Question

I am trying to create a window container for sfml windows. If you are not familiar with sfml, a sfml window is a class which is non-copyable so cannot be put inside a std::vector for example.

I’ve done most of the work, but I am now trying to implement some sort of callback function which will do the open-gl drawing work before window.Display() is called to update the window.

I cannot get my head around the syntax of a callback function (either c or c++ version). Would someone be willing to explain an example to me?

Even better, would someone be willing to explain an example of how to implement a callback function in a class, so that a function (which the user defines) can be passed into a function of the class which will remember what that function was so it can be called by the class later.

To make that above sentence clearer: I think there are three things I need:

1): A function which the user defines, for example:

void doDrawing(){
    // do some open gl stuff
}

2:) A function inside a class which takes the argument of the above, user defined function:

void setDisplayFunc( ?argument of a function? argument ){
    function_pointer pointer = argument;
}

I have no idea what the type of ‘argument’ should be, or what ‘function_pointer’ has to be… I think this is where I am most stuck!

3:) A way in which the class can call the function which was assigned above.

// Do the function which is pointed at by pointer, which is of the type 'function_pointer' ... Whatever function_pointer happens to be!

If anyone has used glut before, then I guess I am trying to implement the glut callback functions which look like:

void someDisplayFunction(){
    // do open gl stuff
}

glutDisplayFunc( someDisplayFunction );

The above function takes the argument of a user-defined function and then does it when we call ‘glutPostRedisplay’ …

I hope this makes sense, I have tried to include as much information as I can to help. Any help is appreciated. (:

Answer 1

You want a function pointer. In your case:

typedef void (*function_pointer)();

With that you can write:

void setDisplayFunc(function_pointer argument){
    function_pointer pointer = argument;
}

To call it you simply do:

pointer();