分配类回调函数到结构？

Question

Recently I learn about callback function and I want to implement it in my app. Here what I have got so far.

// In my image.h

typedef void (__stdcall *DrawingMethod)(IplImage*, HDC, RECT*);

typedef struct _IMAGEPROCESSINGPARAMETER {
    ...
    DrawingMethod draw;
}
IMAGEPROCESSINGPARAMETER,*PIMAGEPROCESSINGPARAMETER,*LPIMAGEPROCESSINGPARAMETER;

class Image {
    public:
        void DrawOriginalSize(IplImage*, HDC, RECT*);
        void DrawToRect(IplImage*, HDC, RECT*);
        void DrawIsotropic(IplImage*, HDC, RECT*);
        int Show();
        IMAGEPROCESSINGPARAMETER ipp;
    ...
};

// In my image.cpp

int Image::Show()
{
    // Get IplImage, HDC, and RECT and finally call the function
    ...
    DrawingMethod d = ipp.draw;
    d(img, dc, &rc);
    return 0;
}

// In main.cpp

#include "image.h"
static Image img;
...
case IDC_FILE_OPEN: {
    img.ipp.draw = img.DrawOriginalSize; // This is ERROR
    img.Show();
    break;
}

How can I make this code to works or is it wrong to write code like this?

Thank in advance

Answer 1

I would suggest you to use std::function if you are using C++11 or boost/function (if not) instead, it gives you much more representation power and is less prone to errors.

In general, it is not advisable to use such c-like techniques, if there is no compelling reason for doing so.

Answer 2

You will need to do more than single simple change to "fix" this.

The problem is that the member functions of Image (such as DrawOriginalSize ) are not the same as regular function - they have an additional this parameter which is implicit, so you don't "see" that parameter. This means that a regular funciton pointer, such as img.ipp.draw can't be a member function.

There are a few solutions. If you make draw a std::function in IMAGEPROCESSINGPARAMETER , it would work straight away.

Alternatively, you will need to make the functions in Image static - if they still need to be member functions (That is, they are using some member variables), you will need to make a static wrapper function - the wrapper function then takes one extra parameter, which is the pointer to the object, which is then made into a this pointer.