在openmp中通过std容器迭代 [英] Iteration through std containers in openmp

问题描述

我试图使用openmp通过std :: set的多线程一个循环。当我写下面的代码 -

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);
    }

我收到此错误：

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

是否有另一种正确的方法使用openmp迭代std容器？
我知道我可以使用 int i 并从 0 到 .size（）和循环体中的迭代器或 operator [] ，但这看起来不那么干净。

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.

推荐答案

stl迭代器的循环并行化仅适用于OpenMP 3.0，并且只适用于随机访问迭代器（例如向量和 deque ）。你应该能够这样做：

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);
      }
   }
}

开销相当大，每个线程遍历整个序列（但是只对其中一些执行 operations ）。您的方法使用 int i 更有效率。

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.

另一个替代方法是查看GCC的并行实现< a href =http://gcc.gnu.org/onlinedocs/libstdc++/manual/parallel_mode.html =nofollow> std :: for_each 。查看我的评论。

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

编辑：STL Parallism TS ，很可能是C ++ 17的一部分，可能是将来对标准容器进行迭代的一个很好的选择。

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.

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