Passing member function to Template function

Question

Given the following function:

template<class F, class... Args>
auto ThreadPool::enqueue(F&& f, Args&&... args) 
    -> std::future<typename std::result_of<F(Args...)>::type>
{
    using return_type = typename std::result_of<F(Args...)>::type;

    auto task = std::make_shared< std::packaged_task<return_type()> >(
            std::bind(std::forward<F>(f), std::forward<Args>(args)...)
        );

    std::future<return_type> res = task->get_future();
    return res;
}

What's the right way to pass a member function to ThreadPool::enqueue as parameter, say the object is:

Foo foo

and the function is:

foo.do_something();

I have tried to use std::bind and std::mem_fn with or without "&" and all failed.

Instead of saying you "tried bind and mem_fun", show us the code you tried. — John Zwinck
– John Zwinck, Commented Jun 7, 2020 at 5:21
For a non-overloaded non-template function you can also use pool.enqueue(&Foo::do_something, &foo, other_args_to_do_something...). — Piotr Skotnicki
– Piotr Skotnicki, Commented Jun 7, 2020 at 11:50

TonySalimi · Accepted Answer · 2020-06-07 11:37:11Z

1

In addition to what @IgorTandetnik has mentioned in the comments, you can also use std::bind with std::mem_fn to pass a member function to your method:

struct Foo
{
   void do_something() {}
   void do_something_else(int x, int y, std::string str) {}
};

int main()
{
   Foo foo;
   ThreadPool pool;

   auto func_sth = std::bind(std::mem_fn(&Foo::do_something), foo);
   auto func_sth_else = std::bind(std::mem_fn(&Foo::do_something_else), foo, 10 , 11, "hi");

   pool.enqueue(func_sth);
   pool.enqueue(func_sth_else);

   return 0;
}

edited Jun 7, 2020 at 11:37

answered Jun 7, 2020 at 8:23

TonySalimi

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4 Comments

BulGali Over a year ago

What is the correct syntax if do_something take parameters, say two ints?

TonySalimi Over a year ago

@BulGali I updated the answer for a function with some parameters.

BulGali Over a year ago

For some reason I get: /functional:467:70: error: no matching function for call to 'std::tuple<AlgoTravel*, std::__cxx11::basic_string...>. Any idea why that might happen? Is there a special syntax if the parameters are passed by reference? say "std::string &str"?

TonySalimi Over a year ago

@BulGali If you are going to pass one parameter (e.g. str) by reference, then pass the parameter with std::ref(str). For const-reference parameters, you can use std::cref() instead.

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