Passing function as universal reference? Why?

[Kofr]

Why Functor&& f is possed as universal ref? this is the function, when the function can become a rvalue?

/**
       *  Calls function <code>f</code> in this thread and returns a future<T> that can
       *  be used to wait on the result.  
       *
       *  @param f the operation to perform
       *  @param prio the priority relative to other tasks
       */
      template<typename Functor>
      auto async( Functor&& f, const char* desc FC_TASK_NAME_DEFAULT_ARG, priority prio = priority()) -> fc::future<decltype(f())> {
         typedef decltype(f()) Result;
         typedef typename fc::deduce<Functor>::type FunctorType;
         fc::task<Result,sizeof(FunctorType)>* tsk = 
              new fc::task<Result,sizeof(FunctorType)>( fc::forward<Functor>(f), desc );
         fc::future<Result> r(fc::shared_ptr< fc::promise<Result> >(tsk,true) );
         async_task(tsk,prio);
         return r;
      }

[VRonin]

Take this functor:

class ExampleFunctor
{
    int* m_longArray;
    int m_longArraySize;
public:
    ExampleFunctor(int sz = 10000)
        :m_longArraySize(sz)
    {
        if (m_longArraySize < 10000) m_longArraySize = 10000;
        m_longArray = new int[m_longArraySize];
        for (int i = 0; i < m_longArraySize; ++i)
            m_longArray[i] = rand();
    }
    ~ExampleFunctor() { if (m_longArray) delete[] m_longArray; }
    ExampleFunctor(const ExampleFunctor& other)
        :m_longArraySize(other.m_longArraySize)
        , m_longArray(new int[other.m_longArraySize])
    {
        for (int i = 0; i < m_longArraySize; ++i)
            m_longArray[i] = other.m_longArray[i];
    }
    ExampleFunctor(ExampleFunctor&& other)
        :m_longArraySize(other.m_longArraySize)
        , m_longArray(other.m_longArray)
    {
        other.m_longArray = nullptr;
    }
    ExampleFunctor& operator=(ExampleFunctor&& other)
    {
        m_longArraySize = other.m_longArraySize;
        std::swap(other.m_longArray, m_longArray);
        return *this;
    }
    ExampleFunctor& operator=(const ExampleFunctor& other)
    {
        if (m_longArray) delete[] m_longArray;
        m_longArraySize = other.m_longArraySize;
        m_longArray = new int[m_longArraySize];
        for (int i = 0; i < m_longArraySize; ++i)
            m_longArray[i] = other.m_longArray[i];
        return *this;
    }
    int operator()()
    {
        int result = 0;
        for (int i = 0; i < m_longArraySize - 1; ++i)
            result += (m_longArray[i] ^= m_longArray[i + 1]);
        return result;
    }
    static ExampleFunctor createFunctor(int size) { return ExampleFunctor(size); }
};

Pass it to your template async(ExampleFunctor::createFunctor(1000000));

Let's see what happens in each case:

• auto async(Functor f : takes a deep copy of the functor
• auto async(const Functor& f : if you want to call the functor (since it's not const) you'll have to take a deep copy of it sooner or later
• auto async(Functor& f: this would work in the example above but if operator()() was const and you create it with const ExampleFunctor temp(1000000); then you can't pass it anymore
• auto async(Functor&& f: accepts all kind of arguments without compromising efficiency

[Konstantin Tokarev]

BTW, keep in mind that there is no such thing as "universal ref" in C++ standard. It's just a way of thinking about rvalue ref T&& in contexts which provide type deduction for T