Details of C ::async in C++11

std::async in C++11 is a template function. std::async calls the function asynchronously, at some point calling Fn with Args as an argument (variable length argument) and returns an std::future object without waiting for Fn to complete. The value returned by Fn is obtained by the get member function of the std::future object. 1 Once Fn is executed, the Shared state will contain the value returned by Fn and ready.

std::async comes in two versions:

1. The specified boot policy is automatically selected without the need to display, so the boot policy is uncertain and can be std::launch::async, std::launch::deferred, or any combination of the two, depending on their system and specific library implementation.

2. Allow the caller to select a specific startup policy.

The launch policy type of std::async is an enumeration class enum class launch, including:

std::launch::async: Asynchronous, starting a new thread to call Fn, the function is called asynchronously by the new thread and its return value is synchronized with the access point of the Shared state.

std::launch::deferred: Delay. This function is called only when the Shared state is accessed. Calls to Fn are deferred until the Shared state of the returned std::future is accessed (using the wait ::future function or get function).

Parameter Fn: Can be a function pointer, a member pointer, or any type of removable function object (that is, an object whose class defines operator()). The return value or exception of Fn is stored in the Shared state for retrieval by the asynchronous std::future object.

Parameter Args: The parameters passed to the Fn call that should be of movable type.

Return value: When the Fn execution ends, the std::future object with the Shared state is ready. The member function of std::future retrieves the value returned by get. When the startup policy is std::launch::async, the returned std::future is linked to the end of the created thread even if the Shared state is never accessed. In this case, the destructor of std::future is synchronized with the return of Fn.

std: : future introduces reference: https: / / www ofstack. com article / 179229. htm

Please refer to the following test code for detailed usage. The following is the test code of copy from other articles, and some adjustments are made. For detailed introduction, please refer to the corresponding reference:

#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>
#include <mutex>
 
namespace future_ {
 
///////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/future/async/
int test_async_1()
{
 auto is_prime = [](int x) {
 std::cout << "Calculating. Please, wait...\n";
 for (int i = 2; i < x; ++i) if (x%i == 0) return false;
 return true;
 };
 
 // call is_prime(313222313) asynchronously:
 std::future<bool> fut = std::async(is_prime, 313222313);
 
 std::cout << "Checking whether 313222313 is prime.\n";
 // ...
 
 bool ret = fut.get(); // waits for is_prime to return
 if (ret) std::cout << "It is prime!\n";
 else std::cout << "It is not prime.\n";
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/future/launch/
int test_async_2()
{
 auto print_ten = [](char c, int ms) {
 for (int i = 0; i < 10; ++i) {
  std::this_thread::sleep_for(std::chrono::milliseconds(ms));
  std::cout << c;
 }
 };
 
 std::cout << "with launch::async:\n";
 std::future<void> foo = std::async(std::launch::async, print_ten, '*', 100);
 std::future<void> bar = std::async(std::launch::async, print_ten, '@', 200);
 // async "get" (wait for foo and bar to be ready):
 foo.get(); //  Note: If you comment out this sentence, it will output '*'
 bar.get();
 std::cout << "\n\n";
 
 std::cout << "with launch::deferred:\n";
 foo = std::async(std::launch::deferred, print_ten, '*', 100);
 bar = std::async(std::launch::deferred, print_ten, '@', 200);
 // deferred "get" (perform the actual calls):
 foo.get(); //  Note: If you comment out this sentence, it will not output '**********'
 bar.get();
 std::cout << '\n';
 
 return 0;
}
 
///////////////////////////////////////////////////////////
// reference: https://en.cppreference.com/w/cpp/thread/async
std::mutex m;
 
struct X {
 void foo(int i, const std::string& str) {
 std::lock_guard<std::mutex> lk(m);
 std::cout << str << ' ' << i << '\n';
 }
 void bar(const std::string& str) {
 std::lock_guard<std::mutex> lk(m);
 std::cout << str << '\n';
 }
 int operator()(int i) {
 std::lock_guard<std::mutex> lk(m);
 std::cout << i << '\n';
 return i + 10;
 }
};
 
template <typename RandomIt>
int parallel_sum(RandomIt beg, RandomIt end)
{
 auto len = end - beg;
 if (len < 1000)
 return std::accumulate(beg, end, 0);
 
 RandomIt mid = beg + len / 2;
 auto handle = std::async(std::launch::async, parallel_sum<RandomIt>, mid, end);
 int sum = parallel_sum(beg, mid);
 return sum + handle.get();
}
 
int test_async_3()
{
 std::vector<int> v(10000, 1);
 std::cout << "The sum is " << parallel_sum(v.begin(), v.end()) << '\n';
 
 X x;
 // Calls (&x)->foo(42, "Hello") with default policy:
 // may print "Hello 42" concurrently or defer execution
 auto a1 = std::async(&X::foo, &x, 42, "Hello");
 // Calls x.bar("world!") with deferred policy
 // prints "world!" when a2.get() or a2.wait() is called
 auto a2 = std::async(std::launch::deferred, &X::bar, x, "world!");
 // Calls X()(43); with async policy
 // prints "43" concurrently
 auto a3 = std::async(std::launch::async, X(), 43);
 a2.wait();           // prints "world!"
 std::cout << a3.get() << '\n'; // prints "53"
 
 return 0;
} // if a1 is not done at this point, destructor of a1 prints "Hello 42" here
 
///////////////////////////////////////////////////////////
// reference: https://thispointer.com/c11-multithreading-part-9-stdasync-tutorial-example/
int test_async_4()
{
 using namespace std::chrono;
 
 auto fetchDataFromDB = [](std::string recvdData) {
 // Make sure that function takes 5 seconds to complete
 std::this_thread::sleep_for(seconds(5));
 //Do stuff like creating DB Connection and fetching Data
 return "DB_" + recvdData;
 };
 
 auto fetchDataFromFile = [](std::string recvdData) {
 // Make sure that function takes 5 seconds to complete
 std::this_thread::sleep_for(seconds(5));
 //Do stuff like fetching Data File
 return "File_" + recvdData;
 };
 
 // Get Start Time
 system_clock::time_point start = system_clock::now();
 
 std::future<std::string> resultFromDB = std::async(std::launch::async, fetchDataFromDB, "Data");
 
 //Fetch Data from File
 std::string fileData = fetchDataFromFile("Data");
 
 //Fetch Data from DB
 // Will block till data is available in future<std::string> object.
 std::string dbData = resultFromDB.get();
 
 // Get End Time
 auto end = system_clock::now();
 auto diff = duration_cast <std::chrono::seconds> (end - start).count();
 std::cout << "Total Time Taken = " << diff << " Seconds" << std::endl;
 
 //Combine The Data
 std::string data = dbData + " :: " + fileData;
 //Printing the combined Data
 std::cout << "Data = " << data << std::endl;
 
 return 0;
}
 
} // namespace future_

GitHub: https: / / github com/fengbingchun/Messy_Test