我有一个以下结构:
struct Data
{
std::string firstMember;
std::string secondMember;
std::string thirdMember;
};
我想按字符串名称选择其中一个成员constexpr,比如
Data instance;
auto& member = getMember(instance, "firstMember");
getMember是constexpr函数/ struct/macros /有问题和表达式应该(我希望它)优化为简单auto& member = instance.firstMember;.我的愿望是能够getMember从另一个constexpr函数调用,而另一个函数又是特定成员的计算名称 - >某种编译时间反射.
我知道,C++中没有反射,因此可以以某种方式注册(部分特殊化？使用一些宏魔法？)有问题的struct成员名称,如:
REGISTER_MEMBER(Data, "firstMember", firstMember);
我想要的只是编译时优化并在运行时不执行任何操作.这可能在C++ 11中如何实现？
GMa*_*ckG 13
如评论中所述,首先看看BOOST_FUSION_ADAPT_STRUCT(和朋友):
#include <boost/fusion/include/adapt_struct.hpp>
#include <string>
struct Data
{
std::string firstMember;
std::string secondMember;
std::string thirdMember;
};
BOOST_FUSION_ADAPT_STRUCT(
Data,
(std::string, firstMember)
(std::string, secondMember)
(std::string, thirdMember)
)
这会将您的Data结构转换为Fusion可用的序列:
#include <boost/fusion/include/at_c.hpp>
int main()
{
Data d = { "firstData", "secondData", "thirdData" };
std::cout << boost::fusion::at_c<0>(d) << std::endl;
}
这打印"firstData".更改索引以按顺序引用成员.
在那里,现在我们可以使用数字在编译时引用成员.但你想要一个名字.在注释中还注意到,处理字符串是一个运行时特性......几乎.C++ 11给了我们constexpr.
这有点棘手,但最终它看起来像这样:
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/repetition/repeat.hpp>
#include <boost/preprocessor/seq.hpp>
#include <boost/preprocessor/tuple/elem.hpp>
#include <stdexcept>
// and repeat for BOOST_FUSION_ADAPT_TPL_STRUCT, etc...
#define REFLECT_STRUCT(NAME, ATTRIBUTES) \
REFLECT_STRUCT_DETAIL(NAME, \
ATTRIBUTES, \
BOOST_PP_SEQ_POP_FRONT( \
BOOST_PP_CAT( \
/* warning: uses fusion implementation details: */ \
BOOST_FUSION_ADAPT_STRUCT_FILLER_0(0,0)ATTRIBUTES, \
_END))) \
#define REFLECT_STRUCT_DETAIL(NAME, ATTRIBUTES, WRAPPEDATTRIBUTES) \
BOOST_FUSION_ADAPT_STRUCT(NAME, ATTRIBUTES) \
\
namespace detail \
{ \
namespace BOOST_PP_CAT(reflect_, NAME) \
{ \
template <int N> \
struct member_name; \
\
BOOST_PP_SEQ_FOR_EACH_I(REFLECT_STRUCT_DETAIL_MEMBER_NAME, \
BOOST_PP_EMPTY, \
WRAPPEDATTRIBUTES) \
\
template <int N> \
constexpr bool member_match_index(const std::size_t index, \
const char* const str, \
const std::size_t len) \
{ \
return index == len || \
(member_name<N>::value()[index] == str[index] \
&& member_match_index<N>(index + 1, str, len)); \
} \
\
template <int N> \
constexpr bool member_match(const char* const str, \
const std::size_t len) \
{ \
return len == member_name<N>::value_length \
&& member_match_index<N>(0, str, len); \
} \
\
constexpr int find_member(const char* const str, \
const std::size_t len) \
{ \
return BOOST_PP_REPEAT(BOOST_PP_SEQ_SIZE(WRAPPEDATTRIBUTES), \
REFLECT_STRUCT_DETAIL_MEMBER_NAME_TEST, \
BOOST_PP_EMPTY) \
throw std::runtime_error("could not find " \
BOOST_PP_STRINGIZE(NAME) \
" member"); \
} \
} \
} \
\
constexpr int BOOST_PP_CAT(indexof_, NAME)(const char* const str, \
const std::size_t len) \
{ \
return detail::BOOST_PP_CAT(reflect_, NAME)::find_member(str, len); \
} \
\
template <std::size_t N> \
constexpr int BOOST_PP_CAT(indexof_, NAME)(const char (&str)[N]) \
{ \
return detail::BOOST_PP_CAT(reflect_, NAME)::find_member(&str[0], N); \
}
#define REFLECT_STRUCT_DETAIL_EXTRACT_NAME(pair) \
BOOST_PP_STRINGIZE(BOOST_PP_TUPLE_ELEM(1, pair))
#define REFLECT_STRUCT_DETAIL_MEMBER_NAME(r, data, n, elem) \
REFLECT_STRUCT_DETAIL_MEMBER_NAME_DETAIL(n, REFLECT_STRUCT_DETAIL_EXTRACT_NAME(elem))
#define REFLECT_STRUCT_DETAIL_MEMBER_NAME_DETAIL(n, name) \
template <> \
struct member_name<n> \
{ \
static constexpr std::size_t value_length = sizeof(name); \
typedef const char value_type[value_length]; \
\
static constexpr const value_type& value() \
{ \
return name; \
} \
};
#define REFLECT_STRUCT_DETAIL_MEMBER_NAME_TEST(z, n, text) \
member_match<n>(str, len) ? n :
它看起来很可怕,但如果你花时间把它分开,它的可读性.
我们必须引入自己的宏来给成员名称提供常量表达式访问; 大多数丑陋来自处理Boost.Preprocessor列表.虽然Fusion确实在改编过程中记录了这些名字(参见参考资料boost::fusion::extension::struct_member_name),但constexpr不幸的是,它们没有被标记为不适用于我们.
这给出了:
#include <boost/fusion/include/at_c.hpp>
#include <iostream>
#include <string>
struct Data
{
std::string firstMember;
std::string secondMember;
std::string thirdMember;
};
REFLECT_STRUCT(
Data,
(std::string, firstMember)
(std::string, secondMember)
(std::string, thirdMember)
)
// your desired code:
// (note the use of at_c ensures this is evaluated at comple-time)
#define GETMEMBER(data, member) boost::fusion::at_c<indexof_Data(member)>(data)
int main()
{
Data d = { "firstData", "secondData", "thirdData" };
std::cout << boost::fusion::at_c<indexof_Data("firstMember")>(d) << std::endl;
std::cout << GETMEMBER(d, "secondMember") << std::endl;
std::cout << GETMEMBER(d, "thirdMember") << std::endl;
/* causes error: std::cout << GETMEMBER(d, "nonexistent_member") << std::endl; */
}
我认为这与你所追求的很接近.
但请记住,这可能并非都是必要的:Boost.Fusion可能已经拥有了您所需要的东西.它位于纯编译时间(Boost.MPL)和常规运行时间之间的区域; 适应你的结构,你已经可以做一些事情,如迭代它(boost::fusion::for_each).
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