Non-constant constant-expressions in C++

http://b.atch.se/posts/non-constant-constant-expressions/

by refp • 11 years ago

71 17 11 years ago

b.atch.se

halayli• 11 years ago

This post demonstrates the problem in C++ community. Trying to do smart wacky things that involves understanding so many C++ rules and aligning them with the moons to figure out what is going on and then just assume that others can easily maintain it.

It intimidates a lot of newcomers and leaves a bad taste. And then we wonder why so many people hate the language.

brandonbloom• 11 years ago

This post also demonstrates the importance of exposing the powerful fundamentals. People are going to do crazy things anyway, so you might as well offer those primitives rather than force people to contort the language to emulate them. This means intentionally designing tractable components rather than accidentally encouraging intractable abuse.

yoklov3• 11 years ago

Agreed. None of this would be an issue with good support for normal compile-time metaprogramming, in the same language that the rest of the code is in, without restrictions (or, with as few restrictions as is possible).

But thats obviously too much, at the very least, I'd want the language designers to understand the implications of the features you add when you add them. You'd think the C++ standards committee would have learned their lesson, after accidentally making templates turing complete.

Even if templates are a great feature (and this is far from a universal opinion), that just means they got lucky.

sz4kerto• 11 years ago

Interestingly, this is because -- after reasonable amount of C++, C#, Java coding I have just settled with Clojure (or Lisp in general). I really enjoyed writing clever C++ template stuff like class hierarchy generators and so on until I became old enough to think that good!=clever, good==maintainable&&it works.

Too much syntax and cleverness is bad for the industry as usually it attracts a lot of smart people who end up writing C++ (or Scala) -- but the problem is that they use a set of patterns+language subset that is unique to the team/company. Therefore knowing C++ does not mean you can just start at a company using C++, you have to learn their way.

Notable exceptions are Java and C#. Java is so simple and verbose that you can't write unreadable code unless you're really evil, it's a shame that the complexity and cleverness is moved to the frameworks (there aren't many "Java programmers", most of them are Spring, etc. specialists). C# works well because MS dictates the patterns (and those patters are usually quite good) while the language itself is very readable and intuitive, without crazy rules.

And there's Lisp/Clojure where there are only a handful simple rules, and cleverness is usually local, e.g. macros or tricky 'functional' data manipulation stuff; but for me that 'local' cleverness is the most understandable.

m0th87• 11 years ago

What is "local cleverness"? And how does lisp's metaprogramming capabilities avoid cleverness run amok?

evincarofautumn• 11 years ago

I find it very difficult to read Java code because it tends to be so long-winded and many-layered.

userbinator• 11 years ago

Java is easy to read at the "micro" level, i.e. the bodies of methods usually don't contain much code; but I think that is precisely what makes it hard to understand at the macro/system level, because that small amount of code depends on a much larger amount throughout the rest of the system, spread across multiple files in different deeply-nested directories. That's when you have to start following chains of method calls, object creations, and class hierarchies.

Latty• 11 years ago

Java is proof that worse is better. It's never completely unreadable, but it's never pleasantly readable either.

barrkel• 11 years ago

Articles like this are one of the reasons I don't profess to know C++ well. I know almost all of the features in isolation, and many in combination, but new combinations are being discovered with novel and surprising effects, like this article demonstrates.

This is not a good advertisement for C++. It would have been much healthier to not have had such a powerful and under-researched template instantiation mechanism, and instead have a genuine compile-time macro language. It's hard to see a way back though. Everything added - like constexpr functions - to make compile-time execution more sane, can be put back into the mix to complect things even further.

MaulingMonkey• 11 years ago

Articles like this are one of the reasons I don't profess to know C++ well.

...any why I profess nobody knows C++ well, even those on the standards committee ;)

yoklov3• 11 years ago

instead have a genuine compile-time macro language

Yep. Jesus christ why didn't they do this. Going further, D (or more recently, JAI) style CTFE would have been great.

But no such luck.

karmakaze• 11 years ago

The demonstrated result is unfortunate, but shouldn't really surprise anyone with a history of C++. 'Template' happens. Attempts to eliminate all such curiosities will only make the language larger and more complex, expanding the cycle.

Someone• 11 years ago

Easier (and bending the rules a bit):

  #include <iostream>
  
  constexpr int f(int line)
  {
      return line;
  }
  
  #define f() f(__LINE__)
  
  int main(int argc, const char * argv[]) {
      constexpr int x = f();
      constexpr int y = f();
      
      std::cout << "x = " << x << "; y = " << y;
      return EXIT_SUCCESS;
  }

Also, I don't think the language designers required

"This lack of dependency must inherently mean that it will yield the same value upon each invocation having the same set of arguments"

to be true. http://en.cppreference.com/w/cpp/language/constexpr only states:

"The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time."

The draft of the C++ standard on my system states that as follows:

"Certain contexts require expressions that satisfy additional requirements as detailed in this sub-clause. Such expressions are called constant expressions. [ Note: Those expressions can be evaluated during translation. — end note ]"

refp• 11 years ago

"This lack of dependency must inherently mean that it will yield the same value upon each invocation having the same set of arguments"

That quote was written in a "this is what most of us used to think" sort of way, it was never intended to be read literally - though maybe I should clearify the intent.

I certainly agree with your post, and your little hack is very cute; +1! :-)

Negitivefrags• 11 years ago

In this comment thread are a bunch of people pretending that just because something can be done means that all C++ programmers will actually do it.

If someone writes a "here is a cool thing" article for another language involving arbitrary obscure details they are praised with "cool hack". If the language is C++ then out comes all the rants about how C++ is inherently unmaintainable because of "this kind of thing".

Why do people react to C++ this way and not other languages?

The reason this happens is because the only thing all the other comments in this thread are really saying is "Preconceptions about C++ confirmed!". It's easy to reap the karma with such comments.

yoklov3• 11 years ago

Why do people react to C++ this way and not other languages?

Probably because stuff like boost::mpl (and so many awful half-implementations of it) exists.

Sadly, if there's a perceived use for a C++ feature, it will be used, no matter how convoluted it is. This is a specific case of a general method for adding state to constexpr, which might be considered useful... I wouldn't be shocked to see this used in the future.

chombier• 11 years ago

So basically, this means c++ template meta-programming is no longer pure (i.e. side-effects can happen during meta-programs) right ?

If so, this is quite big news, similar to when people realised the extent to which SFINAE could be abused. Not exactly sure this is a good thing though.

Ironically, I think some of Stroustrup's goals for c++11/14 were to lower the need for complex template wizardry... Well, now we will have it complex AND stateful !

Non-constant constant-expressions in C++

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