[rabidgnome229]

I'm writing an opengl app (GLUT) in C++ and I'm getting an error when attempting to set the callbacks. The signature for the first is this

Code:

void glutDisplayFunc(void (*func)(void));

In one of my classes I have a function draw declared as such (irrelevant portions omitted)

Code:

#ifndef _GRAPHICS_ENGINE_H
#define _GRAPHICS_ENGINE_H

class GraphicsEngine{
public:
void draw(void);
};

#endif

In my implementation I have the following

Code:

void GraphicsEngine::draw(void){
}

void GraphicsEngine::init(int *argc, char **argv){
glutDisplayFunc(draw);
}

On compilation, I get the following error

Code:

src/GraphicsEngine.cpp:42: error: argument of type 'void (GraphicsEngine::)()' does not match 'void (*)()'

If I'm reading that error correctly its the fact that the function is a member of a class that's causing the problem. If so, how can I set a member function as the glut callback? If not, what is the problem? Thanks in advance

 

[pippolo]

Quote:

Originally Posted by rabidgnome229 If I'm reading that error correctly its the fact that the function is a member of a class that's causing the problem. If so, how can I set a member function as the glut callback? If not, what is the problem? Thanks in advance

Your assumption is correct.
Every function (not static) member of a class A is really a function with an implicit first argument of type A*, and the argument is named this.
So, the signature of your draw function is really:

Code:

void GraphicsEngine::draw(GraphicsEngine *this){
}

and when you write

Code:

GraphicsEngine a;
a.draw();

you are really calling

Code:

draw(a);

and this is incompatible with the glutDisplayFunc signature.

A possible solution (but not the ONLY possible) is to use a static member function. A static function is indipendent from the class object and doesn't have the additional this argument.

Code:

class GraphicsEngine{
public:
static void draw(void);
};

 

[im_not_an_artard]

Quote:

Originally Posted by pippolo Your assumption is correct. Every function (not static) member of a class A is really a function with an implicit first argument of type A*, and the argument is named this. So, the signature of your draw function is really: Code: void GraphicsEngine::draw(GraphicsEngine *this){ } and when you write Code: GraphicsEngine a; a.draw(); you are really calling Code: draw(a); and this is incompatible with the glutDisplayFunc signature. A possible solution (but not the ONLY possible) is to use a static member function. A static function is indipendent from the class object and doesn't have the additional this argument. Code: class GraphicsEngine{ public: static void draw(void); };

^ This

as well you should declare as

Code:

GraphicsEngine *a;

a = new GraphicsEngine;

draw(a);

Edit: wait a sec, i may have misred ur code, you may have to use "this" in ur calls or something

i.e.

Code:

glutDisplayFunc(this::draw);

or

glutDisplayFunc(this->draw);

i think thats the syntax

 

[rabidgnome229]

That's abot what I figured. The class is a singleton class so I ended up putting this in my implementation file

Code:

static void draw_workaround(void){
        GraphicsEngine::instance()->draw();
}

void GraphicsEngine::glInit(int *argc, char **argv){
        glutDisplayFunc(draw_workaround);
 }

One of the first things I tried was this->draw that artard posted, but that didn't work either.

 

[im_not_an_artard]

Quote:

Originally Posted by rabidgnome229 That's abot what I figured. The class is a singleton class so I ended up putting this in my implementation file Code: static void draw_workaround(void){ GraphicsEngine::instance()->draw(); } void GraphicsEngine::glInit(int *argc, char **argv){ glutDisplayFunc(draw_workaround); } One of the first things I tried was this->draw that artard posted, but that didn't work either.

isnt coding fun

 

[pippolo]

Quote:

Originally Posted by im_not_an_artard ^ This as well you should declare as Code: GraphicsEngine *a; a = new GraphicsEngine; draw(a);

my mistake... the last row should be

Code:

draw(&a);

Quote:

Originally Posted by im_not_an_artard Edit: wait a sec, i may have misred ur code, you may have to use "this" in ur calls or something i.e. Code: glutDisplayFunc(this::draw); or glutDisplayFunc(this->draw); i think thats the syntax

No. The problem with a non static member function is that the invocation needs a pointer to a class object, but the invocation is made by the glut library. The glutDisplayFunc asks for a

Code:

void func(void)

function, but a non static member function is really

Code:

void func(obj*)

.

Quote:

Originally Posted by rabidgnome229 That's abot what I figured. The class is a singleton class so I ended up putting this in my implementation file Code: static void draw_workaround(void){ GraphicsEngine::instance()->draw(); } void GraphicsEngine::glInit(int *argc, char **argv){ glutDisplayFunc(draw_workaround); } One of the first things I tried was this->draw that artard posted, but that didn't work either.

the version with the draw_workaround should works (I hope !).

 

[Kirmie]

http://www.parashift.com/c++-faq-lit....html#faq-33.1

This has some info related to pointers to member functions. Might help you out.

Edit: The info I linked to describes the same situation that pippolo is saying so hopefully it will work. Good luck!