References to functions ?

Hello

I have recently read in a fairy good book ('C++ Templates: The Complete
Guide', by
David Vandevoorde, Nicolai M. Josuttis) that "both references to objects and
references to functions are acceptable" as template parameters.

My question is: Is there such thing as a reference to a function ? What
exactly does it mean and how could I use such a reference? Has someone used
this before ?

Thank you
"Timothy Madden"
Romania

re: References to functions ?

Timothy Madden wrote:[color=blue]
> Hello
>
> I have recently read in a fairy good book ('C++ Templates: The Complete
> Guide', by
> David Vandevoorde, Nicolai M. Josuttis) that "both references to objects and
> references to functions are acceptable" as template parameters.
>
> My question is: Is there such thing as a reference to a function ? What
> exactly does it mean and how could I use such a reference? Has someone used
> this before ?[/color]

A reference to a function is no different to any other reference. e.g.

int f(int);

int (*fptr)(int) = f; //or &f
int (&fref)(int) = f; //fref isn't assignable, since functions aren't.

int i = f(10);
int j = fptr(10); //or int j = (*fptr)(10)
int k = fref(10);

So you use a function reference where ever you want a reference to a
function, as opposed to a pointer to a function, or the function itself.

Tom

re: References to functions ?

Timothy Madden wrote:[color=blue]
> Hello
>
> I have recently read in a fairy good book ('C++ Templates: The Complete
> Guide', by
> David Vandevoorde, Nicolai M. Josuttis) that "both references to objects and
> references to functions are acceptable" as template parameters.
>
> My question is: Is there such thing as a reference to a function ? What
> exactly does it mean and how could I use such a reference? Has someone used
> this before ?
>[/color]

Consider this example:

template<typename fun_object>
void Foo(fun_object &f, void *data)
{
f(data);
}

void some_function(void *data)
{
// does something with data
}

class FunObj
{
public:
void operator()(void *data)
{
// does something with data
}
};

int main()
{
void *data;
FunObj f;
Foo(some_function, data);
Foo(f, data);
return 0;
}

Best
Darek

re: References to functions ?

Tom Widmer wrote:[color=blue]
> [...]
> So you use a function reference where ever you want a reference to a
> function, as opposed to a pointer to a function, or the function itself.[/color]

"As opposed"? You mean that you wouldn't use a function pointer where
ever you want a pointer to a function (as opposed to a reference)?

And could you please explain the first part of that sentence? "Use
a function reference where ever you want a reference to a function"?
What does that mean? Is the meaning the same as in "use an int pointer
wherever you want a pointer to int"? Would you say that it's the same
as "use a bar stool wherever you need a stool in a bar"?

I am not a native English speaker, you see, that's why I am asking.

Thanks!

V

re: References to functions ?

void wrote:[color=blue]
> Timothy Madden wrote:
>[color=green]
>> Hello
>>
>> I have recently read in a fairy good book ('C++ Templates: The Complete
>> Guide', by
>> David Vandevoorde, Nicolai M. Josuttis) that "both references to
>> objects and
>> references to functions are acceptable" as template parameters.
>>
>> My question is: Is there such thing as a reference to a function ? What
>> exactly does it mean and how could I use such a reference? Has someone
>> used
>> this before ?
>>[/color]
>
> Consider this example:
>
> template<typename fun_object>
> void Foo(fun_object &f, void *data)[/color]

Here is my question: except due to a mistake, why would one want to have
the '&' here in the first argument? Why not just write

template<typename fun_object>
void Foo(fun_object f, void *data)

???
[color=blue]
> {
> f(data);
> }
>
> void some_function(void *data)
> {
> // does something with data
> }
>
> class FunObj
> {
> public:
> void operator()(void *data)
> {
> // does something with data
> }
> };
>
> int main()
> {
> void *data;
> FunObj f;
> Foo(some_function, data);
> Foo(f, data);
> return 0;
> }[/color]

Yes, in your example, a reference to a function is formed (likely due to
some mistake in the argument declaration). And, yes, it's _legal_. The
question remains, however, why would one _need_ to use a reference to
a function?

Thanks.

V

re: References to functions ?

"Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
news:IcXxd.12517$Ae.8647@newsread1.dllstx09.us.to. verio.net...[color=blue]
> void wrote:[color=green]
> > Timothy Madden wrote:
> >[color=darkred]
> >> Hello
> >>
> >> I have recently read in a fairy good book ('C++ Templates: The Complete
> >> Guide', by
> >> David Vandevoorde, Nicolai M. Josuttis) that "both references to
> >> objects and
> >> references to functions are acceptable" as template parameters.
> >>
> >> My question is: Is there such thing as a reference to a function ? What
> >> exactly does it mean and how could I use such a reference? Has someone
> >> used
> >> this before ?
> >>[/color]
> >
> > Consider this example:
> >
> > template<typename fun_object>
> > void Foo(fun_object &f, void *data)[/color]
>
> Here is my question: except due to a mistake, why would one want to have
> the '&' here in the first argument? Why not just write
>
> template<typename fun_object>
> void Foo(fun_object f, void *data)
>
> ???[/color]
Please do not be so mad. The poster really answered my question and helped
me.

I could want to have the '&' in the first argument if my class in the actual
parameter is not copy-contructable or if it represents or consumes some
external resource and is designed with RAII so the constructor allocates
resources or for the case my fun_object class is a single-ton class. And
there is allways, of course, the case when my fun_object class is huge and I
do not need it copied for the purpose of function Foo

"Timothy Madden"
Romania

re: References to functions ?

Timothy Madden wrote:[color=blue]
> "Victor Bazarov" <v.Abazarov@comAcast.net> wrote in message
> news:IcXxd.12517$Ae.8647@newsread1.dllstx09.us.to. verio.net...
>[color=green]
>>void wrote:
>>[color=darkred]
>>>Timothy Madden wrote:
>>>
>>>
>>>>Hello
>>>>
>>>>I have recently read in a fairy good book ('C++ Templates: The Complete
>>>>Guide', by
>>>>David Vandevoorde, Nicolai M. Josuttis) that "both references to
>>>>objects and
>>>>references to functions are acceptable" as template parameters.
>>>>
>>>>My question is: Is there such thing as a reference to a function ? What
>>>>exactly does it mean and how could I use such a reference? Has someone
>>>>used
>>>>this before ?
>>>>
>>>
>>>Consider this example:
>>>
>>>template<typename fun_object>
>>>void Foo(fun_object &f, void *data)[/color]
>>
>>Here is my question: except due to a mistake, why would one want to have
>>the '&' here in the first argument? Why not just write
>>
>> template<typename fun_object>
>> void Foo(fun_object f, void *data)
>>
>>???[/color]
>
> Please do not be so mad.[/color]

Mad? Do I really come across as mad? I am sorry. It was by no means my
intention. I am trying to learn C++ just like all of us here. That's why
I asked. Perhaps I _am_ mad if my hopes are so high :-)
[color=blue]
> The poster really answered my question and helped
> me.
>
> I could want to have the '&' in the first argument if my class in the actual
> parameter is not copy-contructable or if it represents or consumes some
> external resource and is designed with RAII so the constructor allocates
> resources or for the case my fun_object class is a single-ton class. And
> there is allways, of course, the case when my fun_object class is huge and I
> do not need it copied for the purpose of function Foo[/color]

Hey, that's a very good explanation. Thank you. I've not considered the
use of functors that are not copy-constructible.

V