如何循环非默认构造类的 boost MPL 列表？

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());
}

如果我不将第五个对象包含在列表中，那么它绝对可以正常工作，但是一旦我这样做了，我就会收到以下错误：

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

有什么办法可以做到这一点吗？我尝试删除T来自type_printer调用运算符，所以它看起来像这样：void operator()()我仍然遇到同样的错误。

我通过添加类型包装器结构解决了这个问题，例如the one https://ldionne.github.io/hana/structboost_1_1hana_1_1_type.html见于 Boost.Hana。我将此添加到发现的好主意中这条评论 https://stackoverflow.com/questions/4053436/is-it-possible-to-iterate-an-mplvector-at-run-time-without-instantiating-the-t#comment7166228_4117003其中提到使用boost::mpl::transform自动将此包装类型添加到列表中的每个项目。

下面详细介绍我的解决方案：

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

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

使用这两种新类型，我改变了type_printer函子看起来像这样：

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

And the boost::mpl::for_each现在 main 中的调用看起来像这样：

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());

感谢你们的帮助，我认为这是一个非常好的和优雅的解决方案。