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C++ Creator Wants To Solve 35-Year-Old Generic Programming Issues With Concepts (cio.com)
C++ creator Bjarne Stroustrup is arguing that we can improve code by grounding generic programming in concepts -- what's required by a template's arguments. An anonymous reader quotes Paul Krill's report on a new paper by Stroustrup:
In concepts, Stroustrup sees the solution to the interface specification problem that has long dogged C++, the language he founded more than 35 years ago. "The way we write generic code today is simply too different from the way we write other code," Stroustrup says... Currently an ISO technical specification, concepts provide well-specified interfaces to templates without runtime overhead. Concepts, Stroustrup writes, are intended to complete C++'s support for generic programming as initially envisioned. "The purpose of concepts is to fundamentally simplify and improve design. This leads to fewer bugs and clearer -- often shorter -- code"...
Concepts, Stroustrup believes, will greatly ease engineers' ability to write efficient, reliable C++ code... The most obvious effect will be a massive improvement in the quality of error messages, but the most important long-term effect will be found in the flexibility and clarity of code, Stroustrup says. "In particular, having well-specified interfaces allows for simple, general and zero-overhead overloading of templates. That simplifies much generic code"
Concepts are already available in GNU C Compiler 6.2, and Stroustrup wants them to be included in C++ 20. "In my opinion, concepts should have been part of C++ 17, but the committee couldn't reach consensus on that."
Concepts, Stroustrup believes, will greatly ease engineers' ability to write efficient, reliable C++ code... The most obvious effect will be a massive improvement in the quality of error messages, but the most important long-term effect will be found in the flexibility and clarity of code, Stroustrup says. "In particular, having well-specified interfaces allows for simple, general and zero-overhead overloading of templates. That simplifies much generic code"
Concepts are already available in GNU C Compiler 6.2, and Stroustrup wants them to be included in C++ 20. "In my opinion, concepts should have been part of C++ 17, but the committee couldn't reach consensus on that."
The vagaries and complexities of C++ as it progresses in it's specification is reminiscent of efforts to get epicycles to explain motions of heavenly bodies. Geez, people are snide about Perl syntax. Now we have &ref, &&global_ref, [](args){my_lambda_code();}, copy constructors, move constructors, 'override' to fix virtual function breakage. This is just a mess of a language.
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Bjarne Stroustrup, Doug Lea, Knuth, etc... still make feel like a moron on a almost daily basis....
If someone makes you feel like a moron when they explain something, then maybe they are not as smart as you think they are. If you are a true master, you should be able to explain concepts in a way that even a child can understand. Richard Feynman was famous for this. So was Albert Einstein. Of course you can go too far, and simplify too much, so the children only think they understand. Donald Trump is a good example of that.
Well, part of being a professional programmer, at least IMO, is not going batshit-insane with fancy language features when they're not needed. C++ is a language in which you can write some really, really horrible code if you're determined to do so. And I don't think I've ever heard anyone describe it as a language that's easy to master. But for highly experienced C++ programmers like myself, it's an incredibly powerful language, and that's what's important, at least when I use it professionally.
It's pretty easy to list off a litany of problems with C++. It's bloated, it's ugly, it's hard to learn, full of strange idioms and tricky rules. But it has three characteristics that make it indespensible for certain industries and applications:
* It's ubiquitous. Nearly every platform has a C++ compiler, and there's a lot of sample C or C++ code available to use. It also makes hiring and training easier.
* It's efficient. You don't pay for features you don't use, and it compiles down to fast, efficient, native code.
* It's got reasonably good abstraction features that don't require paying a heavy price for that safety, enabling large, complex programs to be written more easily.
There are a lot of great new languages coming out, but nearly all of those fail on the first point. Unfortunately, that's a deal breaker for many projects.
Irony: Agile development has too much intertia to be abandoned now.
No, not a new fad.
Concepts have been coming in C++ since the 90s.
There are two parts of concepts. On the ideas side, Stapanov came up with the idea with the STL. A concept of X is anything X-like. So loosely speaking if you have an array concept, then anything giving [] and having indexable pointers/iterators matches. So, builtin arrays match, as does std::array and vectors from Eigen would match too.
This makes sense: they're semantically the same and if you wrote an algorithm (eg. sort) using any of them, it would look identical. In this way, it's like a specification. If your class matches the specified "array" concept, then any algorithm written against that spec (the array concept) will work correctly on your class.
It's a great idea.
The other bit is language support. So, much C++ code is written using the idea of concepts, but the language does not assist in any way. Templating is completely generic, you say essentially "accept any class", but if you've written against the array concept, and the class doesn't match, you'll get a compile error right in the guts of the algorithm where you try to use [] on an array.
It would be nice, instead of saying "this function accepts any class" to say "this function accepts any class matching the Array concept", or in short "this function accepts any Array". The compiler knows the types and can tell in advance if the class matches. That way if you try to sort a set with that function, for example, it would simply tell you that the set is not an Array.
You can kind of finesse and finagle something better by utterly abusing a different language feature. The language has a mechanism called SFINAE (substitution failure is not an error), which is designed to make function overloading in the presence of templates sane. When the compiler does overload resolution, it substitutes the current type into any templated functions which match the name in question. If that substitution yields a compiler error, the compiler ignores it and removes that option from the list of overloads. This prevents unrelated templated overloads from breaking builds in irrelevant places. It's supposed to be a hidden detail which makes templating "just work". Except you can abuse it to enable/disable functions based on compile time tests for features of a class. IOW you can use it to implement concepts. Accordding to Stroustrup this means "you know too much :(".
He's right: just because you can do it like that doesn't mean it makes life easy.
Putting concepts into the language gives explicit support, with clean, clear, consistent syntax, not using brittle SFINAE hacks. People have been working out how since the 90s, and there have been several major proposals. It's not actually that trivial. You have to avoid breaking any old code, no matter how perverse. It has to work and work cleanly with old, non concept aware code (otherwise it would get slow/no adoption) and it needs to make life simpler for language users and not be an expert only feature, etc etc.
It looks like the concepts TS is finally hitting enough of the targets to actually work.
So basically, no, concepts is not a fad. It's been a major part of the C++ world for 2 decades.
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