[boost] Re: [Review] Named parameters library

From: David Abrahams (dave_at_[hidden])
Date: 2004-11-18 09:38:24

Daniel Wallin <dalwan01_at_[hidden]> writes:

> Joel de Guzman wrote:
>> 1) WRT DSL, I'm not quite sure about the syntax for
>> defaults. Example:
>>     params[name | "unnamed"]
>> I have a strong feeling that there's a better syntax but I am
>> not sure what it is now. I read in the thread that the syntax
>>     params[name] | "unnamed"
>> is not doable, but I'm not quite sure. Perhaps a clarification
>> will set me straight. Having the default in the brackets seems to
>> rather awkward. Perhaps:
>>     params[name].defaults_to("unnamed")
>> is better?
>
> That has exactly the same problem as having the | outside the brackets.
> The problem is that operator[] would need to return something other than
> the actual parameter reference, something that:
>
>    1) Is convertible to the parameter type.
>
>         We still need params[name], with no default to work.
>
>    2) Has operator|() or defaults_to() member function.
>
> So, we'd need a conversion operator. But if we have a conversion
> operator, we can't reliably pass the arguments to function templates
> because the type deduction will go wrong.

One possible alternative is

    params.defaults_to("unnamed")[name]

but I find that highly illogical to read, because the specification
of the default precedes the keyword it's defaulting.

However, Thorsten's prodding has made me wonder if we need || for the
lazy case at all.  It seems as though

    params.has(name) ? params[name] : something_else;

is superior in every way except brevity.  And there will be
substantial cases where it's briefer as well, because there's no need
to build a function object for something_else.

Am I missing something?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com