Noah Watkins Noah Watkins - 1 year ago 97
C++ Question

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.


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
uses the sequence of pairs to initialize the buffer, and the
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 ...


The only thing I can find for a solution in C++ is to mimic
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.

Answer Source

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 {
  typedef void (PairSequence::*BoolLike)();
  void non_comparable();
  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) {}

  // Safe Bool idiom
  operator BoolLike() const {
    return done ? nullptr : &PairSequence::non_comparable;

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

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


    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);
    return tmp;

  bool done;
  value_type ij;

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

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