How to use std::async on a member function?

Question

How can I operate std::async call on a member function?

Example:

class Person{
public:
    void sum(int i){
        cout << i << endl;
    }
};

int main(int argc, char **argv) {
    Person person;
    async(&Person::sum,&person,4);
}

I want to call to sum async.

Person p;
call async to p.sum(xxx)

I didnt figure out if i can do it with std::async. Dont want to use boost. Looking for a one line async call way.

Answer 1

Something like this:

auto f = std::async(&Person::sum, &p, xxx);

or

auto f = std::async(std::launch::async, &Person::sum, &p, xxx);

where p is a Person instance and xxx is an int.

This simple demo works with GCC 4.6.3:

#include <future>
#include <iostream>

struct Foo
{
  Foo() : data(0) {}
  void sum(int i) { data +=i;}
  int data;
};

int main()
{
  Foo foo;
  auto f = std::async(&Foo::sum, &foo, 42);
  f.get();
  std::cout << foo.data << "\n";
}

Answer 2

There are several ways, but I find it's most clear to use a lambda, like this:

int i=42;
Person p;
auto theasync=std::async([&p,i]{ return p.sum(i);});

This creates a std::future. For a complete example of this, I have a full example including a async-capable setup of mingw here:

http://scrupulousabstractions.tumblr.com/post/36441490955/eclipse-mingw-builds

You need to make sure that p is thread safe and that the &p reference is valid until the async is joined. (You can also hold p with a shared pointer, or in c++14, a unique_ptr or even move p into the lambda.)