Question

I’m trying to use openmp to multithread a loop through std::set. When I write the following code –

    #pragma omp parallel for
    for (std::set<A>::const_iterator i = s.begin(); i != s.end(); ++i) {
            const A a = *i;
            operate(a);
    }

I get this error:

error: invalid type for iteration variable 'i'
error: invalid controlling predicate
error: invalid increment expression.

Is there an another, correct way to iterate through std containers using openmp?
I know I can use int i and iterate from 0 to s.size() and an iterator or operator[] in the loop body, but this looks much less clean.

Answer 1

Loop parallelization for stl iterators only works since OpenMP 3.0, and only for random access iterators (e.g. vector and deque). You should be able to do something like this:

#pragma omp parallel {
   for (std::set<A>::const_iterator i = s.begin(); i != s.end(); ++i) {
      #pragma omp single nowait {
         operate(*i);
      }
   }
}

Overhead is quite big though because each thread iterates over the whole sequence (but only executes operate on some of it). Your method using an int i is more efficient.

As an alternative take a look at GCC’s parallel implementation of std::for_each. See my comment.

EDIT: The STL Parallism TS, which will most likely be part of C++17, might be a good option in the future for iterating over standard containers.