Re: Unidentified subject!

[Erik Mouw <J A K Mouw its tudelft nl>]

On Sun, 16 Jan 2000 13:57:25 -0800 (PST), Kaz Kylheku wrote:
> On Sun, 16 Jan 2000, Jeroen Ruigrok/Asmodai wrote:
>> >
>> >typedef void    (*GtkItemFactoryCallback)  ();
>> >
>> >to
>> >
>> >typedef void    (*GtkItemFactoryCallback)  (void);
>> 
>> I mistook them for empty braces.  But I understand these should be
>> variable argument cases.  Am I correct in this assumption?
> 
> No. The way to declare a variadic function in C and C++ is to use ellipses.  An
> empty parameter list in C++ declares a function that takes no parameters, like
> the (void) list in ANSI C.  An empty parameter list in C declares a function
> which takes a fixed number of arguments, but does not specify that number, or
> their types.

Sorry, but that's not correct.  A declaration like 'int foo()' means that
it is a K&R style function declaration. K&R C didn't have a way to specify
"unspecified number of arguments", so printf could be declared like "int
printf()".

ANSI C introduced a stricter function prototype in which you had to
specify the exact number of parameters. Functions like printf() didn't fit
in this scheme, so the ellipsis notation was invented: "int printf(const
char *format, ...)". See section A7.3.2 "Function Calls" from "The C
Programming Language, second edition" by Kernighan and Ritchie.

As far as I understand from gtkitemfactory.h, GtkItemFactoryCallback is
used a placeholder for two different types of callback functions:

  typedef void    (*GtkItemFactoryCallback1) (gpointer           callback_data,
                                              guint              callback_action,
                                              GtkWidget          *widget);


  typedef void    (*GtkItemFactoryCallback2) (GtkWidget          *widget,
                                              gpointer           callback_data,
                                              guint              callback_action);

Unfortunately, those two types don't share the same first parameter, so it
is impossible to make a vararg function.


Erik

-- 
J.A.K. (Erik) Mouw, Information and Communication Theory Group, Department
of Electrical Engineering, Faculty of Information Technology and Systems,
Delft University of Technology, PO BOX 5031,  2600 GA Delft, The Netherlands
Phone: +31-15-2785859  Fax: +31-15-2781843  Email J.A.K.Mouw@its.tudelft.nl
WWW: http://www-ict.its.tudelft.nl/~erik/