c++ - Boost Fusion 变换类型操作和 as_vector

Question

我正在尝试了解使用 Fusion 的要点，但被这个简单的示例难倒了:

#include <boost/fusion/include/is_sequence.hpp>
#include <boost/fusion/include/as_vector.hpp>
#include <boost/fusion/include/make_vector.hpp>
#include <boost/fusion/include/transform.hpp>

template< typename T >
struct S {
    typedef T type;
};

struct S_f {
    template< typename T >
    struct result {
        typedef typename T::type type;
    };
};

int main () {
    using namespace boost;
    typedef fusion::vector<S<int>> from_type;
    BOOST_MPL_ASSERT((fusion::traits::is_sequence< fusion::vector< int > > ));

    typedef fusion::result_of::transform< from_type, S_f >::type to_type;
    BOOST_MPL_ASSERT((fusion::traits::is_sequence< to_type > ));

    typedef fusion::result_of::as_vector< to_type >::type value_type; // error
}

断言通过但 value_type 的 typedef 失败并出现以下错误。我无法资助代码和文档之间的任何差异，也无法在 stackoverflow 或 boost 邮件列表的其他地方进行补救。

AFAICT 代码是正确的:应用变换元函数的结果是一个 transform_view，而 transform_view 是一个序列，如传递的断言所示。然而，as_vector 元函数在 transform_view 上的应用失败了。什么给了？!

感谢任何帮助。我对混合 mpl 不感兴趣。我知道我可以绕过 MPL 和一些关于 SO 的关于类型操作的 fusion 问题有提倡 MPL 的答案。根据文档，我不需要 MPL。

clang++ -std=c++1z -c t.cpp
In file included from main.cpp:4:
In file included from /usr/local/include/boost/fusion/include/transform.hpp:11:
In file included from /usr/local/include/boost/fusion/algorithm/transformation/transform.hpp:11:
In file included from /usr/local/include/boost/fusion/view/transform_view/transform_view.hpp:15:
In file included from /usr/local/include/boost/fusion/view/transform_view/transform_view_iterator.hpp:18:
/usr/local/include/boost/fusion/view/transform_view/detail/value_of_impl.hpp:37:74: error: no type named 'type' in 'boost::mpl::apply<boost::fusion::detail::apply_transform_result<S_f>, S<int>, mpl_::na, mpl_::na, mpl_::na, mpl_::na>'
                typedef typename mpl::apply<transform_type, value_type>::type type;
                        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
/usr/local/include/boost/fusion/iterator/value_of.hpp:52:15: note: in instantiation of template class 'boost::fusion::extension::value_of_impl<boost::fusion::transform_view_iterator_tag>::apply<boost::fusion::transform_view_iterator<boost::fusion::vector_iterator<boost::fusion::vector<S<int>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>, 0>, S_f> >' requested here
            : extension::value_of_impl<typename detail::tag_of<Iterator>::type>::
              ^
/usr/local/include/boost/fusion/container/vector/detail/cpp03/preprocessed/as_vector10.hpp:19:49: note: in instantiation of template class 'boost::fusion::result_of::value_of<boost::fusion::transform_view_iterator<boost::fusion::vector_iterator<boost::fusion::vector<S<int>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>, 0>, S_f> >' requested here
            typedef typename fusion::result_of::value_of<I0>::type T0;
                                                ^
/usr/local/include/boost/fusion/container/vector/convert.hpp:26:17: note: in instantiation of template class 'boost::fusion::detail::barrier::as_vector<1>::apply<boost::fusion::transform_view_iterator<boost::fusion::vector_iterator<boost::fusion::vector<S<int>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>, 0>, S_f> >' requested here
                template apply<typename result_of::begin<Sequence>::type>::type
                ^
main.cpp:26:32: note: in instantiation of template class 'boost::fusion::result_of::as_vector<boost::fusion::transform_view<boost::fusion::vector<S<int>, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_, boost::fusion::void_>, S_f, boost::fusion::void_> >' requested here
    typedef fusion::result_of::as_vector< to_type >::type value_type; // error
                               ^
1 error generated.

Answer 1

模板元编程的问题在于，当您无法满足元函数的先决条件时，您会得到很多无意义的错误。关于 transform 的要求是那个F是一元 Polymorphic Function Object .文档中的解释有点薄弱，但您可以从示例中看出:这是一个您可以使用参数调用的对象。即 result_of<F(T)>::type需要格式正确。

您传递给 transform 的内容是:

struct S_f {
    template< typename T >
    struct result {
        typedef typename T::type type;
    };
};

那不是多态函数对象。它也不是元函数类。这不是 Boost.Fusion 和 Boost.MPL 都能够理解的东西。这特别令人困惑的是 transform<>元功能是惰性 - 所以看起来你正确地完成了那部分。它只在as_vector<>实际应用了转换，所以看起来这就是失败点。

要将其转换为多态函数对象，只需更改嵌套的 result将类模板转换为调用运算符:

struct S_f {
    template< typename T >
    typename T::type operator()(T );
};

不需要定义，因为您实际上并没有调用它。通过该修复，您的代码可以编译。