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Groups > comp.std.c++ > #680

Re: proposal : keyword default

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On 10/7/2013 3:50 AM, Francis Glassborow wrote:
>
> On 06/10/2013 03:19, Edward Diener wrote:
>>
>> On 10/4/2013 2:58 PM, Paul D. DeRocco wrote:
>>>>
>>>>
>>>> On 10/3/2013 3:25 PM, alexander.stoyan@googlemail.com wrote:
>>>>
>>>> Instead it would be elegant to just use:
>>>>
>>>> thread_t thr_id = 0;
>>>> make_thread(thread_function, default, default, default, &thr_id);
>>>
>>>
>>>
>>> I didn't see the earlier parts of this thread, so this may have come up.
>>> Has the idea of leaving out the arguments entirely come up? E.g.,
>>>
>>> make_thread(thread-function, , , , &thr_id);
>>
>>
>>
>> I have always liked this idea. I see no reason for passing a keyword
>> such as 'default'. Nor do I see a downside for leaving out a value
>> entirely when you want the compiler to use the default value in the
>> function declaration. I do not believe there is any other use for such
>> a function invocation, so there is no chance of breaking existing
>> code.
>>
>> Not passing anything should simply say "Pass the default value". If
>> there is no default value for that positional parameter and the
>> invoker does not pass anything it is a compiler error. Simple,
>> straightforward, and cannot be confused with anything else when
>> invoking a function.
>>
>
> I would lay heavy odds that all this discussion is academic because
> the current rules are not going to change ever :)

How many times has that been said about almost anything only to be
proved wrong !

>
> If you stop to consider it, it should be clear that this would have
> been considered right at the start of the design of default arguments
> for C++. It wasn't that it was hard to do, so there must have been
> other reasons (I am pretty sure they will have been mentioned
> somewhere in 'The Design and Evolution of C++')

You may be crediting Mr. Stroustrup with too much foresight. Nonetheless
if you have any particular discussion to cite about this in D&E or
otherwise I would be glad to peruse it.

>
> If anything, changing the rules now would be more difficult because
> the whole language is so much more complicated.

I do not see anything in the invocation of
somefunction(somedata,,,someotherdata) that could be confused with
anything else in C++. It does not matter that the "whole language is so
much more complicated" for this one situation. Is there any other area
of C++ where the syntax of commas following each other with just
potentially whitespace between them is ever used ?

> In addition, it is
> entirely unnecessary as overloading is generally a more effective
> approach as you can generally decide which parameters to default.
> Inlining means that there is no loss of executable efficiency in
> either space or speed.

I agree that overloading is better than using default parameters in the
vast majority of cases. But since default parameters are still in large
use, especially in APIs not specifically geared toward C++, such as the
Windows header files, why not make it easier in C++ to invoke such
functions.

Furthermore the same technique could be used for class templates which
provide default parameters. Essentially the sequence ",," has no
syntactical meaning in C++. I do realize that the comma operator can be
overloaded in C++ but that has close to zero chance of interfering with
using ",," to specify a default function parameter in a function
invocation or a default template parameter in a class template
instantiation.

Nonetheless I appreciate your (dis)interest and the next step is to
write a proposal for the C++ standards committee.


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Thread

Re: proposal : keyword default alexander.stoyan@googlemail.com - 2013-10-03 13:25 -0600
  Re: proposal : keyword default Daniel Krügler <daniel.kruegler@googlemail.com> - 2013-10-04 11:19 -0700
  Re: proposal : keyword default "Paul D. DeRocco" <pderocco@ix.netcom.com> - 2013-10-04 11:58 -0700
    Re: proposal : keyword default Bo Persson <bop@gmb.dk> - 2013-10-05 19:19 -0700
      Re: proposal : keyword default Edward Diener <eldiener@tropicsoft.invalid> - 2013-10-07 01:50 -0600
    Re: proposal : keyword default Edward Diener <eldiener@tropicsoft.invalid> - 2013-10-05 19:19 -0700
      Re: proposal : keyword default Francis Glassborow <francis.glassborow@btinternet.com> - 2013-10-07 01:50 -0600
        Re: proposal : keyword default Edward Diener <eldiener@tropicsoft.invalid> - 2013-10-07 15:05 -0600
          Re: proposal : keyword default James Kuyper <jameskuyper@verizon.net> - 2013-10-08 12:36 -0700
          Re: proposal : keyword default Helmut Zeisel <zei2011@liwest.at> - 2013-10-12 03:47 -0600
            Re: proposal : keyword default Edward Diener <eldiener@tropicsoft.invalid> - 2013-10-12 11:36 -0600
              Re: proposal : keyword default ootiib@hot.ee - 2013-10-12 23:10 -0600
              Re: proposal : keyword default John Harris <niam@jghnorth.org.uk.invalid> - 2013-10-13 11:33 -0600
                Re: proposal : keyword default Edward Diener <eldiener@tropicsoft.invalid> - 2013-10-16 10:34 -0600
              Re: proposal : keyword default Helmut Zeisel <zei2011@liwest.at> - 2013-10-14 20:37 -0700
  Re: proposal : keyword default Alexander Terekhov <terekhov@web.de> - 2013-10-18 01:20 -0600

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