Passing Member Functions as Function Pointers

[cps]

Hi,

I'm a C programmer taking my first steps into the world of C++.

I'm currently developing a C++ 3D graphics application using GLUT
(OpenGL Utility Toolkit written in C) for the GUI components.

The application is built around a "World" object that contains a "GUI"
object that is a C++ wrapper around GLUT. What I'd like to do is pass a
pointer to a member function in the World class to function in the GUI
class.

Here are the functions I'd like to pass:


void World:isplay()
{
/* clear all pixels */
glClear(GL_COLOR_BUFFER_BIT);

Do OpenGL stuff...
}

void World::Keyboard(unsigned char key, int x, int y)
{
cout << key << endl;

Handle keyboard...
}


I'd like to pass these functions to the constructor of the GUI object
as follows:


p_GUI = new GUI(Display, Keyboard);


I'm getting the following error:

World.cpp(16) : error C2664: 'GUI::GUI(void (__cdecl *)(void),void
(__cdecl *)(unsigned char,int,int))' : cannot convert parameter 1 from
'void (void)' to 'void (__cdecl *)(void)'
None of the functions with this name in scope match the target
type

I understand that (__cdecl *) is a calling convention. What I can't
figure out is how to fix the syntax so that the code compiles.

Any help with this (especially a simple example) would be very much
appreciated.

Cheers,

Chris

 

[Jim Langston]

Hi,

I'm a C programmer taking my first steps into the world of C++.

I'm currently developing a C++ 3D graphics application using GLUT
(OpenGL Utility Toolkit written in C) for the GUI components.

The application is built around a "World" object that contains a "GUI"
object that is a C++ wrapper around GLUT. What I'd like to do is pass a
pointer to a member function in the World class to function in the GUI
class.

Here are the functions I'd like to pass:


void World:isplay()
{
/* clear all pixels */
glClear(GL_COLOR_BUFFER_BIT);

Do OpenGL stuff...
}

void World::Keyboard(unsigned char key, int x, int y)
{
cout << key << endl;

Handle keyboard...
}


I'd like to pass these functions to the constructor of the GUI object
as follows:


p_GUI = new GUI(Display, Keyboard);

I don't see your definition of GUI::GUI anywhere. What are your constructor
parms?

 

[cps]

Here they are:


// GUI.h

#ifndef GUI_H
#define GUI_H

class GUI
{
public:
GUI();
GUI(void (*displayFunctionPtr)(), void
(*keyboardFunctionPtr)(unsigned char key, int x, int y));
~GUI();

private:
};

#endif


GUI::GUI(void (*displayFunctionPtr)(), void
(*keyboardFunctionPtr)(unsigned char key, int x, int y))
{
int argc = 0;
char* argv[30];

glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
glutInitWindowSize(250, 250);
glutInitWindowPosition(100, 100);
glutCreateWindow("Test");
glClearColor(0.0, 0.0, 0.0, 0.0);
glShadeModel(GL_FLAT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0);
glutDisplayFunc(displayFunctionPtr);
glutIdleFunc(displayFunctionPtr);
glutKeyboardFunc(keyboardFunctionPtr);
glutMainLoop();
}

 

[Paul]

Hi,

I'm a C programmer taking my first steps into the world of C++.

Please see the FAQ:
http://www.parashift.com/c++-faq-lite/pointers-to-members.html

- Paul

 

[BobR]

cps wrote in message ...

Here they are:
// GUI.h
#ifndef GUI_H
#define GUI_H
class GUI{ public:
GUI();
GUI(void (*displayFunctionPtr)(), void
(*keyboardFunctionPtr)(unsigned char key, int x, int y));
~GUI();
private:
};
#endif

GUI::GUI(void (*displayFunctionPtr)(), void
(*keyboardFunctionPtr)(unsigned char key, int x, int y)){
int argc = 0;
char* argv[30];
glutInit(&argc, argv); [snip]
glutDisplayFunc(displayFunctionPtr);
glutIdleFunc(displayFunctionPtr);
glutKeyboardFunc(keyboardFunctionPtr);
glutMainLoop();
}

If Paul's suggestion doesn't help, try making the 'call-back' functions
'static'. That worked for me in one Glut pgm I was playing with. See (google
for) 'GlutMaster' [1]

class World{
public:
static void Display();
static void Keyboard(unsigned char key, int x, int y);
};
void World:isplay(){
glClear(GL_COLOR_BUFFER_BIT); // clear all pixels
// Do OpenGL stuff...
}
void World::Keyboard(unsigned char key, int x, int y){
cout << key << endl;
// Handle keyboard...
}

[1] - might still be at URL: http://www.duke.edu/~stetten/GlutMaster/

 

[cpsmusic]

Thanks for the replies.

The link to the "pointers-to-members" page should do the trick.

Also, I tried making the member functions static. This worked, but then
I couldn't access member variables.

Cheers,

Chris

 

[cpsmusic]

I'm still having problems with this.

I've had a look at the "pointers-to-members" page. It says that

Normal C functions can be thought of as having a different calling convention from >member functions, so the types of their pointers (pointer-to-member-function vs. >pointer-to-function) are different and incompatible. C++ introduces a new type of >pointer, called a pointer-to-member, which can be invoked only by providing an object.

Does this mean that the only way I can get this to work is by passing
the object that contains the member functions? In this case, do I need
to pass function pointers at all, as I can access the functions via the
object?

As I mentioned above, I have a World object that contains a GUI object.
The GUI object requires a pointer to a member function ("Display") in
the World object. Is there a better way to do this?

Cheers,

Chris

 

[Paul]

Does this mean that the only way I can get this to work is by passing
the object that contains the member functions? In this case, do I need
to pass function pointers at all, as I can access the functions via the
object?

If there is a void* parameter that is provided for you as a "user defined
data" parameter, you can use the void* as the pointer to the object, and
then retrieve it again when the function gets called.

Of course, the instance of the object must still exist.

Note that this is just one of many ways to do this. The bottom line is that
you have an object instance, and when the API executes the callback, you
retrieve that instance in some way so you can get the member data -- this is
where you have to be creative. That's what it all boils down to.

How to retrieve the instance can be accomplished in many different ways.
The simplest being making a global object (as the C++ FAQ illustrates), or
you can get fancy with maps and handles (this is usually done for OO GUI
classes), or you can use the void* option if the API's callback allows this.

- Paul

 

[BobR]

(e-mail address removed) wrote in message ...

Also, I tried making the member functions static. This worked, but then
I couldn't access member variables.

Cheers,
Chris

I ran into that too. Make the variables 'static' so you can access them from
'Display' and 'Keyboard'. There should not be very many. A static var can be
accessed by non-static member functions.

Otherwise, you need to post a lot more of your code[1] AND the exact errors
you are getting, so the experts in this NG can figure it out.

[1] - less the OpenGL stuff that does not apply to the problem
(glClearColor(), glShadeModel(), etc.). You should separate initialize and
draw into to different functions anyway, it will make a difference when you
animate. Example below is to show you can access static vars from non-static
member functions, not how to do OpenGL (which is off-topic in this NG).

// - in class declaration, define outside -
// ---------------------------------------
static GLfloat RotAngle;
static float xspeed; // X Rotation Speed
static float yspeed; // Y Rotation Speed
static GLfloat AngleX; // Angle For Rotation (deg)
static GLfloat AngleY; // Angle For Rotation (deg)
// ---------------------------------------
// - non-static member function -
// ===== DrawGLScene ===== wxWidgets version
void BobGLCanvas:rawGLScene(){ // Drawing done here
wxClientDC dc( this ); // no 'this' if static
if( !GetContext() ){ return;}
SetCurrent();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity(); // Reset View
gluLookAt(0.0, 0.0, ZoomZ, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); //GLfloat
ZoomZ(1.50F);
glLightfv( GL_LIGHT0, GL_POSITION, lightPosition ); // Set light0
position
if( xspeed || yspeed ){
glRotatef(AngleY*yspeed, 0.0f, 1.0f, 0.0f); // Rotate On Y axis
glRotatef(AngleX*xspeed, 1.0f, 0.0f, 0.0f); // Rotate On X axis
}
else{
glRotatef(RotAngle, 0.0f, 1.0f, 0.0f); // Rotate on Y
axis
}
Material(MatNumber); // Material(2) default
glCallList( listRockNum + listOffset); // Draw the Rock
SwapBuffers(); // Swap Buffers to make rendering visible
return;
} // DrawGLScene()
//---------------------------------------------

 

[cpsmusic]

I've implemented the "static" approach and it seems to be working.

Cheers,

Chris

(e-mail address removed) wrote in message ...

Also, I tried making the member functions static. This worked, but then
I couldn't access member variables.

Cheers,
Chris

I ran into that too. Make the variables 'static' so you can access them from
'Display' and 'Keyboard'. There should not be very many. A static var can be
accessed by non-static member functions.

Otherwise, you need to post a lot more of your code[1] AND the exact errors
you are getting, so the experts in this NG can figure it out.

[1] - less the OpenGL stuff that does not apply to the problem
(glClearColor(), glShadeModel(), etc.). You should separate initialize and
draw into to different functions anyway, it will make a difference when you
animate. Example below is to show you can access static vars from non-static
member functions, not how to do OpenGL (which is off-topic in this NG).

// - in class declaration, define outside -
// ---------------------------------------
static GLfloat RotAngle;
static float xspeed; // X Rotation Speed
static float yspeed; // Y Rotation Speed
static GLfloat AngleX; // Angle For Rotation (deg)
static GLfloat AngleY; // Angle For Rotation (deg)
// ---------------------------------------
// - non-static member function -
// ===== DrawGLScene ===== wxWidgets version
void BobGLCanvas:rawGLScene(){ // Drawing done here
wxClientDC dc( this ); // no 'this' if static
if( !GetContext() ){ return;}
SetCurrent();
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity(); // Reset View
gluLookAt(0.0, 0.0, ZoomZ, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0); //GLfloat
ZoomZ(1.50F);
glLightfv( GL_LIGHT0, GL_POSITION, lightPosition ); // Set light0
position
if( xspeed || yspeed ){
glRotatef(AngleY*yspeed, 0.0f, 1.0f, 0.0f); // Rotate On Y axis
glRotatef(AngleX*xspeed, 1.0f, 0.0f, 0.0f); // Rotate On X axis
}
else{
glRotatef(RotAngle, 0.0f, 1.0f, 0.0f); // Rotate on Y
axis
}
Material(MatNumber); // Material(2) default
glCallList( listRockNum + listOffset); // Draw the Rock
SwapBuffers(); // Swap Buffers to make rendering visible
return;
} // DrawGLScene()
//---------------------------------------------

 

[Diego Martins]

Personally, I abandoned GLUT and switched to SDL

Callbacks suck a lot!

 

[Diego Martins]

Personally, I abandoned GLUT and switched to SDL

Callbacks suck a lot!