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Why can't my class find these callback functions?

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#1 Chrono1081   Members   


Posted 09 May 2011 - 04:16 AM

Hi guys,

I've been working on OpenGL tutorials and now am moving what I'm learning into a class, but I ran into a problem. When I call glutDisplayFunc and glutReshapeFunc they can't find the callback functions (even though they exist). I'm really at a loss here and I either have a syntax error (even though everything works when I take it outside the class) or there is something going on under the hood that I am not aware of? Any help would be greatly appreciated :)

I'll admit I'm pretty rusty with classes but I can't seem to spot what I am doing wrong. Anyone have any idea?

class OGLAppManager
    void reshape(int width, int height);
    void display(void);
    float rotationAngle;

#include "OGLAppManager.h"

    rotationAngle = 0;
    //Set display properties
	glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA | GLUT_DEPTH); //Set up a display buffer, enable double buffering, blending and depth testing
    glutInitWindowSize(1000, 1000); //Set the width and height of the window
    glutInitWindowPosition(100, 100); //Set window position
    glutCreateWindow("GSP 420"); //Set window caption
    //Window Control Functions
    glutDisplayFunc(display); //Tell glut to draw window based on function
    glutReshapeFunc(reshape); //Tell glut to reshape the window when dragged

//Function to reshape the window when dragged
void OGLAppManager::reshape(int width, int height)
    //Set viewport to window size
    glViewport(0, 0, (GLsizei)width, (GLsizei)height); 
    // Switch to projection matrix to manipulate how scene is viewed
    glLoadIdentity(); //Reset projection matrix to avoid artifacts
    //Set FOV, + Near and Far clip planes
    gluPerspective(60, (GLfloat)width / (GLfloat)height, 1.0, 100.0);
    //Switch back to modelview matrix

//Function to draw the scene
void OGLAppManager::display(void)
    //Handle drawing
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f); //Clear background of window
    glClear(GL_COLOR_BUFFER_BIT); //Clear the color buffer
    glLoadIdentity(); //Load identity matrix to reset drawing locations
    //Translate scene back
    glTranslatef(0.0f, 0.0f, -6.0f);
    //Draw a pyramid
	glRotatef(rotationAngle, 0.0, 1.0, 0.0);
    glutSwapBuffers(); //Flush buffers to the window
	rotationAngle++; //Rotation angle for pyramid

#2 Jan-Lieuwe   Members   


Posted 09 May 2011 - 04:31 AM

A class member function will receive the this pointer of the class as its first (hidden) argument, so you're not passing in a 'void func(void)' but a 'void func(OGLAppManager*)'.

The this pointer won't be passed in as first hidden argument when you declare the function as static.

#3 rip-off   Moderators   


Posted 09 May 2011 - 04:37 AM

Member functions are not normal functions, you cannot pass a pointer to a member function to something expecting a pointer to a function. To even refer to a member function, you must use &ClassName::functionName, unlike normal functions. There are many other caveats on member function pointers, but long story short glut cannot interact with c++ member functions. You'll need to write non-member functions. From these you may of course call member functions on any globally accessible object.

I'd recommend using SDL rather than GLUT if you can.

#4 Chrono1081   Members   


Posted 09 May 2011 - 04:58 AM

Thanks so much for the responses guys!

I did notice the pointer thing when I double clicked the error message (and just to try I put a * in front of the name which compiled and linked just fine, but crashed the program on startup :P

I will look into using SDL since I am fairly familiar with it although I've never mixed it with SDL. Are you able to just use the functions of each? (It looked that way when I saw it on Lazyfoo's site).

#5 D.Chhetri   Members   


Posted 09 May 2011 - 10:26 AM

You could do something like so :
class OGLAppManager{
   typedef void (*DisplayFunc)();
  DisplayFunc _display;
   OGLAppManager(): _display( _defaultDispFunc ) {}
   OGLAppManager(const DiplayFunc& f) : _display(f) {}
  void setDisplayFunc(const DisplayFunc& f){ _display = f; }
  void initialize(){ /* all glut initialization here */ }
 static void _defaultDispFunc(){ /* default display function */ }

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