std::async是用来异步的执行一个任务(通过一个函数)。
它的声明如下:
unspecified policy (1)
template <class Fn, class... Args>
future<typename result_of<Fn(Args...)>::type>
async (Fn&& fn, Args&&... args);
specific policy (2)
template <class Fn, class... Args>
future<typename result_of<Fn(Args...)>::type>
async (launch policy, Fn&& fn, Args&&... args);
第二个要传一个启动策略,第一个不用传,默认用 launch::async|launch::deferred
函数参数的含义可以参考:
https://www.cplusplus.com/reference/future/async/
先看一个例子:
#include <iostream> // std::cout
#include <future> // std::async, std::future
#include <unistd.h>
#include <sys/syscall.h>
#define gettid() syscall(SYS_gettid)
// a non-optimized way of checking for prime numbers:
bool is_prime (int x) {
sleep(4);
std::cout << "Thread ID " << gettid() << ": Calculating. Please, wait...\n";
for (int i=2; i<x; ++i) if (x%i==0) return false;
return true;
}
int main ()
{
// call is_prime(313222313) asynchronously:
std::future<bool> fut = std::async ( std::launch::async, is_prime,313222313);
std::cout << "Thread ID " << gettid() << ": 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;
}
执行结果:
Thread ID 28440: Checking whether 313222313 is prime.
Thread ID 28441: Calculating. Please, wait...
It is prime!
可以看到,其实是起了一个线程来执行异步任务。
现在我们稍微改一下main函数代码:
int main ()
{
// call is_prime(313222313) asynchronously:
/*std::future<bool> fut = */std::async ( std::launch::async, is_prime,313222313);
std::cout << "Thread ID " << gettid() << ": 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;
}
输出:
Thread ID 17736: Calculating. Please, wait...
Thread ID 17735: Checking whether 313222313 is prime.
可以看到这个时候,先执行子线程,子线程结束后继续执行主线程。
这是因为std::async返回的是一个std::future 对象, 它的析构函数会一直阻塞直到std::sync启动的线程结束。
上面的代码,没有保存std::async返回的std::future 对象,所以它是一个临时对象,std::async返回就结束,所以返回即调用 析构函数,析构函数阻塞直到线程结束。
所以会出现上面主线程等待子线程完成的打印。