如何循环使用非默认构造类的boost MPL列表？ [英] How do I loop over a boost MPL list of non-default constructed classes?

问题描述

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（））; 
} 
  

如果我没有在列表中包含第五个对象， ，但一旦我这样做，我得到以下错误：

 在/ 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：在构造函数'boost :: initialized< T> :: wrapper :: wrapper（）[with T = five]'：
 /usr/local/include/boost_1_56_0/boost/utility/value_init.hpp:109:7：从'boost :: initialized实例化; T> :: initialized（）[with T = five]'
 /usr/local/include/boost_1_56_0/boost/utility/value_init.hpp:205:12：从'boost :: value_initialized< T& :: value_initialized（）[with T = five]'
 /usr/local/include/boost_1_56_0/boost/mpl/for_each.hpp:81:9：递归实例化为'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， >，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：实例化为'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 /局部/包括/ boost_1_56_0 / boost / mpl / for_each.hpp：104：5：从'void boost :: mpl :: for_each（F，Sequence *，TransformOp *）实例化[with Sequence = boost :: mpl :: list5&一个，两个，三个，四个，五个，TransformOp = boost :: mpl :: identity  / usr / local / include / boost_1_56_0 / boost / mpl / for_each .hpp：118：3：从'void boost :: mpl :: for_each（F，Sequence *）实例化[with Sequence = boost :: mpl :: list5<一，二，三，四，五>，F = type_printer ]'
 mpldef.cpp：37：48：从这里实例化
 /usr/local/include/boost_1_56_0/boost/utility/value_init.hpp:77:12：错误：使用删除的函数'five :: five（）'
 mpldef.cpp：10：15：error：declaration here 
  

有什么办法这样做吗？我尝试从 type_printer 调用操作符中删除 T ，所以它看起来像这样： void

解决方案

我已经解决了这个问题添加类型包装结构，例如 Boost.Hana中显示的一个。我添加了这个在提到使用 boost :: mpl :: transform 自动将这个包装类型添加到列表中的每个项目。 / p>



以下详细信息我的解决方案：

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

使用这两个新类型，我改变了 type_printer 函子看起来像这样：

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

并且 boost :: mpl :: for_each call in main现在看起来像这样：

  using boost :: mpl :: _ 
 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，我认为这是一个非常好的和优雅的解决方案。
感谢您的帮助，我认为这是一个非常好的和优雅的解决方案。
 
I have the following example:
#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());
}
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
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.
解决方案
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.

The following details my solution:
template <typename T>
struct type_
{
  using type = T;
};

template <typename T>
struct add_type_wrapper
{
  using type = type_<T>;
};
Using these two new types, I altered the type_printer functor to look like so:
struct type_printer
{
  template <typename T>
  void operator()(T)
  {
    using type_t = typename T::type;
    print<type_t>();
  }
};
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());
Thanks for your help guys, I think this is a really nice and elegant solution.

                        
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