Re: [boost] determine first arg of callable

Subject: Re: [boost] determine first arg of callable
From: Oliver Kowalke (oliver.kowalke_at_[hidden])
Date: 2013-05-15 12:30:38

2013/5/15 Peter Dimov <lists_at_[hidden]>

> This still doesn't quite explain it. Let's say that my fn takes a
> std::string. How does D know what string to pass? An empty string? "foo"?
> "hello world"? Where does the argument value come from?
>

Fn does takes a Q< std::string > not a plain std::string (I've omitted this
detail in my first example). Q<> might be default constructable.

> template< typename Arg >
> struct Q {...};
>

does something irrelevant

> struct B
> {
>  virtual void run() = 0;
> };
>

interface

> template< typename Fn, typename Arg >
> struct D : public B
> {
>  Fn fn;
>
>  D( Fn fn_) : fn( fn_) {}
>
>  void run() {
>    Q< Arg > q;
>    fn( q)
>  } // argument construction omitted
> };
>

concrete implementation - calls default ctor of Q< Arg > in D::run() and
passes instance of Q< Arg > to fn

> struct X
> {
>  B * b;
>
>  template< typename Fn >
>  X( Fn fn) : b( 0)
>  {
>     typedef typename Magic< Fn >::arg1_type arg_type;
>     b = new D< Fn, arg_type >( fn);
>  }
>
>  void run() { b->run(); }
> };
>

 bundles the stuff

void g( Q< std::string > &) {...}
X x( g);
x.run(); // call g() with an instance of Q< std::string > as arg

struct W
{
   void h( Q< int > &) {...}
};
W w;
X x( bind( & W::h, w, _1) );
x.run(); // call W::h() with an instance of Q< int > as arg