我有以下结构:
struct Data
{
std::string firstMember;
std::string secondMember;
std::string thirdMember;
};
我想以
constexpr的方式通过字符串名称选择成员之一，例如Data instance;
auto& member = getMember(instance, "firstMember");
getMember是
constexpr函数/struct/macros/无论有什么问题，表达式(我希望将其表达)都应优化为简单的auto& member = instance.firstMember;。我在这里的愿望是能够从另一个getMember函数调用constexpr，后者又在计算特定成员的名称->某种编译时反射。我知道，C++中没有反射，因此可以以某种方式注册所讨论的struct成员的名称(部分专门化？使用一些宏魔术吗？)，例如:
REGISTER_MEMBER(Data, "firstMember", firstMember);
我想要的只是进行编译时优化，而在运行时不执行任何操作。在C++ 11中有可能吗？如何？
最佳答案
如评论中所述，首先查看BOOST_FUSION_ADAPT_STRUCT(和 friend ):
#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
Data结构变成Fusion可以使用的序列:#include <boost/fusion/include/at_c.hpp>
int main()
{
Data d = { "firstData", "secondData", "thirdData" };
std::cout << boost::fusion::at_c<0>(d) << std::endl;
}
这将打印
"firstData"。更改索引以按顺序引用成员。在那里，现在我们可以在编译时使用数字引用成员。但是你想要一个名字。注释中还指出，处理字符串几乎是运行时功能。 C++ 11给我们
constexpr。这有点棘手，但最终看起来像这样:
#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 :
它看起来很吓人，但如果您花时间将其拆开，它就很容易读懂。
我们必须引入我们自己的宏，以便对成员名进行常量表达访问。大多数丑陋来自处理Boost.Preprocessor列表。尽管Fusion也确实在改编过程中记录了名称(请参阅
boost::fusion::extension::struct_member_name)，但遗憾的是，它们并未标记为constexpr，因此我们无法使用。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; */
}
我认为这与您追求的目标很接近。
但是请记住，这可能并非全部必要:Boost.Fusion可能已满足您的需求。它位于纯编译时程序段(Boost.MPL)和常规运行时程序段之间的区域。修改您的结构，您已经可以进行迭代(
boost::fusion::for_each)之类的操作。关于c++ - 在C++中模拟编译时反射，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/13830792/