Re: [boost] [lambda] extension question

From: Joel de Guzman (joel_at_[hidden])
Date: 2005-07-19 02:50:15

Jaakko Järvi wrote:
> On Jul 18, 2005, at 6:19 PM, Brock Peabody wrote:
> 
>>When I try to compile my example in VC 7.1 I get an error that seems to
>>indicate that the type being passed to the sig template is
>>boost::tuples::null_type.  Everything works if I hardwire it like:
>>
>>   template <class T> struct sig { typedef std::string type; };
>>
> 
> 
> Yes, a null_type is passed to sig. This is because the signature of the 
> zero argument
> lambda function (in lambda_functor class) signature is instantiated 
> always, regardless
> of whether it is called or not. And there the argument list is just 
> null_type.
> Hence, the return type deduction is not allowed to cause a compilation 
> error in the
> case where the argument tuple to sig is null_type.

FYI, this is a typical problem. Phoenix-2 has fixed this nagging
problem.

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net