Question

I’m thinking of creating a Window class library that uses Win32 API and C++ standard libraries
And I’d like to add signal/slot functionality that handles window messages.

using namespace std;
using namespace boost::signals2;
typedef signal<void (Window*, EventArgs*)> WndEvent; 

class EventArgs { public: HWND hWnd; WPARAM wParam; LPARAM lParam; };

class Window {
    protected: unordered_map<UINT, WndMsg*> msgMap;
    public: void addMsgHandler(UINT msg, void (*handler)(Window*, EventArgs*)) {
        auto iter=msgMap.find(msg);
        if(iter==msgMap.end()) {
            WndEvent* newEvent = new WndEvent();
            newEvent->connect(handler);
            msgMap.insert(make_pair(msg, newEvent));
        }
        else iter->second->connect(handler);
    }

    private: LRESULT wndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lparam) {
        for(auto iter=msgMap.begin(); iter!=msgMap.end(); iter++)
            iter->second->();
    }
};

In using that library:

class Form : public Window {
    void initialize() {// add here pair of 'message' and 'corresponding handler'
        addMsgHandler(WM_LBUTTONUP, boost::bind(&Form::onLButtonUp, this, _1, _2));
        ...
    }
    void onLButtonUp(Window* sender, EventArgs* e) { // event handler
        wchar_t buf[1000]; 
        wsprintf(buf, L"(%d, %d) clicked", GET_X_LPARAM(e->lParam), GET_Y_LPARAM(e->lParam));
        MessageBox(0, buf, L"", MB_OK);
    }
}

Here the problem is onLButtonUp’s signature doesn’t match since it’s a class member function.
So I tried to use boost::bind and I got compile error like this:

 Window::addMsgHandler': cannot convert parameter 2 'boost::_bi::bind_t<R,F,L>' to 'void (__cdecl *)(Window *,EventArgs *)'

which means I have to change (from simple function pointer to boost function object?) the signature of addMsgHandler() but

I cannot determine the data type (with very long template parameters) to be passed.

The compile error message says

R=void,            
F=boost::_mfi::mf2<void,Form,Window *,EventArgs *>,1>                           
L=boost::_bi::list3<boost::_bi::value<Form*>,boost::arg<1>,boost::arg<2>>                

Furthermore, since many other classes will be derived from class Window, I cannot know the template type when I define class Window.

Is there a proper way to pass boost.bind object as function parameter?

Answer 1

Define it like this:

void addMsgHandler(UINT msg, const function<void(Window* , EventArgs*)>& slot);

Or even like this:

void addMsgHandler(UINT msg, const WndEvent::slot_type &slot);

Now, when calling this function, slot parameter should be a “callable” accepting 2 arguments of types Window * and EventArgs *, and you can create such a slot using mem_fn, boost::bind, std::bind, or just passing an appropriate raw function ptr.