...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::or_ — For matching one of a set of alternate grammars. Alternates are tried in order to avoid ambiguity.
When used as a transform, proto::or_<>
applies the transform
associated with the first grammar that matches the expression.
// In header: <boost/proto/matches.hpp>
template<typename... G>
struct or_ : proto::transform<or_<G...> > {
// types
typedef or_ proto_base_expr;
// member classes/structs/unions
template<typename Expr, typename State, typename Data>
struct impl : proto::transform_impl< Expr, State, Data > {
// types
typedef unspecified
result_type;
// public member functions
result_type operator()(typename impl::expr_param,
typename impl::state_param,
typename impl::data_param) const;
};
};
An expression type E
matches
proto::or_<G0,G1,...Gn>
if E
matches any Gx
for
x
in [0,n]
.
When applying
proto::or_<G0,G1,...Gn>
as a transform with an expression e
of type E
,
state s
and data d
, it is equivalent to
Gx()(e, s, d)
, where
x
is the lowest number such that
proto::matches<E, Gx>::value
is true
.
The maximun number of template arguments proto::or_<>
accepts
is controlled by the BOOST_PROTO_MAX_LOGICAL_ARITY
macro.