C++11中的std::async是个模板函数。std::async异步调用函数，在某个时候以Args作为参数(可变长参数)调用Fn，无需等待Fn执行完成就可返回，返回结果是个std::future对象。Fn返回的值可通过std::future对象的get成员函数获取。一旦完成Fn的执行，共享状态将包含Fn返回的值并ready。
std::async有两个版本：
1.无需显示指定启动策略，自动选择，因此启动策略是不确定的，可能是std::launch::async，也可能是std::launch::deferred，或者是两者的任意组合，取决于它们的系统和特定库实现。
2.允许调用者选择特定的启动策略。
std::async的启动策略类型是个枚举类enum class launch，包括：
1. std::launch::async：异步，启动一个新的线程调用Fn，该函数由新线程异步调用，并且将其返回值与共享状态的访问点同步。
2. std::launch::deferred：延迟，在访问共享状态时该函数才被调用。对Fn的调用将推迟到返回的std::future的共享状态被访问时(使用std::future的wait或get函数)。
参数Fn：可以为函数指针、成员指针、任何类型的可移动构造的函数对象(即类定义了operator()的对象)。Fn的返回值或异常存储在共享状态中以供异步的std::future对象检索。
参数Args：传递给Fn调用的参数，它们的类型应是可移动构造的。
返回值：当Fn执行结束时，共享状态的std::future对象准备就绪。std::future的成员函数get检索的值是Fn返回的值。当启动策略采用std::launch::async时，即使从不访问其共享状态，返回的std::future也会链接到被创建线程的末尾。在这种情况下，std::future的析构函数与Fn的返回同步。
std::future介绍参考：https://blog.csdn.net/fengbingchun/article/details/104115489
详细用法见下面的测试代码，下面是从其他文章中copy的测试代码，部分作了调整，详细内容介绍可以参考对应的reference：
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#include "future.hpp"
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#include <iostream>
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#include <future>
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#include <chrono>
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#include <utility>
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#include <thread>
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#include <functional>
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#include <memory>
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#include <exception>
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#include <numeric>
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#include <vector>
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#include <cmath>
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#include <string>
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#include <mutex>
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namespace future_ {
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///////////////////////////////////////////////////////////
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// reference: http://www.cplusplus.com/reference/future/async/
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int test_async_1()
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{
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auto is_prime = [](
int x) {
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std::
cout <<
"Calculating. Please, wait...\n";
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for (
int i =
2; i < x; ++i)
if (x%i ==
0)
return
false;
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return
true;
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};
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// call is_prime(313222313) asynchronously:
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std::future<
bool> fut =
std::async(is_prime,
313222313);
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std::
cout <<
"Checking whether 313222313 is prime.\n";
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// ...
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bool ret = fut.get();
// waits for is_prime to return
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if (ret)
std::
cout <<
"It is prime!\n";
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else
std::
cout <<
"It is not prime.\n";
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return
0;
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}
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///////////////////////////////////////////////////////////
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// reference: http://www.cplusplus.com/reference/future/launch/
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int test_async_2()
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{
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auto print_ten = [](
char c,
int ms) {
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for (
int i =
0; i <
10; ++i) {
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std::this_thread::sleep_for(
std::chrono::milliseconds(ms));
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std::
cout << c;
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}
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};
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std::
cout <<
"with launch::async:\n";
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std::future<
void> foo =
std::async(
std::launch::async, print_ten,
'*',
100);
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std::future<
void> bar =
std::async(
std::launch::async, print_ten,
'@',
200);
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// async "get" (wait for foo and bar to be ready):
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foo.get();
// 注：注释掉此句，也会输出'*'
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bar.get();
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std::
cout <<
"\n\n";
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std::
cout <<
"with launch::deferred:\n";
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foo =
std::async(
std::launch::deferred, print_ten,
'*',
100);
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bar =
std::async(
std::launch::deferred, print_ten,
'@',
200);
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// deferred "get" (perform the actual calls):
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foo.get();
// 注：注释掉此句，则不会输出'**********'
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bar.get();
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std::
cout <<
'\n';
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return
0;
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}
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///////////////////////////////////////////////////////////
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// reference: https://en.cppreference.com/w/cpp/thread/async
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std::mutex m;
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struct X {
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void foo(int i, const std::string& str) {
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std::lock_guard<
std::mutex> lk(m);
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std::
cout << str <<
' ' << i <<
'\n';
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}
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void bar(const std::string& str) {
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std::lock_guard<
std::mutex> lk(m);
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std::
cout << str <<
'\n';
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}
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int operator()(int i) {
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std::lock_guard<
std::mutex> lk(m);
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std::
cout << i <<
'\n';
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return i +
10;
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}
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};
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template <
typename RandomIt>
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int parallel_sum(RandomIt beg, RandomIt end)
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{
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auto len = end - beg;
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if (len <
1000)
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return
std::accumulate(beg, end,
0);
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RandomIt mid = beg + len /
2;
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auto handle =
std::async(
std::launch::async, parallel_sum<RandomIt>, mid, end);
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int sum = parallel_sum(beg, mid);
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return sum + handle.get();
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}
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int test_async_3()
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{
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std::
vector<
int> v(
10000,
1);
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std::
cout <<
"The sum is " << parallel_sum(v.begin(), v.end()) <<
'\n';
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X x;
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// Calls (&x)->foo(42, "Hello") with default policy:
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// may print "Hello 42" concurrently or defer execution
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auto a1 =
std::async(&X::foo, &x,
42,
"Hello");
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// Calls x.bar("world!") with deferred policy
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// prints "world!" when a2.get() or a2.wait() is called
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auto a2 =
std::async(
std::launch::deferred, &X::bar, x,
"world!");
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// Calls X()(43); with async policy
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// prints "43" concurrently
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auto a3 =
std::async(
std::launch::async, X(),
43);
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a2.wait();
// prints "world!"
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std::
cout << a3.get() <<
'\n';
// prints "53"
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return
0;
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}
// if a1 is not done at this point, destructor of a1 prints "Hello 42" here
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///////////////////////////////////////////////////////////
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// reference: https://thispointer.com/c11-multithreading-part-9-stdasync-tutorial-example/
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int test_async_4()
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{
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using
namespace
std::chrono;
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auto fetchDataFromDB = [](
std::
string recvdData) {
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// Make sure that function takes 5 seconds to complete
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std::this_thread::sleep_for(seconds(
5));
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//Do stuff like creating DB Connection and fetching Data
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return
"DB_" + recvdData;
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};
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auto fetchDataFromFile = [](
std::
string recvdData) {
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// Make sure that function takes 5 seconds to complete
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std::this_thread::sleep_for(seconds(
5));
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//Do stuff like fetching Data File
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return
"File_" + recvdData;
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};
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// Get Start Time
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system_clock::time_point start = system_clock::now();
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std::future<
std::
string> resultFromDB =
std::async(
std::launch::async, fetchDataFromDB,
"Data");
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//Fetch Data from File
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std::
string fileData = fetchDataFromFile(
"Data");
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//Fetch Data from DB
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// Will block till data is available in future<std::string> object.
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std::
string dbData = resultFromDB.get();
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// Get End Time
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auto end = system_clock::now();
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auto diff = duration_cast <
std::chrono::seconds> (end - start).count();
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std::
cout <<
"Total Time Taken = " << diff <<
" Seconds" <<
std::
endl;
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//Combine The Data
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std::
string data = dbData +
" :: " + fileData;
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//Printing the combined Data
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std::
cout <<
"Data = " << data <<
std::
endl;
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return
0;
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}
-
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}
// namespace future_
GitHub：https://github.com/fengbingchun/Messy_Test
转载：https://blog.csdn.net/fengbingchun/article/details/104133494
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