C++ 中类成员上的类和 std::async
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Class and std::async on class member in C++
提问by UldisK
I'm try to write a class member which calls another class member multiple times in parallel.
我正在尝试编写一个类成员，它并行地多次调用另一个类成员。
I wrote a simple example of the problem and can't even get to compile this. What am I doing wrong with calling std::async? I guess the problem would be with how I'm passing the the function.
我写了一个简单的问题示例，甚至无法编译它。我在调用 std::async 时做错了什么？我想问题在于我如何传递函数。
#include <vector>
#include <future>
using namespace std;
class A
{
int a,b;
public:
A(int i=1, int j=2){ a=i; b=j;}
std::pair<int,int> do_rand_stf(int x,int y)
{
std::pair<int,int> ret(x+a,y+b);
return ret;
}
void run()
{
std::vector<std::future<std::pair<int,int>>> ran;
for(int i=0;i<2;i++)
{
for(int j=0;j<2;j++)
{
auto hand=async(launch::async,do_rand_stf,i,j);
ran.push_back(hand);
}
}
for(int i=0;i<ran.size();i++)
{
pair<int,int> ttt=ran[i].get();
cout << ttt.first << ttt.second << endl;
}
}
};
int main()
{
A a;
a.run();
}
compilation:
汇编：
g++ -std=c++11 -pthread main.cpp
回答by JohannesD
do_rand_stfis a non-static member function and thus cannot be called without a class instance (the implicit thisparameter.) Luckily, std::asynchandles its parameters like std::bind, and bindin turn can use std::mem_fnto turn a member function pointer into a functor that takes an explicit thisparameter, so all you need to do is to pass thisto the std::asyncinvocation and use valid member function pointer syntax when passing the do_rand_stf:
do_rand_stf是一个非静态成员函数，因此在没有类实例（隐式this参数）的情况下不能被调用。幸运的是，std::async像 一样处理它的参数std::bind，bind然后可以std::mem_fn用来将成员函数指针转换为采用显式this参数的函子，所以您需要做的就是传递this给std::async调用并在传递时使用有效的成员函数指针语法do_rand_stf：
auto hand=async(launch::async,&A::do_rand_stf,this,i,j);
There are other problems in the code, though. First off, you use std::coutand std::endlwithout #includeing <iostream>. More seriously, std::futureis not copyable, only movable, so you cannot push_backthe named object handwithout using std::move. Alternatively, just pass the asyncresult to push_backdirectly:
但是，代码中还有其他问题。首先，您使用std::cout和std::endl不使用#includeing <iostream>。更为严重的是，std::future是不是可复制，只有移动的，所以你不能push_back命名对象hand，而无需使用std::move。或者，只需将async结果push_back直接传递给：
ran.push_back(async(launch::async,&A::do_rand_stf,this,i,j));
回答by Andrew
You can pass the thispointer to a new thread:
您可以将this指针传递给新线程：
async([this]()
{
Function(this);
});
