boost :: mpl转换操作元函数参数错误

Question

在Boost MPL web-documentation中，它讨论了将元函数类作为参数传递给boost :: mpl :: transform。在这种情况下，元函数参数应该是某种执行mpl :: ForwardSequence的操作。但是，当使用简单的元函数类将mpl :: transform应用到mpl :: map时，我得到模板错误。 （由于这些错误是相当广泛的，我只包括什么，我相信要在培训相关的位我很乐意，如果请求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 

在我的具体情况我的代码如下所示：

/* 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映射到本地C++类型，然后在from_native_tmap反向映射。

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

一个大'谢谢！'任何愿意帮助的人都可以参加。

Answer 1

/* 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; }