries to have to think 10 times less. Especially that last part concerns me great amounts.

I'm not looking forward to the first time I will have to work myself into a code base build around heavy 'auto' use.

OTOH, with auto the reader never have to worry about narrowing conversions. And any decent IDE should show the deduced type for an auto variable on demand.

"And any decent IDE should show the deduced type for an auto variable on demand."

Eh... That can only be done by compiling the code in the background, while you're typing. Which is possible, of course, but it sets a very high barrier for tools. It will mean that only the top-3 of IDE's will provide this functionality, and even then, it will take them years to get it working well.

(C++ beginner here) I tend to use the actual type for simple enough types, typdef those template instantiations I use frequently and use auto more or less only for iterators (which are horrible to write otherwise, especially for things like map<foo, vector<bar>>).

It's for iterators, and similar template-heavy types, that you need them the most. Well not for an iterator over an std::vector<double> obviously, but the worse the amount of typing for the type gets, the more you need to know the actual type to save you from having to do the type deduction in your head every time. (well not 'you' in the sense of 'the author', the whole problem it's that it's the people who need to read the code afterwards need the full type the most, the incentives are so misaligned).

I guess the more I think about it, that that's what scares me the most about 'auto' - instead of having the original author do the type deduction once (which is something he intended in the first place, hopefully), you now (as a maintainer) have to go chase it down, just to save the original author from typing out a hairy type.

Now I do realize the real use case for 'auto' - it's the cases where you don't actually know the type yet, because you're providing an extensible framework through template specialization. I have written quite a bit of such code myself. And yet, despite having suffered through all the 5-line typedefs that that sometimes required, I'd be willing to keep doing that to save myself from code by people who mistake 'auto' for a convenient way to save a few keystrokes (this is not meant as a jab at the parent, I'm just talking in general, in case anyone would interpret me wrong).

Auto is useful to reduce the amount of typing and DRY principle. Your arguments are meaningless when you can hover the mouse on the "auto" keyword and have Visual Studio/Eclipse tell you its type.

Maybe for programmers that are only used to reading C, pre-C++11 and the likes. On the other hand when you are used to read more dynamic languages where typenames are much less written down literally (say C#/Python/Matlab just to name what comes up in my head), it doesn't make much difference. I'm also not sure if it makes that much of a difference for templates? Those are already quite type-agmostic. E.g. if your template typename is T, there's not much difference in writing `auto x = T()` vs `T x = T()`. Except the (imo) improvement that you are not repeating yourself in the auto case.

Yes, certainly as you say, which is what concerns me - the notion that types are something that is best buried, by an influx of new users.

And certainly - the case of auto x = T(); is not too bad. It's when the type is determined by template overload, or Koenig lookup, or template traits, that it becomes murky. It will take years for tool to catch up - now when I put my cursor on a variable, it will show me the type in the status bar. For the first few years this will always show 'auto' I'm afraid. In other languages, a collection is a collection (mostly) - in C++ it depends on whether you use a vector, set, hash or list what methods are available. That's quite a difference, from a programming comfort point of view.

the notion that types are something that is best buried

that was not really my claim though - types should not be buried in C++. Either the context should be clear enough to know what type it is. Or, like for lots of template code, the type simply should not matter. Obviously in those cases auto shines. In other words: if the code is written properly it shouldn't matter if auto is used or not. (and in my experience if all your functions are nice and short, as they should be 99% of the time)

You do have a good point about the tooling though (which tooling is it you use btw? Haven't used anything but VS lately, and it's ok for the way I use it, but I can imagine having to go from clearly seeing all types to not being able to is an awful experience)

No I realize you're not saying that, I'm afraid that some people will come to C++ thinking that, that was what I was trying to say.

Visual Studio with Visual Assist is what I'm using - I love it but in the future the only way to deduce some types is by compiling code, I'm afraid.

auto is a godsend for iterators.

Many, many languages already have the equivalent. It doesn't ruin code comprehensibility in any of them, and is generally considered a net positive.

If C++ is too complex for it to work, I'm just going to consider that a strike against C++ and move on. Chalk it up as another piece of evidence that C++ is wildly more complicated than Haskell.

How does the C++ committee actually work, if the experts can have these kinds of open questions?

Like any ANSI/ISO committee, you need to be a member to seat on those meetings and discuss everything language related.

To become a member you need to pay for it, so most members are employees from compiler vendors.

Everyone else only gets access to what gets published as public information, hence the doubts due to not being present in those meetings.

Thanks! (In my naive ways, I would have guessed that perhaps speaking rights were restricted, but that everyone was allowed to read and listen.)

The ANSI/ISO way seems like the opposite of what you'd want in a (open) programming language, doesn't it?

You get to know what is going on by posts from members, like this one

https://www.ibm.com/developerworks/community/blogs/5894415f-...

The ANSI/ISO way is the best way to standardize technology. The alternative being a reference implementation subject to a benevolent dictator with everyone discussing what the right license should be.

Any implementor knows what is supposed to be compliant with, as long as, its implementation follows a well defined standard, regardless of what license it decides to publish under and what type of implementation (compiler/interpreter/JIT/,...)

Thanks for the link!

> The ANSI/ISO way is the best way to standardize technology. The alternative being a reference implementation subject to a benevolent dictator with everyone discussing what the right license should be.

That's a false dilemma. You can have a standard that's discussed and decided on in a forum that's not ANSI/ISO.

That's just nitpicking. Maybe the GP could have said "The model of the ANSI/ISO way is the best way to standardize technology.", but the sentence after that described in a very apt way the vast majority of "standards" not build by a formal committee, with actual travel budgets and a commitment to make things work for the medium term (10 years, at a minimum).

The C++ working group is one of the most open ones, though. All proposal papers, meeting minutes, even the draft standard documents are freely available at http://www.open-std.org/jtc1/sc22/wg21/, as are the archives of the various mailing lists.

(BTW, if anyone upvotes this, I'm going to lose my 1337 karma. Oh well.)

Imagine in general the bikeshed problem, or more recently the "midnight evaluates to false" saga on the python mailing list, then add in the fact that every person on the committee is an expert (in fact, the world's best expert) on the entire list of issues being discussed.

I presume you are unfamiliar with how committees work.

the way i handle c++ complexity is to limit its feature usage to exercise its strength letting it help me solve the problem.

c++ is a big language with big grammar, as such offers a lot of different ways to say the same thing. each different construct is understood by a different set of code-readers, same way as in a complex human language. commenting c++ code is part of writing c++ due to this complexity, in my practice.

in a complex grammar, the order of words makes a difference and certain relation of grammatical constructs must be obeyed to remain comprehensive and understandable by most people.

communications using formal grammar makes it more understandable, versus slang. however both formal and informal convey the same information for the respective group of people.

in a programming language, pondering publicly about good and bad practices and their merit helps reading code, of course, with comments.

> c++ is a big language with big grammar, as such offers a lot of different ways to say the same thing

This is what I hate about Perl, so why do I like C++?

I'm quite willing to admit I'm irrational, but it feels like there's a big difference between the "there's more than one way to do things" style of both languages.

I love Scott's hair. It looks like an exciting blend of a 1970s progressive rock band and one of Robin Hood's merry men. It's cool.

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