How to parallel_reduce a C++ map key or value?

[khteh]

I don't find it from google. How can I parallel_reduce either the key or value of a map? For example:

       map<T, size_t> counts;
	size_t count = parallel_reduce(
		blocked_range<pair<T, size_t>>(0, counts.size()), 0,
		[&](tbb::blocked_range<pair<T, size_t>> const &r, size_t running_total)
		{
			for (auto it = r.begin(); it != r.end(); it++)
				running_total += it->second;
			return running_total;
		},
		std::plus<pair<T, size_t>>());