...one of the most highly
regarded and expertly designed C++ library projects in the
world.
— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards
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_ : proto::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. Fun > {};
With recurse_if_<>
as defined above,
proto::fold_tree<Sequence, State0, Fun>()(expr, state, data)
is equivalent to:
proto::fold< Sequence, State0, 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.