Equivalent C++ to Python generator pattern

I've got some example Python code that I need to mimic in C++. I do not require any specific solution (such as co-routine based yield solutions, although they would be acceptable answers as well), I simply need to reproduce the semantics in some manner.

Python

This is a basic sequence generator, clearly too large to store a materialized version.

def pair_sequence():
    for i in range(2**32):
        for j in range(2**32):
            yield (i, j)

The goal is to maintain two instances of the sequence above, and iterate over them in semi-lockstep, but in chunks. In the example below the first_pass uses the sequence of pairs to initialize the buffer, and the second_pass regenerates the same exact sequence and processes the buffer again.

def run():
    seq1 = pair_sequence()
    seq2 = pair_sequence()

    buffer = [0] * 1000
    first_pass(seq1, buffer)
    second_pass(seq2, buffer)
    ... repeat ...

C++

The only thing I can find for a solution in C++ is to mimic yield with C++ coroutines, but I haven't found any good reference on how to do this. I'm also interested in alternative (non general) solutions for this problem. I do not have enough memory budget to keep a copy of the sequence between passes.

Generators exist in C++, just under another name: Input Iterators. For example, reading from std::cin is similar to having a generator of char.

You simply need to understand what a generator does:

there is a blob of data: the local variables define a state
there is an init method
there is a "next" method
there is a way to signal termination

In your trivial example, it's easy enough. Conceptually:

struct State { unsigned i, j; };

State make();

void next(State&);

bool isDone(State const&);

Of course, we wrap this as a proper class:

class PairSequence:
    // (implicit aliases)
    public std::iterator<
        std::input_iterator_tag,
        std::pair<unsigned, unsigned>
    >
{
  // C++03
  typedef void (PairSequence::*BoolLike)();
  void non_comparable();
public:
  // C++11 (explicit aliases)
  using iterator_category = std::input_iterator_tag;
  using value_type = std::pair<unsigned, unsigned>;
  using reference = value_type const&;
  using pointer = value_type const*;
  using difference_type = ptrdiff_t;

  // C++03 (explicit aliases)
  typedef std::input_iterator_tag iterator_category;
  typedef std::pair<unsigned, unsigned> value_type;
  typedef value_type const& reference;
  typedef value_type const* pointer;
  typedef ptrdiff_t difference_type;

  PairSequence(): done(false) {}

  // C++11
  explicit operator bool() const { return !done; }

  // C++03
  // Safe Bool idiom
  operator BoolLike() const {
    return done ? 0 : &PairSequence::non_comparable;
  }

  reference operator*() const { return ij; }
  pointer operator->() const { return &ij; }

  PairSequence& operator++() {
    static unsigned const Max = std::numeric_limts<unsigned>::max();

    assert(!done);

    if (ij.second != Max) { ++ij.second; return *this; }
    if (ij.first != Max) { ij.second = 0; ++ij.first; return *this; }

    done = true;
    return *this;
  }

  PairSequence operator++(int) {
    PairSequence const tmp(*this);
    ++*this;
    return tmp;
  }

private:
  bool done;
  value_type ij;
};

So hum yeah... might be that C++ is a tad more verbose :)

In C++ there are iterators, but implementing an iterator isn't straightforward: one has to consult the iterator concepts and carefully design the new iterator class to implement them. Thankfully, Boost has an iterator_facade template which should help implementing the iterators and iterator-compatible generators.

Sometimes a stackless coroutine can be used to implement an iterator.

P.S. See also this article which mentions both a switch hack by Christopher M. Kohlhoff and Boost.Coroutine by Oliver Kowalke. Oliver Kowalke's work is a followup on Boost.Coroutine by Giovanni P. Deretta.

P.S. I think you can also write a kind of generator with lambdas:

std::function<int()> generator = []{
  int i = 0;
  return [=]() mutable {
    return i < 10 ? i++ : -1;
  };
}();
int ret = 0; while ((ret = generator()) != -1) std::cout << "generator: " << ret << std::endl;

Or with a functor:

struct generator_t {
  int i = 0;
  int operator() () {
    return i < 10 ? i++ : -1;
  }
} generator;
int ret = 0; while ((ret = generator()) != -1) std::cout << "generator: " << ret << std::endl;

P.S. Here's a generator implemented with the Mordor coroutines:

#include <iostream>
using std::cout; using std::endl;
#include <mordor/coroutine.h>
using Mordor::Coroutine; using Mordor::Fiber;

void testMordor() {
  Coroutine<int> coro ([](Coroutine<int>& self) {
    int i = 0; while (i < 9) self.yield (i++);
  });
  for (int i = coro.call(); coro.state() != Fiber::TERM; i = coro.call()) cout << i << endl;
}

Since Boost.Coroutine2 now supports it very well (I found it because I wanted to solve exactly the same yield problem), I am posting the C++ code that matches your original intention:

#include <stdint.h>
#include <iostream>
#include <memory>
#include <boost/coroutine2/all.hpp>

typedef boost::coroutines2::coroutine<std::pair<uint16_t, uint16_t>> coro_t;

void pair_sequence(coro_t::push_type& yield)
{
    uint16_t i = 0;
    uint16_t j = 0;
    for (;;) {
        for (;;) {
            yield(std::make_pair(i, j));
            if (++j == 0)
                break;
        }
        if (++i == 0)
            break;
    }
}

int main()
{
    coro_t::pull_type seq(boost::coroutines2::fixedsize_stack(),
                          pair_sequence);
    for (auto pair : seq) {
        print_pair(pair);
    }
    //while (seq) {
    //    print_pair(seq.get());
    //    seq();
    //}
}

In this example, pair_sequence does not take additional arguments. If it needs to, std::bind or a lambda should be used to generate a function object that takes only one argument (of push_type), when it is passed to the coro_t::pull_type constructor.

Equivalent C++ to Python generator pattern

Python

C++

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