Question

I am writing an application using the OpenCV libraries, the Boost libraries and a pieve of code that I have downloaded from this LINK. I have created a project under the same solution with Thunk32 and I have the following files:

MainProject.cpp

#include "stdafx.h"

int main( int argc, char** argv )
{
    IplImage *img = cvLoadImage( "C:/Users/Nicolas/Documents/Visual Studio 2010/Projects/OpenCV_HelloWorld/Debug/gorilla.jpg" );
    Window::WindowType1 *win = new Window::WindowType1("Something");
    cvNamedWindow( "window", CV_WINDOW_AUTOSIZE );
    cvShowImage( "window", img );
    cvSetMouseCallback( "oonga", (CvMouseCallback)win->simpleCallbackThunk.getCallback(), NULL );  

    while( true )
    {
        int c = waitKey( 10 );

        if( ( char )c == 27 )
        { break; }
    }
    return 0;
}

Window.h

class Window {

public:
    Window();
    virtual ~Window();

    //virtual void mouseHandler( int event, int x, int y, int flags, void *param );

private:
    void assignMouseHandler( CvMouseCallback mouseHandler );

    class WindowWithCropMaxSquare;
    class WindowWithCropSelection;
    class WindowWithoutCrop;

public:
    typedef WindowWithCropMaxSquare WindowType1;
    typedef WindowWithCropSelection WindowType2;
    typedef WindowWithoutCrop WindowType3;

protected:
    
};

class Window::WindowWithCropMaxSquare : public Window {

public:
    indev::Thunk32<WindowType1, void _cdecl ( int, int, int, int, void* )> simpleCallbackThunk;

    WindowWithCropMaxSquare( char* name );
    ~WindowWithCropMaxSquare();

    void _cdecl mouseHandler( int event, int x, int y, int flags, void *param );

private:

protected:
    
};

and Window.cpp

#include "stdafx.h"

Window::Window()
{

}

Window::~Window()
{

}

void Window::assignMouseHandler( CvMouseCallback mouseHandler )
{

}

Window::WindowWithCropMaxSquare::WindowWithCropMaxSquare( char* name )
{
    simpleCallbackThunk.initializeThunk(this, &Window::WindowWithCropMaxSquare::mouseHandler); // May throw std::exception
}

Window::WindowWithCropMaxSquare::~WindowWithCropMaxSquare()
{

}

void _cdecl Window::WindowWithCropMaxSquare::mouseHandler( int event, int x, int y, int flags, void *param )
{
    printf("entered mousehandler");
}

Now, when I run this, If I don’t move the mouse inside the window, it’s ok and the callback has been successfully passed to the cvSetMouseCallback function. The cvSetMouseCallback function has three parameters 1. the name of the window, 2. the CvMouseCallback and the NULL character. The CvMouseCallback is defined as

typedef void (CV_CDECL *CvMouseCallback )(int event, int x, int y, int flags, void* param);

and the CV_CDECL is just a redefinition of the _cdecl calling convention.

#define CV_CDECL __cdecl

Now, my mouseHandler function is a class member function, which I assume conforms to the _thiscall calling convention.

My question is, why do I get the following error just when I put my mouse on the window, if it has managed to get into the method at least once? I guess there’s a change the second moment my mouse moves within the windoow. Can anyone help me please?

Here’s an image with what I am doing:

Answer 1

That thunk code uses the __stdcall convention, not __cdecl. In this case, since cvSetMouseCallback takes a void* which it passes through to the callback, I would recommend that you use a static callback function and use this data pointer to pass the this pointer. You may then put your logic in this static function or else just call an instance version of the callback using the pointer that was passed in.

class Window {
public:
    void _cdecl staticMouseHandler( int event, int x, int y, int flags, void *param ) {
        ((MouseHandler*)param)->mouseHandler(event, x, y, flags, NULL);
    }
// ...
}

// ...

cvSetMouseCallback( "oonga", &Window::staticMouseHandler, win );