Class and std::async on class member in C++

Question

I'm try to write a class member which calls another class member multiple times in parallel.

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.

#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:

g++ -std=c++11 -pthread main.cpp  

Answer 1

do_rand_stf is a non-static member function and thus cannot be called without a class instance (the implicit this parameter.) Luckily, std::async handles its parameters like std::bind, and bind in turn can use std::mem_fn to turn a member function pointer into a functor that takes an explicit this parameter, so all you need to do is to pass this to the std::async invocation and use valid member function pointer syntax when passing the 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::cout and std::endl without #includeing <iostream>. More seriously, std::future is not copyable, only movable, so you cannot push_back the named object hand without using std::move. Alternatively, just pass the async result to push_back directly:

ran.push_back(async(launch::async,&A::do_rand_stf,this,i,j)); 

Answer 2

You can pass the this pointer to a new thread:

async([this]() {     Function(this); });