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Struct template fold_tree

boost::proto::fold_tree — A PrimitiveTransform that recursively applies the proto::fold<> transform to sub-trees that all share a common tag type.


// In header: <boost/proto/transform/fold_tree.hpp>

template<typename Sequence, typename State0, typename Fun> 
struct fold_tree :  proto::transform< fold_tree<Sequence, State0, Fun> > {
  // member classes/structs/unions
  template<typename Expr, typename State, typename Data> 
  struct impl :  
    proto::fold<Sequence, State0, recurse_if_<typename Expr::proto_tag, Fun> >
      ::template impl<Expr, State, Data>


proto::fold_tree<> is useful for flattening trees into lists; for example, you might use proto::fold_tree<> to flatten an expression tree like a | b | c into a Fusion list like cons(c, cons(b, cons(a))).

proto::fold_tree<> is easily understood in terms of a recurse_if_<> helper, defined as follows:

 template<typename Tag, typename Fun>
struct recurse_if_ :
    // If the current node has type type "Tag" ...
    boost::is_same<proto::tag_of<proto::_>, Tag>(),
    // ... recurse, otherwise ...
    proto::fold<proto::_, proto::_state, recurse_if_<Tag, Fun> >,
    // ... apply the Fun transform.

With recurse_if_<> as defined above, proto::fold_tree<Sequence, State0, Fun>()(expr, state, data) is equivalent to:

  recurse_if_<typename Expr::proto_tag, Fun>
>()(expr, state, data).

It has the effect of folding a tree front-to-back, recursing into child nodes that share a tag type with the parent node.