Boost::mpl转换操作元函数参数错误

boost::mpl transform operation meta-function argument error

本文关键字：函数 参数 错误 操作 mpl 转换 Boost      更新时间：2023-10-16

在Boost MPL web文档中，它讨论了将元函数类作为Boost:: MPL::transform的参数传递。在这种情况下，元函数参数应该是在mpl::ForwardSequence上执行的某种操作。但是，当使用简单的元函数类将mpl::transform应用到mpl::map时，我得到了模板错误。(由于这些错误相当广泛，我只包括了我认为相关的部分。如果需要，我很乐意发布更广泛的错误报告。

错误:

/usr/include/boost/mpl/aux_/preprocessed/gcc/bind.hpp:207:21: error: no type named     ‘type’ in ‘struct boost::mpl::apply_wrap2<boost::mpl::push_front<mpl_::na, mpl_::na>, boost::mpl::map0<>, boost::mpl::pair<unsigned int, INT32U> >’
test_boost_mpl.cpp:106:1: error: ‘from_native_tmap’ was not declared in this scope

在我的特殊情况下，我的代码如下所示:

/* stl includes */
#include <cstdint>
/* boost includes */
#include <boost/type_traits.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/mpl/transform.hpp>
struct Boolean {
  enum { tag_value = 0x83 };
};
struct INT32U {
  enum { tag_value = 0x84 };
};
typedef mpl::map
< 
  mpl::pair<Boolean, bool>,
  mpl::pair<INT32U, std::uint32_t>
> to_native_tmap;
struct swap_f {
  template<typename PAIR>
  struct apply {
    typedef typename mpl::pair<typename PAIR::second, typename PAIR::first> type;
  };
};
typedef mpl::transform<to_native_tmap, swap_f>::type from_native_tmap;
BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, bool>::type, Boolean> ));
BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, std::uint32_t>::type, INT32U> ));
int main(void) { return 0; }

我的意图是在to_native_tmap中映射到本地c++类型，然后在from_native_tmap中反向映射。

此代码在BOOST_MPL_ASSERT()上失败，或者如果我尝试实例化to_native_tmap mpl::map类型。

大大的"谢谢你!"

/* stl includes */
#include <cstdint>
/* boost includes */
#include <boost/type_traits.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/mpl/at.hpp>
#include <boost/mpl/map.hpp>
#include <boost/mpl/empty.hpp>
#include <boost/mpl/assert.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/mpl/inserter.hpp> //ADDED
#include <boost/mpl/insert.hpp> //ADDED
namespace mpl= boost::mpl; //ADDED
using boost::is_same; //ADDED
struct Boolean {
  enum { tag_value = 0x83 };
};
struct INT32U {
  enum { tag_value = 0x84 };
};
typedef mpl::map
< 
  mpl::pair<Boolean, bool>,
  mpl::pair<INT32U, std::uint32_t>
> to_native_tmap;
struct swap_f {
  template<typename PAIR>
  struct apply {
    typedef typename mpl::pair<typename PAIR::second, typename PAIR::first> type;
  };
};
//The default inserter used by mpl::transform requires that the container have a push_front algorithm
//This isn't the case for mpl::map (as the assertion in g++4.8.0 reveals: REQUESTED_PUSH_FRONT_SPECIALIZATION_FOR_SEQUENCE_DOES_NOT_EXIST)
//In this case you need to add a custom inserter
typedef mpl::inserter<mpl::map0<>, mpl::insert<mpl::_1,mpl::_2> > map_inserter; //ADDED
typedef mpl::transform<to_native_tmap, swap_f, map_inserter >::type from_native_tmap; //CHANGED
BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, bool>::type, Boolean> ));
BOOST_MPL_ASSERT(( is_same
                   <mpl::at<from_native_tmap, std::uint32_t>::type, INT32U> ));
int main(void) { return 0; }
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