Generators for C++

Iterators in C++ is a good but not a perfect abstraction. The concept of foreach() (D, Python, Ruby, etc.) appears as a more generic solution. At least, foreach() does not require an artificial iterator::end() to be defined for the collection.

The foreach() abstraction can be imagined as some function/object that returns the next value of collection/sequence each time it gets invoked. Such functions are known as generators.

Generators are functional objects that persist internal state between invocations thus they can be considered as corroutines too.

This version of $generator's for C++ is based on the bright idea of Simon Tatham - "coroutines in C". In particular, on the idea of using switch/case for the implementation.

To declare a generator, you will use $generator, $yield, $emit, and $stop "keywords" that are macro definitions in fact.

Here is a typical implementation of a generator. In this particular example our generator will emit numbers from 10 to 1 in descending order:

include "generator.hpp"

$generator(descent)
{
   // place for all variables used in the generator
   int i; // our counter

   // place the constructor of our generator, e.g. 
   // descent(int minv, int maxv) {...}
   
   // from $emit to $stop is a body of our generator:
    
   $emit(int) // will emit int values. Start of body of the generator.

      for (i = 10; i > 0; --i)
         $yield(i); // a.k.a. yield in Python,
                    // returns next number in [1..10], reversed.
   $stop; // stop, end of sequence. End of body of the generator.
};

Having such a descending generator declared, we will use it as:

int main(int argc, char* argv[])
{
  descent gen;
  for(int n; gen(n);) // "get next" generator invocation
    printf("next number is %d\n", n);
  return 0;
}

The gen(n) there is in fact an invocation of the bool operator()(int& v) method defined "under the hood" of our generator object. It returns true if the parameter v was set, and false if our generator cannot provide more elements - was stopped.

$yield cannot be placed inside a switch statement as $emit() declares a switch by itself.

The generator.hpp is licensed under The BSD License.

Please check async.hpp if you need to use this approach in asynchronous code.