HOW TO：使用取消來中斷平行迴圈

發行項
08/12/2011

這個範例將示範如何使用取消來實作平行搜尋演算法。

範例

下列範例會使用取消來搜尋陣列中的項目。 parallel_find_any 函式會使用 Concurrency::parallel_for 演算法和 Concurrency::structured_task_group 物件來搜尋包含給定值的位置。當工作函式找到此值時，它會呼叫 Concurrency::structured_task_group::cancel 方法來取消未來的工作。執行階段會取消任何作用中工作，而且不會啟動新的工作。

// parallel-array-search.cpp
// compile with: /EHsc
#include <ppl.h>
#include <iostream>
#include <random>

using namespace Concurrency;
using namespace std;

// Returns the position in the provided array that contains the given value, 
// or -1 if the value is not in the array.
template<typename T>
int parallel_find_any(const T a[], size_t count, const T& what)
{
   // The position of the element in the array. 
   // The default value, -1, indicates that the element is not in the array.
   int position = -1;

   // Use parallel_for to search for the element. 
   // The task group enables a work function to cancel the overall 
   // operation when it finds the result.

   structured_task_group tasks;
   tasks.run_and_wait([&]
   {
      parallel_for(std::size_t(0), count, [&](int n) {
         if (a[n] == what)
         {
            // Set the return value and cancel the remaining tasks. 
            position = n;            
            tasks.cancel();
         }
      });
   });

   return position;
}

int wmain()
{
   const size_t count = 10000;
   int values[count];

   // Fill the array with random values.
   mt19937 gen(34);
   for (size_t i = 0; i < count; ++i)
   {
      values[i] = gen()%10000;
   }

   // Search for any position in the array that contains value 3123.
   const int what = 3123;
   int position = parallel_find_any(values, count, what);
   if (position >= 0)
   {
      wcout << what << L" is at position " << position << L'.' << endl;
   }
   else
   {
      wcout << what << L" is not in the array." << endl;
   }  
}

下列顯示這個範例 (Example) 的範例 (Sample) 輸出。