如何循環非默認構造類的boost MPL列表？

Question

我有以下示例：

#include <iostream>

#include <boost/mpl/for_each.hpp>
#include <boost/mpl/list.hpp>

struct one {};
struct two {};
struct three {};
struct four {};
struct five { five() = delete; };

template <typename T>
void print()
{
  std::cout << "hello " << typeid(T).name() << std::endl;
}

struct type_printer
{
  template <typename T>
  void operator()(T)
  {
    print<T>();
  }
};

int main()
{
  typedef boost::mpl::list<
    one,
    two,
    three,
    four,
    five
  >::type type_list;

  boost::mpl::for_each<type_list>(type_printer());
}

如果我不在列表中包含第五個對象，哪個工作絕對正常，但一旦我這樣做，我會收到以下錯誤：

In file included from /usr/local/include/boost_1_56_0/boost/mpl/for_each.hpp:29:0,
                 from mpldef.cpp:3:
/usr/local/include/boost_1_56_0/boost/utility/value_init.hpp: In constructor ‘boost::initialized<T>::wrapper::wrapper() [with T = five]’:
/usr/local/include/boost_1_56_0/boost/utility/value_init.hpp:109:7:   instantiated from ‘boost::initialized<T>::initialized() [with T = five]’
/usr/local/include/boost_1_56_0/boost/utility/value_init.hpp:205:12:   instantiated from ‘boost::value_initialized<T>::value_initialized() [with T = five]’
/usr/local/include/boost_1_56_0/boost/mpl/for_each.hpp:81:9:   recursively instantiated from ‘static void boost::mpl::aux::for_each_impl<false>::execute(Iterator*, LastIterator*, TransformFunc*, F) [with Iterator = boost::mpl::l_iter<boost::mpl::list4<two, three, four, five> >, LastIterator = boost::mpl::l_iter<boost::mpl::l_end>, TransformFunc = boost::mpl::identity<mpl_::na>, F = type_printer]’
/usr/local/include/boost_1_56_0/boost/mpl/for_each.hpp:81:9:   instantiated from ‘static void boost::mpl::aux::for_each_impl<false>::execute(Iterator*, LastIterator*, TransformFunc*, F) [with Iterator = boost::mpl::l_iter<boost::mpl::list5<one, two, three, four, five> >, LastIterator = boost::mpl::l_iter<boost::mpl::l_end>, TransformFunc = boost::mpl::identity<mpl_::na>, F = type_printer]’
/usr/local/include/boost_1_56_0/boost/mpl/for_each.hpp:104:5:   instantiated from ‘void boost::mpl::for_each(F, Sequence*, TransformOp*) [with Sequence = boost::mpl::list5<one, two, three, four, five>, TransformOp = boost::mpl::identity<mpl_::na>, F = type_printer]’
/usr/local/include/boost_1_56_0/boost/mpl/for_each.hpp:118:3:   instantiated from ‘void boost::mpl::for_each(F, Sequence*) [with Sequence = boost::mpl::list5<one, two, three, four, five>, F = type_printer]’
mpldef.cpp:37:48:   instantiated from here
/usr/local/include/boost_1_56_0/boost/utility/value_init.hpp:77:12: error: use of deleted function ‘five::five()’
mpldef.cpp:10:15: error: declared here

有沒有辦法做到這一點？ 我嘗試從type_printer調用操作符中刪除T ，所以它看起來像這樣： void operator()() ，我仍然得到相同的錯誤。

Answer 1

我已通過添加一種類型的包裝結構，就像解決了這個一中Boost.Hana看到。 我將此添加到此注釋中找到的好主意，提到使用boost::mpl::transform將此包裝類型自動添加到列表中的每個項目。

以下詳述我的解決方案：

template <typename T>
struct type_
{
  using type = T;
};

template <typename T>
struct add_type_wrapper
{
  using type = type_<T>;
};

使用這兩種新類型，我將type_printer函數改為：

struct type_printer
{
  template <typename T>
  void operator()(T)
  {
    using type_t = typename T::type;
    print<type_t>();
  }
};

現在，main中的boost::mpl::for_each調用如下所示：

using boost::mpl::_1;
using wrapped_list = boost::mpl::transform<type_list, add_type_wrapper<_1>>::type;
boost::mpl::for_each<wrapped_list>(type_printer());

感謝您的幫助，我認為這是一個非常好的和優雅的解決方案。

Answer 2

您還可以向類型列表中的項添加一個間接級別，如下所示：

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

struct type_printer
{
  template <typename T>
  void operator()(T)
  {
    print<T>();
  }
  template <typename T>
  void operator()(type_ref<T>)
  {
    print<T>();
  }
};

int main()
{
  typedef boost::mpl::list<
    one,
    two,
    three,
    four,
    type_ref<five>
  >::type type_list;

  boost::mpl::for_each<type_list>(type_printer());
}

Answer 3

你可以做自己喜歡的事情：

namespace detail {

    template <typename Tuple>
    structure my_for_each_t;

    template <typename... Ts>
    structure my_for_each_t<boost::mpl::list<Ts...>>
    {
        template <typename F>
        void operator()(F f) const
        {
             initializer_list<int>{(static_cast<void>(f<Ts>()), 0)...};
        }
    };

}

template <typename Tuple, typename F>
void my_for_each(F f)
{
    detail::my_for_each_t<Tuple>()(f);
}

或者轉換列表以添加指向類型的指針，並更改type_printer ：

struct type_printer
{
  template <typename T>
  void operator()(T*) // pointer added.
  {
    print<T>();
  }
};