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Q&A Confusion about using std::async with class member function

So,I'm having a doubt when I call std:async() on a member function of a class. The book I'm reading says (emphasis mine) You can also pass a pointer to a member function to async(). In that case...

1 answer · posted 4y ago by hamsasimon‭ · last activity 4y ago by C. K. Young‭

Question c++ multithreading

#2: Post edited by

hamsasimon‭ · 2020-12-26T03:07:55Z (almost 4 years ago)
Typo in title.

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~~Confusion about ssing std::async with class member function~~

Confusion about using std::async with class member function

So,I'm having a doubt when I call `std:async()` on a member function of a class. The book I'm reading says (emphasis mine)
> You can also pass a pointer to a member function to async(). In that case, the first argument after the member function has to be a **reference** or a **pointer** to the object for which the member function gets called:
However, in the example given the object of the class is passed by value, so I thought that perhaps it was a small typo and I decided to tryout myself:
#include <future>
#include <thread>
#include <iostream>
// Example of calling a member functions with std::async()
class X
{
public:
void add (int num1, int num2)
{
int result = num1 + num2;
std::cout << "The result is " << result << "\n";
}
};
int main()
{
X x;
auto a = std::async(&X::add, x, 42,5); // try call x.add(42,5) asynchronously
//auto a = std::async(&X::add, &x, 42,5); // this works as well
}
It turns out at least on my environment it works if the object is passed by reference and by value. I'm compiling in windows with `g++.exe (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 8.1.0`
Also, this answer on stack overflow seems to indicate that the object should only be passed by reference: [https://stackoverflow.com/questions/13669094/how-to-use-stdasync-on-a-member-function](https://stackoverflow.com/questions/13669094/how-to-use-stdasync-on-a-member-function)

So,I'm having a doubt when I call `std:async()` on a member function of a class. The book I'm reading says (emphasis mine)
> You can also pass a pointer to a member function to async(). In that case, the first argument after the member function has to be a **reference** or a **pointer** to the object for which the member function gets called:
However, in the example given the object of the class is passed by value, so I thought that perhaps it was a small typo and I decided to tryout myself:
#include <future>
#include <thread>
#include <iostream>
// Example of calling a member functions with std::async()
class X
{
public:
void add (int num1, int num2)
{
int result = num1 + num2;
std::cout << "The result is " << result << "\n";
}
};
int main()
{
X x;
auto a = std::async(&X::add, x, 42,5); // try call x.add(42,5) asynchronously
//auto a = std::async(&X::add, &x, 42,5); // this works as well
}
It turns out at least on my environment it works if the object is passed by reference and by value. I'm compiling in windows with `g++.exe (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 8.1.0`
Also, this answer on stack overflow seems to indicate that the object should only be passed by reference: [https://stackoverflow.com/questions/13669094/how-to-use-stdasync-on-a-member-function](https://stackoverflow.com/questions/13669094/how-to-use-stdasync-on-a-member-function)

#1: Initial revision by

hamsasimon‭ · 2020-12-26T03:06:46Z (almost 4 years ago)

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Confusion about ssing std::async with class member function

So,I'm having a doubt when I call `std:async()` on a member function of a class. The book I'm reading says (emphasis mine)
 > You can also pass a pointer to a member function to async(). In that case, the first argument after the member function has to be a **reference** or a **pointer** to the object for which the member function gets called:

However, in the example given the object of the class is passed by value, so I thought that perhaps it was a small typo and I decided to tryout myself:

    #include <future>
    #include <thread>
    #include <iostream>


    // Example of calling a member functions with std::async()
    class X
    {
    public:
      void add (int num1, int num2)
      {
          int result = num1 + num2;

          std::cout << "The result is " << result << "\n";
      }

    };

    int main()
    {
      X x;
      auto a = std::async(&X::add, x, 42,5); // try call x.add(42,5) asynchronously
    //auto a = std::async(&X::add, &x, 42,5); // this works as well
    }

It turns out at least on my environment it works if the object is passed by reference and by value. I'm compiling in windows with `g++.exe (x86_64-posix-seh-rev0, Built by MinGW-W64 project) 8.1.0`

Also, this answer on stack overflow seems to indicate that the object should only be passed by reference: [https://stackoverflow.com/questions/13669094/how-to-use-stdasync-on-a-member-function](https://stackoverflow.com/questions/13669094/how-to-use-stdasync-on-a-member-function)

c++ multithreading

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