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References to functions ?

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Timothy Madden
Guest

Posted: Tue Dec 21, 2004 2:17 pm Post subject: 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

Tom Widmer
Guest

Posted: Tue Dec 21, 2004 2:59 pm Post subject: Re: References to functions ?

Timothy Madden wrote:

Quote:

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 ?

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

void
Guest

Posted: Tue Dec 21, 2004 3:10 pm Post subject: Re: References to functions ?

Timothy Madden wrote:

Quote:

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

Victor Bazarov
Guest

Posted: Tue Dec 21, 2004 3:18 pm Post subject: Re: References to functions ?

Tom Widmer wrote:

Quote:

[...]
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.

"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

Victor Bazarov
Guest

Posted: Tue Dec 21, 2004 3:22 pm Post subject: Re: References to functions ?

void wrote:

Quote:

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)

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 void Foo(fun_object f, void *data)

???

Quote:

{
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;
}

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

Timothy Madden
Guest

Posted: Tue Dec 21, 2004 5:11 pm Post subject: Re: References to functions ?

"Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote

Quote:

void wrote:
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 void Foo(fun_object &f, void *data)

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 void Foo(fun_object f, void *data)

???
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

Victor Bazarov
Guest

Posted: Tue Dec 21, 2004 5:16 pm Post subject: Re: References to functions ?

Timothy Madden wrote:

Quote:

"Victor Bazarov" <v.Abazarov (AT) comAcast (DOT) net> wrote in message
news:IcXxd.12517$Ae.8647 (AT) newsread1 (DOT) dllstx09.us.to.verio.net...

void wrote:

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 void Foo(fun_object &f, void *data)

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 void Foo(fun_object f, void *data)

???

Please do not be so mad.

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 :-)

Quote:

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

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

V

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