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.tuohang.net/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/asyncstd::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 ///////////////////////////////////////////////////////////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
