Re: [boost] call_traits

From: Graeme Prentice (gp.kiwi_at_[hidden])
Date: 2005-09-24 21:40:51

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Jason Hise
> Sent: Sunday, 25 September 2005 2:11 p.m.
> To: boost_at_[hidden]
> Subject: [boost] call_traits
> 
> 
> Why doesn't the following compile?  I get the error "foo: 
> function does 
> not take 1 arguments" on VC8.
> 
> #include <boost/call_traits.hpp>
> 
> template < typename P1 >
> void foo ( typename ::boost::call_traits < P1 >::param_type p1 )
> {
> }
> 
> int main (  )
> {
>     foo ( 42 ); // error here
>     return 0;
> }

This is the infamous non-deduced context.  A nested name specifier
(call_traits) is a non-deduced context so P1 cannot be deduced.  You have to
write foo<int>(42).  This is like an invisible cast, so you have to remember
to change all the explicit type specifiers if you change the type of an
argument being passed in such a function.

> 
> (Background info: I would like to use this technique to generate 
> optimized forwarding functions that do not know anything about the 
> function being forwarded to.  Overloads taking any number of params 
> would be generated with the boost preprocessor library.)
> 
> -Jason

It might be better to write specializations for foo rather than specify the
types at the point of call.

Graeme