我如何遍历非默认的构造类的提振MPL列表？ [英] How do I loop over a boost MPL list of non-default constructed classes?

问题描述

我有下面的例子：

#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());
}

如果我不包括在列表中的第五个对象，作品精绝，但一旦我做我得到以下错误：

Which works absolutely fine if I don't include the fifth object in the list, but once I do I get the following errors:

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

是否有这样做的方法吗？我试着删除 T 从 TYPE_PRINTER 电话运营商，所以它看起来是这样的：无效运算符（）（），我仍然得到了同样的错误。

Is there any way of doing this? I tried removing T from the type_printer call operator, so it looked something like this: void operator()() and I still got the same error.

推荐答案

我加入一个类型的包装结构，像的在Boost.Hana看到一个。我说这在<一个发现了伟大的想法href=\"http://stackoverflow.com/questions/4053436/is-it-possible-to-iterate-an-mplvector-at-run-time-without-instantiating-the-t#comment7166228_4117003\">this即使用提到评论的boost :: MPL ::变换来此包装类型添加到每个项目中的自动列表中。

I've solved this by adding a type wrapper struct, like the one seen in Boost.Hana. I added this to the great idea found in this comment that mentions using boost::mpl::transform to add this wrapper type to each item in the list automatically.

下面详细介绍我的解决方法：

The following details my solution:

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

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

使用这两种新的，我改变了 TYPE_PRINTER 仿函数看起来像这样：

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

以及的boost :: MPL :: for_each的主呼叫现在看起来是这样的：

And the boost::mpl::for_each call in main now looks like this:

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());

感谢您的帮助家伙，我认为这是一个非常好的和优雅的解决方案。

Thanks for your help guys, I think this is a really nice and elegant solution.

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