...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::functional::flatten — A PolymorphicFunctionObject type that returns a "flattened" view of a Proto expression tree.

// In header: <boost/proto/fusion.hpp> struct flatten : proto::callable { // member classes/structs/unions template<typename This, typename Expr> struct result<This(Expr)> : result< This(Expr const &) > { }; template<typename This, typename Expr> struct result<This(Expr &)> : proto::result_of::flatten< Expr > { }; // public member functions template<typename Expr> typename proto::result_of::flatten< Expr >::type const operator()(Expr &) const; template<typename Expr> typename proto::result_of::flatten< Expr const >::type const operator()(Expr const &) const; };

A PolymorphicFunctionObject type that returns a "flattened" view
of a Proto expression tree. For a tree with a top-most node tag of type
`T`

, the elements of the flattened sequence are determined by
recursing into each child node with the same tag type and returning those nodes of different
type. So for instance, the Proto expression tree corresponding to the expression
`a | b | c`

has a flattened view with elements [a, b, c], even
though the tree is grouped as `((a | b) | c)`

.

`flatten`

public member functionstemplate<typename Expr> typename proto::result_of::flatten< Expr >::type const operator()(Expr & expr) const;

template<typename Expr> typename proto::result_of::flatten< Expr const >::type const operator()(Expr const & expr) const;