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：http://www.zzvips.com/article/183327.html
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：
?| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 |
#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(); // 注：注释掉此句，也会输出'*'
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(); // 注：注释掉此句，则不会输出'**********'
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
以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持服务器之家。
原文链接：https://blog.csdn.net/fengbingchun/article/details/104133494
如果您对该产品感兴趣，请填写办理（客服微信：xiaoxiongyidong）
发表评论
◎欢迎参与讨论，请在这里发表您的看法、交流您的观点。