OpenMP custom reduction example

openmpcustom.cpp

//  g++-mp-4.9 -fopenmp --std=c++11 x.cpp  -lgomp 
#include <iostream>
#include <omp.h>
#include <unistd.h>

class Custom
{
public:
	Custom(int y = 0) : x(y) {}

	Custom& operator += (const Custom & other)
	{
		x += other.x;
		return *this;
	}


	int x = 0;
};

// not needed
Custom operator + (const Custom & a, const Custom & other)
{
	return Custom(a.x+other.x);
}


//https://software.intel.com/en-us/node/583439
//page 180 of: http://www.openmp.org/mp-documents/OpenMP4.0.0.pdf
#pragma omp declare reduction(CustomPlus: Custom: \
	omp_out += omp_in) 
//	initializer (omp_priv=Custom(100))

int main(int argc, char * argv[])
{
	Custom sum(0);
	Custom a[100];
	for(int i = 0; i < 100; i++)
		a[i].x = 100;

	{
		#pragma omp parallel for reduction(CustomPlus:sum)
		for(int i = 0; i < 100; i ++)
		{
			#pragma omp critical
			{
				std::cout << " " << i << " " << omp_get_thread_num() << std::endl;
			}
			usleep(1000);
			Custom val(i*2+a[i].x);
			sum += val;
		}
	}
	std::cout << sum.x << std::endl;
	return 0;
}