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Subject: Re: [boost] [afio] Formal review of Boost.AFIO
From: Thomas Heller (thom.heller_at_[hidden])
Date: 2015-08-28 01:47:59

On 08/27/2015 04:04 AM, Niall Douglas wrote:
A lot of things about future vs. shared_future...
> Thoughts?

Ok, after reading all the other messages in this and other threads I am
starting to understand what the real problem is (all this is IMHO, of

The biggest mistake in your design is that of the functions taking
"preconditions". This brought you all this mess. The problem is, those
are actually not preconditions but actually arguments to the function,
more specifically, your "precondition" is always (?) the handle (or a
future to the handle so to speak) to the actual operation that is being
performend for which you need to get the value. That all is a result of
the non-existing single responsibility pattern in your code. After
getting rid of that, you will see that most of the points you brought up
against using std::future (or boost::future) will actually go away.
Here is my concrete suggestion:
  - Let your async functions return a future to their result! (Not a
tuple to the handle and the result...)
  - Don't use futures as arguments to your async functions. Say what you
want as input!

Let's take on your read example... and suppose we have the following two
function signatures:

future<handle> open(std::string name);
future<std::vector<char>> read(handle const & h, std::size_t bytes,
std::size_t offset);

See how easy, clear and succinct those functions are? Almost self
explaining without any further documentation.

Now, on to your example about a 100 parallel reads, so let's encapsulate
it a bit to concentrate on the important part:

future<void> read_items(handle const & h)
     vector<future<vector<char>>> reads;
     for(std::size_t i = 0; i != 100; ++i)
         reads.push_back(async_read(h, 4096, i * 4096));
     // handle reads in some form ... for simplicity, just wait on all
     //of them ...
     return when_all(std::move(reads));

No continuations, cool, eh?
(Well, that's a lie, when_all attaches continuations to the passed
futures as well as the conversion from the result of when_all to

In order to call it, you now have several options, as already outlined
in another mail:
1. read_items(open("niall.txt").get());
2. open("niall.txt").then([](future<handle> fh){ return
3. read_items(await open("niall.txt"));

As you can see, the question about shared_future vs. future doesn't even
Was it already mentioned that an rvalue (unique) future can be
implicitly converted to a shared_future?

Now I hear you saying, but what if one of my operations depend on other
operations? How do I handle those preconditions?
The answer is simple: Use what's already there!
Utilities you have to your disposal are:
  - when_all, when_any, when_n etc
  - .then

This allows for the greatest possible flexibility and a clean API!

So to repeat myself, as a guideline to write a future based API:
1. You should always return a single (unique) future that represents the
result of the asynchronous operation.
2. Never return shared_future from your API functions
3. Use the standard wait composure functions instead of trying to be
clever and hide all that from the user inside your async functions.

(NB: In HPX we have one additional function called dataflow, it models
await on a library level and executes a passed function whenever all
input futures are ready, this reminded me a lot of your "preconditions").

> Niall

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