{

struct find_element

{

void operator()(Iterator begin,Iterator end,MatchType match,

std::promise<Iterator>* result,

std::atomic<bool>* done_flag)

{

try

{

for(;(begin!=end) && !done_flag->load();++begin)

{

if(*begin == match)

{

result->set_value(begin);

done_flag->store(true);

return;

}

}

}

catch(...)

{

try

{

result->set_exception(std::current_exception());

done_flag->store(true);

}

catch(...)

{}

}

}

};

unsigned long const length=std::distance(first,last);

if(!length)

return last;

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::promise<Iterator> result;

std::atomic<bool> done_flag(false);

std::vector<std::thread> threads(num_threads-1);

//你通过在块中附入线程链接的代码，使得检查结构之前需要等待所有线程结束。

{

join_threads joiner(threads);

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(find_element(),block_start,block_end,match,&result,&done_flag);

block_start=block_end;

}

find_element()(block_start,last,match,&result,&done_flag);

}

//检查是否有匹配项的时候，所有线程都被联合起来了

if(!done_flag.load())

{

return last;

}

return result.get_future().get();

}