Struct template protect

boost::proto::protect — A PrimitiveTransform which prevents another PrimitiveTransform from being applied in an ObjectTransform.

Synopsis

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

template<typename PrimitiveTransform> 
struct protect :  proto::transform< protect<PrimitiveTransform> > {
  // member classes/structs/unions
  template<typename , typename , typename > 
  struct impl {
    // types
    typedef PrimitiveTransform result_type;
  };
};

Description

When building higher order transforms with proto::make<> or proto::lazy<>, you sometimes would like to build types that are parameterized with Proto transforms. In such lambda-style transforms, Proto will unhelpfully find all nested transforms and apply them, even if you don't want them to be applied. Consider the following transform, which will replace the proto::_ in Bar<proto::_>() with proto::terminal<int>::type:

template<typename T>
struct Bar
{};

struct Foo :
  proto::when<proto::_, Bar<proto::_>() >
{};

proto::terminal<int>::type i = {0};

int main() {
  Foo()(i);
  std::cout << typeid(Foo()(i)).name() << std::endl;
}

If you actually wanted to default-construct an object of type Bar<proto::_>, you would have to protect the _ to prevent it from being applied. You can use proto::protect<> as follows:

// OK: replace anything with Bar<_>()
struct Foo :
  proto::when<proto::_, Bar<proto::protect<proto::_> >() >
{};