speculative_execution_using_wait_for.cpp

#include <future>
#include <iostream>
#include <chrono>

//Speculative execution using std::future<> wait_for()

int quick_but_gestimation()
{
  for (int i=0; i < 2; i++)
	{
		std::this_thread::sleep_for(std::chrono::seconds(1));
		std::cout << "[quick func]: time elapsed : " << i << " second" << std::endl;
	}

	return 10;
}

int slow_but_accurate()
{
	for (int i=0; i < 6; i++)
	{
		std::this_thread::sleep_for(std::chrono::seconds(1));
		std::cout << "[slow func]: time elapsed : " << i << " second" << std::endl;
	}

	return 11;
}

std::future<int> f;

int bestResult()
{
	f = std::async(std::launch::async, slow_but_accurate);

	int guestimation = quick_but_gestimation();
   
	std::future_status s = f.wait_for(std::chrono::seconds(3)); 
   
	if(s == std::future_status::ready)
		return f.get();
	else
		return guestimation;
}

int main()
{
	std::cout << "Best result: " << std::endl;
	std::cout << bestResult() << std::endl;
}
/*
//output from g++ 4.7.2

// When future object is declared and defined INSIDE bestResult() function
  
  
Best result: 
[slow func]: time elapsed : 0 second[quick func]: time elapsed : 
0 second
[quick func]: time elapsed : 1 second
[slow func]: time elapsed : 1 second
[slow func]: time elapsed : 2 second
[slow func]: time elapsed : 3 second
[slow func]: time elapsed : 4 second
[slow func]: time elapsed : 5 second
10
Press ENTER to continue...

 
  
  
// When future object is declared and defined OUTSIDE bestResult() function
  
  
 Best result: 
[slow func]: time elapsed : [quick func]: time elapsed : 0 second
0 second
[slow func]: time elapsed : 1 second
[quick func]: time elapsed : 1 second
[slow func]: time elapsed : 2 second
[slow func]: time elapsed : 3 second
10
[slow func]: time elapsed : 4 second
[slow func]: time elapsed : 5 second
Press ENTER to continue...
 
*/

Be cautious VS2012 has notable behavioral difference with gcc 4.7, which is that both cases works exactly same regardless of location of future object. Another difference is enum class std::future_status should be written as std::future_status::future_status or it's typedef std::_Future_status in VC++11.

GCC4.7

  /// Launch code for futures
  enum class launch 
  { 
    any, 
    async, 
    sync 
  };

  /// Status code for futures
  enum class future_status 
  {
    ready,
    timeout,
    deferred
  };

VS2012

namespace launch {
enum launch {   // names for launch options passed to async
    async = 0x1,
    deferred = 0x2,
    any = async | deferred, // retained
    sync = deferred
    };


...

}

typedef launch::launch _Launch_type;



    namespace future_status {
enum future_status {    // names for timed wait function returns
    ready,
    timeout,
    deferred
    };
    }   // namespace future_status

typedef future_status::future_status _Future_status;