Re: [Boost-users] [boost parameter] misusing deduced - thanks, but sill unsure about the next step

25 Feb 2009

      David Abrahams wrote:
...
on Sun Feb 22 2009, er  wrote:
...
I've realized I wasn't using deduced properly. Something like this?
typedef parameter::parameters<
      parameter::required<
            parameter::deduced,
            is_samempl::_1,self_type
        >
...
params;
Should allow and ArgPack of the form ((tag = x)) or ((x)), provided x is of type
self_type, right?
Yes, that looks right to me.
Thank you for your answer, but I would still need a bit of help:

I'm trying to fetch an argument only by its type, not its name or position.

For example, I can't replace ((x)) above by say ((x,y)) where only x has 
the desired type, because apparently, this is not a proper way to form 
an ArgPack.

I've tried to simplify my intent in the code below.

*.hpp
struct no_name_{};
namespace tag{
     ::boost::parameter::keyword&
             no_name =
             ::boost::parameter::keyword::get();
}//tag
struct A{
	typedef parameter::parameters<
		parameter::required<
             parameter::deduced,
             is_samempl::_1,A
         >
	> params;

	explicit A(double x):x_(x){}

	template<typename Args>
	A(const Args& args):x_(0){
		const A& a = (params()(args))[tag::no_name];
		x_ = a.x_;
	};
	double x_;
};

*.cpp
     A a0(0.9);
     A a1((a0,0.5));// left-hand operand of comma has no effect
     std::cout << "a1.x_=" << a1.x_ << std::endl; // I want 0.9 here