What I am trying to do is to iterate over all elements of a container in parallel comparable to #pragma omp for; however the container in question does not offer a random access iterator. I therefore use a workaround via tasks described in this stackoverflow answer:
for (std::map<A,B>::iterator it = my_map.begin();
it != my_map.end();
++it)
{
#pragma omp task
{ /* do work with it */ }
}
My problem is that a 'scratch space' object is needed in each iteration; this object is expensive to construct or copy into the data environment of the task. It would only be necessary to have a single thread local object for each thread; in the sense that each task uses the object of the thread it is executed on. private requires a copy and shared results in a race condition. Is there a way to realize this with OpenMP?
I researched #pragma omp threadprivate, however the objects are not static, as the structure of the program looks something like this:
method(int argument_for_scratch_object){
#pragma omp parallel
{
Object scratch(argument_for_scratch_object);
//some computations are done here...
#pragma omp single nowait
{
//here goes the for loop creating the tasks above
//each task uses the scratch space object
}
}
}
If scratch was declared static (and then made threadprivate) before the parallel region, it would be initialized with argument_for_scratch_object of the first method call; which might not be correct for the subsequent method calls.
According to your update, I would suggest to use a global/static thread-private pointer and then initialize it by each thread within your parallel section.
static Object* scratch_ptr;
#pragma omp threadprivate(scratch_ptr);
void method(int argument_for_scratch_object)
{
#pragma omp parallel
{
scratch_ptr = new Object(argument_for_scratch_object);
...
delete scratch_ptr;
}
}