Partitioners

All algorithms also have an optional extra parameter for custom partitioners. A partitioner_for an iterator is simply a callable object that receives two iterators representing a range and returns an iterator indicating where this range should be split for a parallel algorithm. For instance, this would be a partitioner that always splits the problem in half:

Defining a custom partitioner

auto p = [](auto first, auto last) {
    return std::next(first, (last - first) / 2);
};

When we execute an algorithm with our custom partitioner, the algorithm would recursively split the input in half and launch a task for each of these parts.

Using a custom partitioner

auto it = find(ex, p, v, 3000);
if (it != v.end()) {
    assert(*it == 3000);
    std::ptrdiff_t pos = it - v.begin();
    assert(pos >= 0 && pos <= 50000);
}

Hint

A partitioner_for can indicate the problem is too small to be partitioned by returning last, which means the problem should not be split.

When no partitioner is provided, the default_partitioner is used, which imposes a minimum grain size to split the problem and stops recommending the problem to be split if threads are busy with the problem.