Variadic functions with no parameters

[peter_ammon]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? And what
was the motivation for allowing functions of this type where C forbids
them?

I also notice that C++ makes the comma optional, whereas it's mandatory
in C. That is, this is a legal prototype:

void function2(int ...);

What is the motivation for this change?

Followups set to comp.lang.c++

Thanks,
-Peter

 

[Victor Bazarov]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? [..]

There is no way.

 

[Howard]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? [..]

There is no way.

Eh?

Isn't that what the macros va_list, va_start, va_arg, and va_end are for?

-Howard

 

[Ken Human]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? [..]

There is no way.

Eh?

Isn't that what the macros va_list, va_start, va_arg, and va_end are for?

-Howard

Yes, but va_start requires a pointer to the first argument in the list.
The function's format must be similiar to "void function(T
pFirstElement, ...);"

 

[Victor Bazarov]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? [..]

There is no way.

Eh?

Isn't that what the macros va_list, va_start, va_arg, and va_end are for?

Yes they are. Please explain how you'd use them in the case where
there are no _named_ arguments.

 

[Howard]

(e-mail address removed) wrote:

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? [..]

There is no way.

Eh?

Isn't that what the macros va_list, va_start, va_arg, and va_end are for?

Yes they are. Please explain how you'd use them in the case where
there are no _named_ arguments.

Oh, I get your point now. I forgot that you need one named parameter to get
started. Thanks.
-Howard

 

[James Dennett]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

Yes, it's allowed.

In such a function, how do you access any of the parameters?

You cannot portably do so. It may be possible in
implementation-specific ways.

And what
was the motivation for allowing functions of this type where C forbids
them?

I don't know what the motivation was (apart from it being simpler
than disallowing them), but this does have useful consequences in
providing a worst-match for overload resolution, which can be
useful in situations such as template metaprogramming.

I also notice that C++ makes the comma optional, whereas it's mandatory
in C. That is, this is a legal prototype:

void function2(int ...);

What is the motivation for this change?

D&E might answer that, though I'm not sure if it does and don't
have my copy handy right now to check.

-- James

 

[Krzysztof Zelechowski]

The C++ grammar appears to admit (and g++ accepts)

void function(...);

In such a function, how do you access any of the parameters? And what
was the motivation for allowing functions of this type where C forbids
them?

The reason is overloading. C language forbids overloaded functions, so
there is no point of declaring formal parameters and not using them
afterwards. You can just do without them.
Chris