add original source

source/listing_2.8.cpp

@@ -0,0 +1,67 @@
+#include <thread>
+#include <numeric>
+#include <algorithm>
+#include <functional>
+#include <vector>
+#include <iostream>
+
+template<typename Iterator,typename T>
+struct accumulate_block
+{
+    void operator()(Iterator first,Iterator last,T& result)
+    {
+        result=std::accumulate(first,last,result);
+    }
+};
+
+template<typename Iterator,typename T>
+T parallel_accumulate(Iterator first,Iterator last,T init)
+{
+    unsigned long const length=std::distance(first,last);
+
+    if(!length)
+        return init;
+
+    unsigned long const min_per_thread=25;
+    unsigned long const max_threads=
+        (length+min_per_thread-1)/min_per_thread;
+
+    unsigned long const hardware_threads=
+        std::thread::hardware_concurrency();
+
+    unsigned long const num_threads=
+        std::min(hardware_threads!=0?hardware_threads:2,max_threads);
+
+    unsigned long const block_size=length/num_threads;
+
+    std::vector<T> results(num_threads);
+    std::vector<std::thread>  threads(num_threads-1);
+
+    Iterator block_start=first;
+    for(unsigned long i=0;i<(num_threads-1);++i)
+    {
+        Iterator block_end=block_start;
+        std::advance(block_end,block_size);
+        threads[i]=std::thread(
+            accumulate_block<Iterator,T>(),
+            block_start,block_end,std::ref(results[i]));
+        block_start=block_end;
+    }
+    accumulate_block<Iterator,T>()(block_start,last,results[num_threads-1]);
+    
+    std::for_each(threads.begin(),threads.end(),
+        std::mem_fn(&std::thread::join));
+
+    return std::accumulate(results.begin(),results.end(),init);
+}
+
+int main()
+{
+    std::vector<int> vi;
+    for(int i=0;i<10;++i)
+    {
+        vi.push_back(10);
+    }
+    int sum=parallel_accumulate(vi.begin(),vi.end(),5);
+    std::cout<<"sum="<<sum<<std::endl;
+}