Re: [Boost-users] [Proto] Building a mpl::vector from an expression

Subject: Re: [Boost-users] [Proto] Building a mpl::vector from an expression
From: christophe henry (henry_christophe_at_[hidden])
Date: 2009-08-19 10:23:33

"Eric Niebler" schrieb im Newsbeitrag news:<4A8BB56F.7050706_at_[hidden]>... 
> Steven Watanabe wrote: 
> > christophe henry wrote: 
> >> I am trying to build a mpl::vector from an expression like 
> >> 
> >> (string(),ClassA(),ClassB()) 
> >> 
> >> To get a mpl::vector 
> >> 
> >> 
> >> Is there a way to do this or I should give up the idea? 
> > 
> > Does fold_tree help? 
> 
> It does. 
> 

*snip*

 

 

Ah It see. Thanks a lot, I think I got the idea. 

I just have a concern with:

 

     template<class T> struct is_my : mpl::false_ {};
     template<> struct is_my<std::string> : mpl::true_ {};
     template<> struct is_my<ClassA> : mpl::true_ {};
     template<> struct is_my<ClassB> : mpl::true_ {};

As it forces extra declarations which not everybody is willing to make. But based on the same fold idea I can allow something like:

attributes_ << int() << std::map<int,int>() << bool()

 

Where attributes_ is made a proto terminal<my_tag> with proto::extends and the following grammar (which I shamelessly stole from the doc):

 

struct BuildAttributes

: proto::or_<

// Don't add the flag terminal to the list

proto::when<

     proto::terminal< my_tag >

   , proto::_state

  >

// Put all other terminals at the head of the

// list that we're building in the "state" parameter

, proto::when<

    proto::terminal<proto::_>

  , fusion::cons<proto::_value, proto::_state>(proto::_value, proto::_state)

  >

// For left-shift operations, first fold the right

// child to a list using the current state. Use

// the result as the state parameter when folding

// the left child to a list.

, proto::when<

     proto::shift_left<BuildAttributes, BuildAttributes>

   , proto::fold<

        proto::_

      , fusion::nil()

      , BuildAttributes>()

  >

>

{};

 

I then just have to invert the order again as it gets reversed (ok a few seconds more to compile...):

 

boost::mpl::copy<typename boost::result_of<BuildAttributes(Attr)>::type,boost::mpl::front_inserter<boost::mpl::vector<> > >::type

 

Correct?

 

Thanks a lot,

 

Christophe

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