[Boost-users] [variant] apply_visitor/static access to which()

Subject: [Boost-users] [variant] apply_visitor/static access to which()
From: Stefan Strasser (strasser_at_[hidden])
Date: 2009-11-18 15:00:55

Hi,

I have a problem regarding the static visitor of boost::variant.

there is a variant and a mpl vector:

variant<A1,A2,A3> var;
typedef mpl::vector<B1,B2,B3> B_types;

the variant and the vector have the same number of types, but not the same 
types.

I need to "visit" the element of the variant, and access the B_types element 
at the same position as the element of the variant:

struct visitor : static_visitor<void>{
  template<class A>
  void operator()(A a) const{
     typedef typename mpl::at_c<B_types,var.which()>::type B; //error
  }
};

so if var.which() == 1, I need a typedef of type B2. obviously the code above 
doesn't work because which() is a runtime function. but the value of which() 
is known statically at this point (that's the point of visitation).

is there a way to access it, with boost::variant visitors?

I like neither of the solutions I can come up with:
1. write my own visitation pattern that not only instantiates the visitor with 
the variant type but also with the vector type.

2.
convert the variant to

template<class A,class B>
struct A_holder{
  typedef B B_type;
  A value;
};
variant<A_holder<A1,B1>,A_holder<A2,B2>,A_holder<A3,B3> > var;

3. do a seperate "visition" of the mpl vector inside the variant visitor:
struct visitor : static_visitor<void>{
  template<class A>
  void operator()(A a) const{
     switch(var.which()){
       case...
       case...
     }
  }
};

is there a better way to do this?