C++11中std::packaged_task的使用详解

#include "future.hpp"
#include <iostream>
#include <future>
#include <chrono>
#include <utility>
#include <thread>
#include <functional>
#include <memory>
#include <exception> 
#include <numeric>
#include <vector>
#include <cmath>
#include <string>
 
namespace future_ {
 
///////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/future/packaged_task/
int test_packaged_task_1()
{
 
{ // constructor/get_future/operator=/valid
 std::packaged_task<int(int)> foo; // default-constructed
 std::packaged_task<int(int)> bar([](int x) { return x * 2; }); // initialized
 
 foo = std::move(bar); // move-assignment
 std::cout << "valid: " << foo.valid() << "\n";
 std::future<int> ret = foo.get_future(); // get future
 std::thread(std::move(foo), 10).detach(); // spawn thread and call task
 
 int value = ret.get(); // wait for the task to finish and get result
 std::cout << "The double of 10 is " << value << ".\n";
}
 
{ // reset/operator()
 std::packaged_task<int(int)> tsk([](int x) { return x * 3; }); // package task
 
 std::future<int> fut = tsk.get_future();
 tsk(33);
 std::cout << "The triple of 33 is " << fut.get() << ".\n";
 
 // re-use same task object:
 tsk.reset();
 fut = tsk.get_future();
 std::thread(std::move(tsk), 99).detach();
 std::cout << "Thre triple of 99 is " << fut.get() << ".\n";
}
 
{ // constructor/get_future
 auto countdown = [](int from, int to) {
 for (int i = from; i != to; --i) {
 std::cout << i << '\n';
 std::this_thread::sleep_for(std::chrono::seconds(1));
 }
 std::cout << "Lift off!\n";
 return from - to;
 };
 
 std::packaged_task<int(int, int)> tsk(countdown); // set up packaged_task
 std::future<int> ret = tsk.get_future(); // get future
 
 std::thread th(std::move(tsk), 5, 0); // spawn thread to count down from 5 to 0
 
 int value = ret.get(); // wait for the task to finish and get result
 std::cout << "The countdown lasted for " << value << " seconds.\n";
 
 th.join();
}
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: https://en.cppreference.com/w/cpp/thread/packaged_task
int test_packaged_task_2()
{
{ // lambda
 std::packaged_task<int(int, int)> task([](int a, int b) { return std::pow(a, b);});
 std::future<int> result = task.get_future();
 
 task(2, 9);
 std::cout << "task_lambda:\t" << result.get() << '\n';
}
 
{ // bind
 std::packaged_task<int()> task(std::bind([](int x, int y) { return std::pow(x, y); }, 2, 11));
 std::future<int> result = task.get_future();
 
 task();
 std::cout << "task_bind:\t" << result.get() << '\n';
}
 
{ // thread
 std::packaged_task<int(int, int)> task([](int x, int y) { return std::pow(x, y); });
 std::future<int> result = task.get_future();
 
 std::thread task_td(std::move(task), 2, 10);
 task_td.join();
 std::cout << "task_thread:\t" << result.get() << '\n';
}
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: https://thispointer.com/c11-multithreading-part-10-packaged_task-example-and-tutorial/
struct DBDataFetcher {
 std::string operator()(std::string token)
 {
 // Do some stuff to fetch the data
 std::string data = "Data From " + token;
 return data;
 }
};
 
int test_packaged_task_3()
{
 // Create a packaged_task<> that encapsulated a Function Object
 std::packaged_task<std::string(std::string)> task(std::move(DBDataFetcher()));
 
 // Fetch the associated future<> from packaged_task<>
 std::future<std::string> result = task.get_future();
 
 // Pass the packaged_task to thread to run asynchronously
 std::thread th(std::move(task), "Arg");
 
 // Join the thread. Its blocking and returns when thread is finished.
 th.join();
 
 // Fetch the result of packaged_task<> i.e. value returned by getDataFromDB()
 std::string data = result.get();
 std::cout << data << std::endl;
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: https://stackoverflow.com/questions/18143661/what-is-the-difference-between-packaged-task-and-async
int test_packaged_task_4()
{
 // sleeps for one second and returns 1
 auto sleep = []() {
 std::this_thread::sleep_for(std::chrono::seconds(1));
 return 1;
 };
 
{ // std::packaged_task
 // >>>>> A packaged_task won't start on it's own, you have to invoke it
 std::packaged_task<int()> task(sleep);
 
 auto f = task.get_future();
 task(); // invoke the function
 
 // You have to wait until task returns. Since task calls sleep
 // you will have to wait at least 1 second.
 std::cout << "You can see this after 1 second\n";
 
 // However, f.get() will be available, since task has already finished.
 std::cout << f.get() << std::endl;
}
 
{ // std::async
 // >>>>> On the other hand, std::async with launch::async will try to run the task in a different thread :
 auto f = std::async(std::launch::async, sleep);
 std::cout << "You can see this immediately!\n";
 
 // However, the value of the future will be available after sleep has finished
 // so f.get() can block up to 1 second.
 std::cout << f.get() << "This will be shown after a second!\n";
}
 
 return 0;
}
 
} // namespace future_