what is the difference in :

typedef void (fn) ();

and

typedef void (*fn) ();

the first one is used by my course reader for passing callback
functions as arguments to other functions.
the second one is used in a command dispatch table which is basically
a map with the strings as keys(names of functions) and function
pointers as values.

On 2007-12-10 05:22:25 -0500, "riddhi.mittal@gmail.com"
<riddhi.mittal@gmail.com> said:

>
> what is the difference in :
>
> typedef void (fn) ();
>
> and
>
> typedef void (*fn) ();
>
> the first one is used by my course reader for passing callback
> functions as arguments to other functions.

No, it's not. Those other functions don't take this type, they take
pointers to this type.

> the second one is used in a command dispatch table which is basically
> a map with the strings as keys(names of functions) and function
> pointers as values.

Yup. The first is a function type and the second is a pointer to that
same function type. It's exactly the same as the difference between int
and int* Well, almost; a function type can't be used in as many places
as an int.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

On Dec 10, 2:09 pm, Pete Becker <p...@versatilecoding.com> wrote:
> On 2007-12-10 05:22:25 -0500, "riddhi.mit...@gmail.com"
> <riddhi.mit...@gmail.com> said:
> > what is the difference in :

> > typedef void (fn) ();

> > and

> > typedef void (*fn) ();

> > the first one is used by my course reader for passing callback
> > functions as arguments to other functions.

> No, it's not. Those other functions don't take this type, they take
> pointers to this type.

> > the second one is used in a command dispatch table which is basically
> > a map with the strings as keys(names of functions) and function
> > pointers as values.

> Yup. The first is a function type and the second is a pointer to that
> same function type. It's exactly the same as the difference between int
> and int* Well, almost; a function type can't be used in as many places
> as an int.

What's probably confusing him is the fact that when used to
declare a parameter to a function, a function type is remapped
to a pointer to function type, much in the same way an array is
remapped to a pointer. So if he has something like:

void registerCallback( fn ) ;

, this is implicitly converted to:

void registerCallback( fn* ) ;

if the first typedef is used.

It's one of those obfuscation features C++ inherited from C.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
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