How do I pass my member function as a functor parameter (call back) in a template class?

Question

I now have the following template class (partially formed here for brevity) DDS_Interface.hpp:

MyClass DDS_Interface{
public:
    DDS_Interface(){... mp_reader = ...}; // not complete

    template <typename Functor>
    void start_reader(const Functor & handler) // <-- HERE
    {
        dds::sub::cond::ReadCondition rc(*mp_reader,
                                         dds::sub::status::DataState::any(),
                                         [ &handler ]()
                                         {
                                             // do stuff (populate sample)
                                             handler(sample.data()); // <-- HERE
                                         }; );
    }

private:
    dds::sub::DataReader* mp_reader;
}

Then I have some code in main.cpp:

void handler(MyDataType data)
{
    printf("cols: %d, rows: %d\r\n", data.cols(), data.rows());
}

int main()
{
    DDS_Interface<MyDataType> dds();
    dds.start_reader(handler); // <-- HERE
}

Added some HERE comments to point out key parts of the code. Now this appears to work fine (I am new-ish to lambda and functors so if you see anything silly in this code - please comment!)

However, now that I have managed to get this bit to work - the main thing here is that I am passing a functor into dds.start_reader(...) - I want to pass a member function (or pointer to one).

So now I change main.cpp to this:

class Test
{
public:
    Test() : m_dds() {}; // init m_dds

    void handler(MyDataType data)
    {
        printf("cols: %d, rows: %d\r\n", data.cols(), data.rows());
    }

    void start()
    {
        m_dds.start_reader(this->handler); // this does not work
    }
private:
    DDS_Interface<MyDataType> m_dds;
}

So you can see that instead of passing a basic function-pointer I am trying to pass a member function pointer.

I know that the signature for a member function pointer is something like Test::*handler. But I want start_reader to take a member function of from any class, so that is why I have (now probably wrongly):

template <typename Functor>
void start_reader(const Functor & handler)

I have read quite a lot about this, but I still can't quite figure out how to resolve this part... so my question is sort of "how do I get a square peg into a round hole"? - i.e. how do I pass any member function into start_reader() effectively as a call-back. I can change the start_reader signature to pass the class instance (or anything else) as required...

note: I am using MSVC 2012 which is mostly c++11 compliant (but not fully). I don't want to use boost, but I think MSVC 2012 supports things like bind and std::function if that helps?!

A member function requires an object to call it on, so `handler(sample.data());` cannot compile, you need `(some_test_object.*handler)(sample.data());`. Do you have an instance of `Test` inside DDS_Interface? — nwp, Mar 07 '18 at 09:39
@nwp no, not inside the DDS_Interface - but I could pass one in at construction or when I call `start_reader()`? — code_fodder, Mar 07 '18 at 09:40
@juanchopanza yeah, I know the title is similar - but if you look at the final result of the linked question we only passed in a member function within the same class - not one that was passed in from a different class... (which is where I am stuck) — code_fodder, Mar 07 '18 at 09:41
Typically getting an instance is too complicated. Staying with a non-member function and passing `[this](MyDataType data) { handler(data); }` is easier because you just bind the instance to call the member function to the functor you pass. — nwp, Mar 07 '18 at 09:42
That doesn't really matter. The solution is the same: use a lambda. — juanchopanza, Mar 07 '18 at 09:42
@nwp / juanchopanza do you mean like: `m_dds.start_reader(this->handler);` becomes: `m_dds.start_reader([this](MyDataType data) { handler(data) });` ? — code_fodder, Mar 07 '18 at 09:44
Yes, that looks reasonable. Assuming `this` in that context is the correct instance to call `handler` on. — nwp, Mar 07 '18 at 09:46
@nwp yeehaww! (this is the end of a few days struggle)... that is so awesome, I really didn't like lambdas much before this week.... but they are soo cool! as are you guys : )). So I had no idea that the answer is the same - but I invite you to write your comment into an answer just so I can mark it up : ) — code_fodder, Mar 07 '18 at 09:53

How do I pass my member function as a functor parameter (call back) in a template class?

0 Answers0