Re: [Boost-users] [variant] Variant call forwarding?

Subject: Re: [Boost-users] [variant] Variant call forwarding?
From: Mathias Gaunard (mathias.gaunard_at_[hidden])
Date: 2010-02-23 14:31:30

Rutger ter Borg a écrit :
> Hello,
> 
> I'm wondering what's the easiest way to enable call forwarding by a variant 
> to contained function objects accepting the same arguments?
> 
> I.e.,
> 
> variant< F, G > m_funcs;
> 
> m_funcs = f;
> m_funcs( a, b, c );
> 
> m_funcs = g;
> m_funcs( a, b, c );

Let's consider that's the signature is R (T1, T2, T3).

struct call_visitor : static_visitor<R>
{
     call_visitor(T1&& a_, T2&& b_, T3&& c_) : a(a_), b(b_), c(c_)
     {
     }

     template<typename F>
     R operator()(F& f) const
     {
         return f(a, b, c);
     }

     T1&& a;
     T2&& b;
     T3&& c;
};

Then you could do what you want with

apply_visitor(call_visitor(a, b, c), m_funcs);

If you want to do it for arbitrary signatures and provide perfect 
forwarding without rvalue references, it is going to be a bit more work.

With polymorphic lambdas (an extension to C++0x lambdas), it is quite 
simpler:

apply_visitor([&](auto& f) { return f(a, b, c); }, m_funcs);