没有匹配的 function 来调用：错误：必须使用 '.*' 或 '->*' 来调用指向成员 function 在 'f (...)' 中的指针，例如 '(... ->* f) (...)'

Question

I'm trying to call a non-static member function from a thread pool. The submit function tells me error: must use '. ' or '-> ' to call pointer-to-member function in 'f (...)', eg '(... ->* f) (...)' for the std::future<decltype(f(args...)) portion on the first line. I'm trying to not create a static function. I've tried a few combinations but I don't think I understand what it is asking for. Would anyone help?

auto submit(F&& f, Args&&... args) -> std::future<decltype(f(args...))> {
        // Create a function with bounded parameters ready to execute
        std::function<decltype(f(args...))()> func = std::bind(std::forward<F>(f), std::forward<Args>(args)...);
        // Encapsulate it into a shared ptr in order to be able to copy construct / assign 
        auto task_ptr = std::make_shared<std::packaged_task<decltype(f(args...))()>>(func);

        // Wrap packaged task into void function
        std::function<void()> wrapper_func = [task_ptr]() 
        {
            (*task_ptr)(); 
        };

        // Enqueue generic wrapper function
        m_queue.enqueue(wrapper_func);

        // Wake up one thread if its waiting
        m_conditional_lock.notify_one();

        // Return future from promise
        return task_ptr->get_future();
    }

Compiler Output

Update: I changed auto submit(F&& f, Args&&... args) -> std::future<decltype(f(args...))> to auto submit(F&& f, Args&&... args) -> std::future<std::invoke_result_t<F, Args...>>

Now I'm getting a new compiler error "no type named 'type'". Picture below.

no type named 'type' compiler error

Answer 1

To correctly get the result type of a call to a member function or a normal function you must use std::invoke_result_t :

auto submit(F&& f, Args&&... args) -> std::future<std::invoke_result_t<F, Args...>> {
    // ...
}

That way, both member function and non member function will work.

Consider that when sending a member function, you must also pass the instance:

// object of type MyClass -----v
submit(&MyClass::member_func, my_class, param1, param2);