Re: [boost] [parameter] Lack of safety?

From: David Abrahams (dave_at_[hidden])
Date: 2006-04-12 17:34:44

"Fred Bertsch" <fred.bertsch_at_[hidden]> writes:

> There are cases where boost::parameter can translate its arguments
> to the wrong arguments at the function implementation side.  I'm
> hoping that I'm doing something wrong, but it seems like a problem
> with the parameter library.

I guess it depends on your point of view, but I don't think this can
reasonably be viewed as a library problem

> The parameter library suggests creating global variables named with
> the names of the arguments to your function. 

Well, namespace-scope constants, yeah.

> If you have an inner scope at the calling site with a variable with
> the same name, things don't work. 

It seems natural to me that if you have something declared at an inner
scope with the same name and you use that name without qualification,
the compiler picks up the inner declaration (except in the case of
ADL, which is an abomination ;->).  That's just the normal C++.

> I'm okay if this simply results in a compiler error, but in some
> fairly simple cases, it can correctly compile to the wrong call to
> the implemented function.
>
> Here's an example:
>
> void FooImpl( int x, int y ) {
>   // Gets x = 8, y = 14. We wanted x = 12, y = 8.
> }
>
> BOOST_PARAMETER_KEYWORD( tags, x )
> BOOST_PARAMETER_KEYWORD( tags, y )
>

<schnipp most of the Boost.Parameter related stuff because it's
not relevant to the problem>

> void Bar( int y ) {
>   ...
>   Foo( y = 8 );
         ^^^^^
The problem happens right here, before the parameter library can
possibly get a chance to intervene and do something wrong (or right).

I realize why, in a language with built-in support for named
parameters, you might expect this to work, but we can't work magic
here --- we can only get pretty close :^).  In C++, what would you
expect the expression

       y = 8

to do in the context of this function body?  If you want to reference
the keyword object, you either need to avoid hiding it (pick a
different name for your function parameter), or use qualification to
reference the other symbol.  In this case, since your keyword objects
are in the global namespace, it would be

   ::y = 8

HTH,

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com