Hacker News Reader

Comment by CamperBob2 | Hacker News Reader

  file_stream<unsignedchar>::open_istream(L"myfile.txt").then([](basic_istream<unsignedchar> fileStream)

I don't think I've ever seen this C++ idiom before (either the ".then" method or the use of anonymous brackets like that.) Is this common everywhere or just in Windows circles?

View on HN

.then() is just a method. The "anonymous brackets" is a c++11 lambda function. I don't think chaining it like that is common anywhere, yet.

The .then() method appears to be something on concurrency::task, which is part of the Microsoft's Parallel Pattern Library. This article says:

"we've started a new project called Casablanca to build a modern C++ library for cloud-based client-server communication, in which all concurrent operations are exposed as PPL tasks."

http://www.drdobbs.com/parallel/improving-futures-and-callba...

Link to Microsoft's C++ committee proposal (N3327): http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n332...

That looks like Microsoft's new "async everywhere" approach to API design. It's probably a pretty good idea in concept, but I hope they don't expect C++ developers to actually write those awful nested lambda .then() blocks for what are conceptually sequential actions.

I expect you can define named functions for your "conceptually sequential actions" if you prefer.

Sure, but is that much better? It reminds me of having to create named struct classes to simulate closures prior to C++11 lambdas. The language syntax just doesn't support that style of programming.

So how many languages actually do support this style directly?

The C# async keyword looks pretty clean, just give C++ a decade or so to catch up.

Is there a better approach available in C++? In other languages you'd use monads or other special compiler forms that can rewrite sequential looking code into the horrible multi-nested callback/error handling code it needs to be.

Boost Coroutines perhaps? http://www.boost.org/doc/libs/1_53_0/libs/coroutine/doc/html...

I've used that style of programming in some of my hobby projects (mixed with other obvious sequencing operations like wait_until(predicate) and delay(time)) for doing timeline scripting. In my experience, it tended to become a mess when I needed things more complex than linear action sequences (e.g. conditionals, loops, etc. are better suited to a real interpreter).

As far as common uses of chaining for sequencing in C++, the only common use that comes to mind is iostreams operator <<, but that predates lambdas.

I think the use of then() makes sense as this is an async call and not serial. So the function passed to then() is a completion handler, i.e. it is run when the call is complete.

So do this call, and then when its finsihed do this.

I like it.

This is C++11 code.

You should look at it, it makes C++ feel a new language.

Reply on news.ycombinator.com