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在Boost MPL 网络文档中，它谈到了将元函数类作为参数传递给 boost::mpl::transform。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 应用于 mpl::map 时，我遇到了模板错误。（由于这些错误非常广泛，我只包括了我认为是相关的部分。如果需要，我非常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; }

我的意图是映射到本地 c++ 类型，to_native_tmap然后在from_native_tmap.

BOOST_MPL_ASSERT()如果我尝试实例化to_native_tmapmpl::map 类型，此代码将失败。

非常感谢你！' 任何愿意提供帮助的人。

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1 回答 1

5 
/* 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; }
于 2013-04-03T17:41:03.837 回答