Re: When ... argument should be used?

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 7 Dec 2009 02:15:02 -0800 (PST)

Message-ID:

<5d6e3888-7e01-4584-b596-2bc6d534f9d1@p30g2000vbt.googlegroups.com>

On Dec 5, 4:19 pm, Peng Yu <pengyu...@gmail.com> wrote:

On Dec 5, 8:18 am, Victor Bazarov <v.Abaza...@comAcast.net> wrote:

Jonathan Lee wrote:

On Dec 4, 8:47 pm, Peng Yu <pengyu...@gmail.com> wrote:

I'm wondering what is the general
guideline in use '...' as the function argument.

My own guideline would be "don't do it". I can't speak
about the history of the <cstdarg> functions, but I would
guess they're obsolete in the face of passing classes,
overloading operators, etc. Probably for every use of it,
there would be a "more C++" way of doing the same thing.

I've personally never encountered a use for it.

Aside from the need to reuse the output mechanism provided
by 'printf' (which is, after all, quite convenient if you
think about it), there has never been other serious need in
the ellipsis in all my years either.

This prompts me the following questions:

1. What is the original purpose of having ... argument in C

Support for things like printf and execl.

2. For what disadvantages, ... argument become deprecated in C++?

Can't interface some Posix system functions. (In other words,
it ain't gonna happen.)

3. What language construct in C++ are used to fulfill the
original purpose of ... argument in C?

The usual solution is to provide some sort of collector class,
and use it as an argument. So you could write something like:

f( collector.with(a).with(b).with(c) );

I have been using C++ for a long time. It is natural for me
not to use ... argument.

However, '...' is extensively used in R (www.r-project.org),
which is the language that I am learning now. The usage of
'...' in R is similar to the usage of '...' in C, but not
exactly. The fact intrigues me that the similar language
construct (i.e., '...') gets deprecated in one language but
gets widespread in another. Is there a summary on the pros and
cons of using '...' (at least in C++)?

Pros: none that I know of.
Cons: no type safety (even at runtime).

--
James Kanze