CPP►experience

C++中的异步操作

实际上，C++11中对异步的支持不比Python和JavaScript弱，就是有点长。

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#include <iostream>
#include <vector>
#include <algorithm>
#include <future>
#include <numeric>
template <typename RAIter>
int parallel_sum(RAIter front, RAIter beg, RAIter end)
{
  auto len = end - beg;
  if (len < 1000)
    {
      char out[50];
      sprintf(out, "Calculate from %d to %d\n", beg - front, end - front);
      std::cout << out;
      return std::accumulate(beg, end, 0);
    }
  RAIter mid = beg + len / 2;
  auto handle_right = std::async(std::launch::async, parallel_sum<RAIter>, front, mid, end);
  auto handle_left = std::async(std::launch::async,parallel_sum<RAIter>, front, beg, mid);
  return handle_left.get() + handle_right.get();
}
int main()
{
  std::vector<int> v(20000, 1);
  std::cout << "The sum is " << parallel_sum(v.begin(), v.begin(), v.end()) << '\n';
}

一个小插曲，这里编译的时候博主用g++直接-o，结果疯狂出错。最后发现要加上-pthread参数才能正常使用，看来gcc操作也要多学学啊。。。

结果1：

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Calculate from 9375 to 10000
Calculate from 6875 to 7500
Calculate from 6250 to 6875
Calculate from 14375 to 15000
Calculate from 5625 to 6250
Calculate from 16875 to 17500
Calculate from 8750 to 9375
Calculate from 8125 to 8750
Calculate from 5000 to 5625
Calculate from 19375 to 20000
Calculate from 11875 to 12500
Calculate from 13125 to 13750
Calculate from 16250 to 16875
Calculate from 7500 to 8125
Calculate from 13750 to 14375
Calculate from 11250 to 11875
Calculate from 1250 to 1875
Calculate from 1875 to 2500
Calculate from 10625 to 11250
Calculate from 4375 to 5000
Calculate from 10000 to 10625
Calculate from 18125 to 18750
Calculate from 17500 to 18125
Calculate from 3750 to 4375
Calculate from 18750 to 19375
Calculate from 15625 to 16250
Calculate from 15000 to 15625
Calculate from 12500 to 13125
Calculate from 625 to 1250
Calculate from 2500 to 3125
Calculate from 3125 to 3750
Calculate from 0 to 625
The sum is 20000

结果2：

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Calculate from 18125 to 18750
Calculate from 9375 to 10000
Calculate from 4375 to 5000
Calculate from 18750 to 19375
Calculate from 13750 to 14375
Calculate from 6875 to 7500
Calculate from 15625 to 16250
Calculate from 11875 to 12500
Calculate from 3750 to 4375
Calculate from 1875 to 2500
Calculate from 6250 to 6875
Calculate from 16250 to 16875
Calculate from 11250 to 11875
Calculate from 10625 to 11250
Calculate from 13125 to 13750
Calculate from 19375 to 20000
Calculate from 14375 to 15000
Calculate from 12500 to 13125
Calculate from 8750 to 9375
Calculate from 8125 to 8750
Calculate from 17500 to 18125
Calculate from 16875 to 17500
Calculate from 5625 to 6250
Calculate from 10000 to 10625
Calculate from 15000 to 15625
Calculate from 1250 to 1875
Calculate from 7500 to 8125
Calculate from 625 to 1250
Calculate from 0 to 625
Calculate from 2500 to 3125
Calculate from 3125 to 3750
Calculate from 5000 to 5625
The sum is 20000

如果在输出中间信息的时候不先在字符串里输出会很乱，因为同一行cout输出不同项的操作不是原子的，中间可能会转到其他线程去。还是要看编译器在汇编时如何工作比较好。