假设我想在我的c ++代码中使用std::async
并行运行一个计算量很大的函数func
。现在因为它是一个繁重的功能,我们可能首先使用std::launch::deferred
策略,因为情况是我们可能根本不需要运行。
但是,如果我们将来需要突然执行它们,我们希望并行运行。那么我们如何在之后修改std::launch
政策。
[嗯,有人可以说,为什么你突然创建std::async
,因为突然你需要执行。但我假设我不能这样做。]
或者,除了使用std::async
之外,还有更好更清洁的方法吗?
非常感谢任何帮助。提前谢谢。
#include <future>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <iostream>
std::vector<double> func(size_t n) // a computationally heavy function
{
std::vector<double> vec(n);
std::generate_n(vec.begin(), n, std::rand);
return vec;
}
int main()
{
// create asyncs, now deferred for lazy execution
auto v1 = std::async(std::launch::deferred, func, 200); // deferred for lazy execution
auto v2 = std::async(std::launch::deferred, func, 250); // deferred for lazy execution
// only after sometime we decide to execute both of them
// but we also now want them to execute in parallel
// so how can we now change the launch policy?
// to get the values as quickly as can be
auto n1 = v1.get().size();
auto n2 = v2.get().size();
std::cout<<"Got "<<n1<<" and "<<n2<<" random numbers in parallel!"<<std::endl;
return 0;
}
多思考一点可以解决这个问题:
使用std::async
定义std::launch::deferred
后,当一个人调用.get()
函数时,是否保证运行 async (即并行)当然,不是。 http://en.cppreference.com/w/cpp/thread/launch说
它在调用线程上执行。
然后 async 的概念被破坏了,对吧?
答案 0 :(得分:1)
如果std::async
使用std::launch::deferred
,则会在调用返回的std::future
对象的get()
函数时运行。
这表明您可以强制std::launch::async
这样:
int s1 = 0;
int s2 = 0;
auto v1 = std::async(std::launch::deferred, []{ return 1; });
auto v2 = std::async(std::launch::deferred, []{ return 2; });
// some fancy coding ...
if(need_to_upgrade_launch_policy())
{
auto v1a = std::async(std::launch::async, [&]{ return v1.get(); });
auto v2a = std::async(std::launch::async, [&]{ return v2.get(); });
s1 = v1a.get();
s2 = v2a.get();
}
// more clever coding ...
if(v1.valid()) // was never upgraded
s1 = v1.get();
if(v2.valid()) // was never upgraded
s2 = v2.get();