C++ : if constexpr isn't broken

https://brevzin.github.io/c++/2019/01/15/if-constexpr-isnt-broken/

by Bl4ckb0ne • 8 years ago

74 80 8 years ago

brevzin.github.io

roywiggins• 8 years ago

"a somewhat more awkward and uglier, yet slightly more correct way... all the functionality is right there. We’re not missing anything" might as well be C++'s motto :)

aassddffasdf• 8 years ago

"The amount of things you have to know in C++ is arguably a bit larger."

Arguably, lol.

nivexous• 8 years ago

In Bitcoin SV, there is a saying "devs gotta dev" to refer to the mass of crypto developers that add unnecessary complexity to their coins to solve minor or even nonexistant problems while not seeing the bigger (usually economic) picture. Everyone wants to make their mark on the space and there aren't enough leaders to direct the energy. C++ looks to be doing the same and not able to see the impact this will have on adoption. Oh well. I personally moved from C++ to Rust last year and haven't looked back.

carlmr2• 8 years ago

It's already there, but it's really ugly and hard to read, would be my conclusion from this.

paulddraper• 8 years ago

"And if we are missing it, let's add it".

gHosts• 8 years ago

Yup.

And while the rest of the world knew how to find a file size... how long was it before C++ cleaned that very very simple corner up?

And if you look under the hood at the implementation of std::conditional.... I feel ill.

The thing I like about Alexandrescu is he invented this meta programming stuff... super super super clever magic...

...and then backed away saying it shouldn't have to be so hard, we shouldn't need magic to program.

wyldfire• 8 years ago

Slides from Alexandrescu's referenced talk [1].

[1] http://erdani.com/MeetingC++2018Keynote.pdf

greesil• 8 years ago

I think I just got used to C++11. What is this dark magic?

felixguendling• 8 years ago

If you refer to `if constexpr`: There is no dark magic. Most of it is already possible with C++11 meta programming (i.e. with std::enable_if [0]). However, it's much more readable now (and it's more fun to write :D). I think we're on a good way that compile time programming will be more easily accessible.

[0] https://en.cppreference.com/w/cpp/types/enable_if

gpderetta• 8 years ago

Most of this stuff was possible in C++03. Newer standards only make it more convenient and (arguably) saner.

mattnewport10• 8 years ago

I had similar thoughts when looking through the slides for Andrei's talk. A main argument in favor of static if over the mechanisms C++ offers to do the same things seems to be familiarity of if statements but having scope not be respected is such an unintuitive difference for anyone with experience in curly braces languages that it seems like a net loss in clarity to me. It looks more like using macros for conditional compilation.

Overuse of if statements is generally something of a code smell and tends to make code harder to follow. To my taste the C++ way of doing things is generally preferable as a result even if more verbose. Given this is probably somewhat a matter of taste it is hard to justify changing C++ to match the D way IMO.

mark-r• 8 years ago

Why is it every time I see a new article about C++ features my eyes glaze over? Am I just getting old, or has C++ jumped the shark?

moron4hire• 8 years ago

I had a similar feeling. As a long-time C# programmer (how old is C#? That many years), I look at this and get really wide-eyed, "how is that at all a good idea?!"

A lot of these features seem like ways to hack around the fact that C++ templates are not generics, they are literally templates for writing copies of classes. It seems like features like this are going to make code size explode.

A lot of the examples also seem very smelly from an OOP perspective. We should one class be able to have different fields depending on template parameters? That seems like something one should do in a subclass.

mattnewport10• 8 years ago

I'm very familiar with C++ and C# and overall I find I like C++ a lot better. There are a few nice convenience features of C# but knowing both languages I tend to find whenever they differ I feel like C++ made the better design choice.

And C# going all in on OOP is a part of that. It's reasonably clear now that OOP was mostly a big mistake and a dead end. C++ is less infected by going all in on it as it has always been a multi paradigm language.

adrianN2• 8 years ago

I'm not sure what you mean by generics if you think that C++ templates are not a kind of generics. Do you prefer that everything happens at runtime?

Similarly, subclasses as opposed to templates carry a runtime cost. Virtual function calls are not free. Why pay that cost if you can deduce the right code from the types at compiletime? Besides, and this is probably a matter of opinion, I find code hard to follow that uses inheritance heavily.

dasloop• 8 years ago

Probably because you are used to the paralysed C++ pre C++11. Nothing happens for years and now we have new functionalities every three years supported by all major compilers. But, also, many new functionalities are not for global consumption. The audience of many meta programming functionalities are library developers for example.

jupp0r2• 8 years ago

Isn't every developer in a non-trivial software project a library developer? As soon as you have a common piece of functionality you want to reuse - that's a library.

p0nce5• 8 years ago

And that's exactly what's different in D, where the audience for meta-programming is "everyone". It's not just about more power, but how accessible this power is. To think you need to suffer to have this power is not true.

munchbunny• 8 years ago

It's because C++ is coming up with more complex ways to solve emergent problems from C++'s own complexity. So to appreciate why a proposal exists in the first place, first you have to catch up on the context that current C++ practitioners already have.

cjhanks2• 8 years ago

I agree with this blog writer. Allowing typedefs to cross static if boundaries would be a major change to the way I read code. Unless there's a more substantive example demonstrating why using the existing metaprogramming is insufficient, its benefits are imo outweighed by its cost.

renox• 8 years ago

Well you already have to remember that 'if' introduces a scope but '#if' doesn't..

cjhanks2• 8 years ago

I guess that's true. But I can't remember the last time I have used `#if` aside from header guards and optimization paths. So they do exist, but I almost never do this.

choeger3• 8 years ago

Oh my. If only C++ would have used proper macros from the start...

adev_2• 8 years ago

There is nothing as "a proper macro". There is proper meta-programming support or a badly design language.

choeger3• 8 years ago

A macro is a metal-level abstraction. So proper macros require proper metaprogramming support. And, I would argue, the other way around.

gumby5• 8 years ago

I discussed this once with Stroustrup. He mourned that the word "macro" had been "polluted" (his word, though I completely agreed) by the preprocessor and said "templates was the best we could do within the constraints of C".

This was in the mid 90s and things are a lot better these days but still, I had better macro support writing Lisp in the 1970s.

choeger3• 8 years ago

I am ready to believe that hygienic macros would have been difficult, but just using lisp macros should have been straightforward. Tbh, I think that templates were an adhoc, amateurish design decision.

JabavuAdams• 8 years ago

I used to be a C++ guy from the early 90s 'till about 2010. Every few months, I think "I should really learn the new C++ hotness", and then I read something like this. It's like a bunch of priests arguing about how best to fit a bunch of ugly angels on a pointless pin. It just feels so pointless.

If you must use C++, just use another language to generate the C++. All this template mumbo jumbo. Who says your meta-programming language has to be the same as your target language?

I used to spend some time looking for the perfect vector (as in linear-algebra) library. Templatized for vector dimension, data-type, blah blah blah. Dude, just use Python to generate it. Duplication doesn't violate DRY if the duplication is generated from a single higher-level source.

Doing meta-programming in C++ is a really limited way of thinking.

nealabq• 8 years ago

fit a bunch of ugly angels on a pointless pin. It just feels so pointless.

A pointless pin would indeed be pointless.

mattnewport10• 8 years ago

I don't think anyone on the C++ committee is proposing banning you from generating C++ code with Python. I also doubt anyone who's interested in C++ metaprogramming has failed to consider the possibility of doing codegen via other means. In light of these facts what seems rather pointless is your comment.

flukus• 8 years ago

Who says your meta-programming language has to be the same as your target language?

I'd lay blame at IDE's and they way the take over the build system along with intellisense. They don't play well with anything generated before the compile step. Even adding a build step is and alien concept to many, so we end up with the build step shoe horned into languages.

C++ : if constexpr isn't broken

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