...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::lazy — A PrimitiveTransform that uses
proto::make<>
to build a
CallableTransform, and then uses
proto::call<>
to apply it.
// In header: <boost/proto/transform/lazy.hpp>
template<typename T>
struct lazy : proto::transform< lazy<T> > {
// member classes/structs/unions
template<typename Expr, typename State, typename Data>
struct impl : proto::transform_impl<Expr, State, Data> {
// types
typedef see-below
result_type;
// public member functions
result_type operator()(typename impl::expr_param,
typename impl::state_param,
typename impl::data_param) const;
};
};
proto::lazy<>
is useful as a higher-order transform,
when the transform to be applied depends on the current state of the transformation. The
invocation of the
proto::make<>
transform evaluates any nested transforms, and the resulting type is treated
as a CallableTransform, which is evaluated with
proto::call<>
.
For the full description of the behavior of the
proto::lazy<>
transform, see the documentation for the nested
proto::lazy::impl<>
class template.