"unix pipes" or automatic concurrent pipelining in C++
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Latest commit cced5c7 Jan 8, 2013 @KholdStare Adding scons makefile



Most programmers are familiar with the concept of a Unix pipe. Besides allowing easy function composition, they provide an easy way to implicitly run each task concurrently. This library aims to do just that in C++, allowing for type-safe, concurrent and parallelized pipelines of functions "for free".

Synopsis (API Proposal)

As an example, say you have a nice set of composable functions, and you want to feed data through them:

Image<RGB> loadImage(std::string path);
Image<RGB> processImage(Image<RGB> const& image);
void saveImage(Image<RGB>const& image);

To process several images, the simple imperative approach is a for loop:

std::vector<std::string> paths{"tree.jpg", "mountain.jpg", "car.jpg"};

for (auto&& path : paths)
    Image<RGB> image = loadImage(path);

It gets the job done but is single threaded and not concurrent. Most of the time the program will be blocked, waiting for IO to complete. Since it is single threaded, parallelism cannot be exploited. Wouldn't it be nice if one image was being read, while another was being processed, while another was being saved?

Enter Plumbing++, which automatically creates threads for each processing step, and joins each with a concurrent FIFO:

std::vector<std::string> paths{"tree.jpg", "mountain.jpg", "car.jpg"};

(Plumbing::makeSource(paths) >> loadImage >> processImage >> saveImage).wait();

Done! Each connection create a concurrent FIFO, called a Pipe. The read side of this Pipe is returned in the form of an iterable object:

Plumbing::PipeSource<Image<RGB>> imageSource = (paths >> loadImage);

These can be freely iterated over, or used in STL algorithms:

std::vector<Image<RGB>> imageCopies;
std::copy(imageSource.begin(), imageSource.end(), std::back_inserter(imageCopies));

for (auto&& elem : anotherSource)
    std::cout << elem << std::endl;

Each PipeSource (being iterable) thus becomes the input to the next connection. If the last function in the pipeline returns void, the connect call returns a future, so the caller can wait for the whole computation to complete:

std::future<void> future = (source >> loadImage >> processImage >> saveImage);

Any exceptions thrown from processing functions are caught in their threads and propagated through the pipeline, cleanly closing all threads, and leaving the culprit exception in the future:

std::future<void> future = (source >> loadImage >> processImage >> saveImage);
try {
catch (std::runtime_exception& e)
    // deal with exception

For those opposed to operator overloading, a variadic function "connect" that does the same thing:

                  loadImage, processImage, saveImage).wait();

Any single input/single output callable object is supported, including ordinary functions, lambdas, and any function object.

If one of your functions is required to be assymetric, such as combining several input values into one output, this can be done using iterators. As an example, to merge several images into a single HDR, you might have a function object:

// As an example, creates one image for every three,
// effectively merging them
struct mergeHDR
    template <typename InputIterator, typename OutputIterator>
    void operator() (InputIterator first,
                     InputIterator last,
                     OutputIterator dest);

Such a function object can now be used in a pipeline:

std::futute<void> fut =
    >> loadImage
    >> Plumbing::makeIteratorFilter<Image<RGB>, Image<RGB>>(mergeHDR())
    >> saveImage;


Unfortunately, since working with iterators requires function templates, input/return types cannot be deduced automatically as with other examples, and have to be provided manually as template arguments to makeIteratorFilter.


The above features are currently implemented, but I am seeking constructive criticism on the API and implementation. There is a reddit discussion thread here with lots of ideas and suggestions. Everything after this point is sort of a "status report meets stream of conciousness" ramble.

Currently, several improvements can be made to the implementation regarding move semantics, to save expensive copies. It should be possible to move objects all the way through the pipeline.

Operator overloading

A concern with the API is the use of operator overloading. The operator >> template function may accept too broad a range of inputs, and also leaks into the global name space. Not cool.

As mentioned above, a variadic template function "connect" exists that avoids the problem of operator overloading. As suggested in the reddit thread, the influence of operator >> can be narrowed by only allowing it to act on a special object, requiring the user to create that object to jump-start the process, e.g.:

( Plumbing::MakeSink(vals) >> getFirstChar >> printLine<char> ).wait();

This means operator >> would essentially only act on these Sinks:

template <typename T, typename Func>
// return type elided to spare you of ugly template voodoo
operator >> (Sink<T>& sink, Func func);

This is nice, however this Sink type now has to wrap all types of iterable objects (not just Pipes as it does currently). This can be done through type erasure, but would incur an extra virtual method call for every dereference of the underlying iterable (be it a Pipe<T>, or std::vector<int>).

The alternative is to have two distinct types:

  • A Sink<T> that encapsulates the end of Pipe<T>
  • An Input<Container<T>> that is a thin wrapper around an iterable object in order to restrict when operator >> gets triggered.

This is less elegant requiring more types and more functions definitions, but will be more efficient, incurring no calls to virtual methods, while restricting the use of operator >>

UPDATE I have decided to kill two birds with one stone. The new Source class encapsulates any iterable, including a Pipe- however, it is polymorphic at compile time, so has zero overhead. A PipeSource is just a type alias for a Source over Pipes.

Future Work/Ideas

This section covers some "TODOs", and things to look into.

Connecting non-trivial functions

Single input, single output functions are a small subset of the functions one would want to pipeline, so here are a few ideas on how to incorporate those into the library as well.

Some functions could take several arguments of different types, and return something else:

 * Takes a background, and a foreground with transparency and overlays the
 * two to make a new image
Image<RGB> overlay(Image<RGB>& background, Image<RGBA>& foregroundWithMask);

Plumbing::combine(backgroundSink, foregroundSink) >> overlay;

Other ways of connecting

User plhk in this this comment on reddit suggests using the (>>=) operator to assemble the pipeline. This is very fitting as it is the bind operator in Haskell (which this library is inspired from), effectively porting Haskell syntax directly to C++.

However since this is C++, the (>>=) operator is right associative, so at present you would have to write:

(((paths >>= loadImage) >>= processImage) >>= saveImage).wait();

Of course, this can be "remedied" by changing the semantics of the operator so it aggregates functions from right to left until you finally bind to an iterable, at which point it will trigger execution. This is useful in "pre assembling" pipelines and then reusing/combining them later:

pipeline = (loadImage >>= processImage >>= saveImage);
(paths >>= pipeline);
(otherPaths >>= pipeline);

This could also allow aggregation of the execution graph of the whole pipeline, that can be used for task scheduling, if/when work-stealing is implemented.