C++ Futures and Promises std::future and std::async
Example
In the following naive parallel merge sort example, std::async is used to launch multiple parallel merge_sort tasks. std::future is used to wait for the results and synchronize them:
#include <iostream>
using namespace std;
void merge(int low,int mid,int high, vector<int>&num)
{
vector<int> copy(num.size());
int h,i,j,k;
h=low;
i=low;
j=mid+1;
while((h<=mid)&&(j<=high))
{
if(num[h]<=num[j])
{
copy[i]=num[h];
h++;
}
else
{
copy[i]=num[j];
j++;
}
i++;
}
if(h>mid)
{
for(k=j;k<=high;k++)
{
copy[i]=num[k];
i++;
}
}
else
{
for(k=h;k<=mid;k++)
{
copy[i]=num[k];
i++;
}
}
for(k=low;k<=high;k++)
swap(num[k],copy[k]);
}
void merge_sort(int low,int high,vector<int>& num)
{
int mid;
if(low<high)
{
mid = low + (high-low)/2;
auto future1 = std::async(std::launch::deferred,[&]()
{
merge_sort(low,mid,num);
});
auto future2 = std::async(std::launch::deferred, [&]()
{
merge_sort(mid+1,high,num) ;
});
future1.get();
future2.get();
merge(low,mid,high,num);
}
}
Note: In the example std::async is launched with policy std::launch_deferred. This is to avoid a new thread being created in every call. In the case of our example, the calls to std::async are made out of order, the they synchronize at the calls for std::future::get().
std::launch_async forces a new thread to be created in every call.
The default policy is std::launch::deferred| std::launch::async, meaning the implementation determines the policy for creating new threads.
