Question

我正在尝试使用Fusion，我对这个简单的例子感到难过：

#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
}

断言传递，但value_type的typedef失败，错误如下。我无法为代码和文档之间的任何差异提供资金，也无法在stackoverflow或boost邮件列表的其他地方补救。

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

感谢任何帮助。我对混合mpl不感兴趣。我知道我可以通过MPL和一些关于类型操作的融合问题绕道而行，有关于提倡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，当你未能满足元函数的前提条件时，你会得到许多无意义的错误。 transform的要求是F是一元Polymorphic Function Object。对文档中的内容的解释有点弱，但您可以从示例中看出：这是一个可以用参数调用的对象。也就是说，result_of<F(T)>::type需要格式良好。

您传递给transform的是：

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

这不是多态函数对象。它也不是一个元函数类。这不是Boost.Fusion和Boost.MPL能够理解的东西。特别令人困惑的是transform<>元函数 lazy - 所以看起来你正确地做了那个部分。仅在as_vector<>中才实际应用了转换，因此它看起来就像故障点所在。

要将其转换为多态函数对象，只需将嵌套的result类模板更改为调用运算符：

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

没有必要定义，因为你实际上没有调用它。使用该修复程序，您的代码将进行编译。

Boost Fusion变换类型操作和as_vector

1 个答案: