std::async in C++11 is a template function. std::async calls the function asynchronously. At some time, it calls Fn with Args as a parameter (variable length parameter). It can return without waiting for Fn to complete execution. The returned result is an std::future object. The value returned by Fn can be obtained through the get member function of std::future object. Once the execution of Fn is completed, the shared state will contain the value returned by Fn and ready.
std::async has two versions:
1. There is no need to display the specified startup policy, which is automatically selected. Therefore, the startup policy is uncertain. It may be std::launch::async, std::launch::deferred, or any combination of the two, depending on their system and specific library implementation.
2. Allows the caller to select a specific startup policy.
The startup policy type of std::async is enum class launch, which includes:
1. std::launch::async: asynchronously, start a new thread to call Fn, which is asynchronously called by the new thread, and synchronize its return value with the access point in the shared state.
2. std::launch::deferred: delay. This function is called only when the shared state is accessed. The call to Fn is deferred until the shared state of the returned std::future is accessed (using the wait or get function of std::future).
Parameter Fn: can be function pointer, member pointer, any type of movable constructed function object (that is, the 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.
Args: parameters passed to Fn call, their type should be of movable construction.
Return value: when Fn execution ends, the std::future object in the shared state is ready. The value retrieved by get, a member function of std::future, is the value returned by Fn. When the start policy adopts std::launch::async, the returned std::future will be 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 introduction reference: https://blog.csdn.net/fengbingchun/article/details/104115489
See the following test code for detailed usage. The following is the test code copied from other articles. Some of them have been adjusted. For details, 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 this sentence is commented out, it will also 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 this sentence is commented out, 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_
