c++ - boost fusion 类型 offsetof

Question

我目前正在尝试计算 boost fusion 适应结构中数据成员的偏移量，但我不确定是否有一种优雅的方法可以做到这一点。我想做如下事情:

#include <iostream> 
#include <boost/fusion/adapted/struct/adapt_struct.hpp>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/sequence/intrinsic/at.hpp>
#include <boost/fusion/include/at.hpp>
#include <cstddef.h>

   struct test {
      int a;  
      char c;  
      double b;  
    };

BOOST_FUSION_ADAPT_STRUCT(
  test, 
  (int, a)
  (char, c)
  (double, b)
)

int main() {
test s{1, 2, 3.0}; 
// The following code doesn't work... I'm just trying to get my point across
std::cout << "offset is :" << offsetof(test, at<1>(s)) << std::endl;
}

重点是我不想在 offsetof 函数中明确声明“a”或“b”或“c”。这本身并不是那么有用，但我想在 boost for_each 循环中使用它，这样我就可以在编译时计算任何结构的所有数据成员的偏移量。

如果您有任何想法，我很想听听!

Answer 1

我不是专家，但我认为由于 offsetof 的性质，如果不使用宏，就无法实现您想要的效果。以下示例简单地使用一个宏为结构的每个成员创建一个特征，该特征在您访问它时返回其预先计算的偏移量。

Running on Wandbox

#include <iostream> 
#include <string>
#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/at.hpp>
#include <boost/fusion/include/for_each.hpp>
#include <boost/fusion/include/mpl.hpp>

#include <boost/mpl/range_c.hpp>

#include <cstddef>

namespace not_fusion
{
    template <typename StructName, int N>
    struct offset_of;
}

//Actually creates each respective 'offset_of' trait 
//you could change 'BOOST_STATIC_CONSTANT(std::size_t,...)'
//with a 'static constexpr std::size_t ...' or whatever you like
#define CREATE_OFFSET_TRAIT(R,STRUCT_NAME,INDEX,MEMBER) \
template <> struct offset_of<STRUCT_NAME, INDEX>{ BOOST_STATIC_CONSTANT(std::size_t, value = offsetof(STRUCT_NAME,MEMBER)); };

//Iterates the struct members in order to create the corresponding 'offset_of' traits
#define NOT_FUSION_SAVE_OFFSETS(STRUCT_NAME,MEMBERS) \
namespace not_fusion { \
BOOST_PP_SEQ_FOR_EACH_I(CREATE_OFFSET_TRAIT,STRUCT_NAME,MEMBERS) \
}

//Simply "invokes" 'BOOST_FUSION_ADAPT_STRUCT' and 'NOT_FUSION_SAVE_OFFSETS'
#define ADAPT_STRUCT_AND_SAVE_OFFSETS(TYPE,...) \
BOOST_FUSION_ADAPT_STRUCT(TYPE,__VA_ARGS__) \
NOT_FUSION_SAVE_OFFSETS(TYPE,BOOST_PP_VARIADIC_TO_SEQ(__VA_ARGS__))

struct test1 {
    int a;  
    char c;  
    double b;  
};

struct test2 {
    char c;
    short s;
    int i;
};

ADAPT_STRUCT_AND_SAVE_OFFSETS(test1,a,c,b);
ADAPT_STRUCT_AND_SAVE_OFFSETS(test2,c,s,i);


template <typename Struct>
void print_offsets(const std::string& name,const Struct&)
{
    //This could be changed to use 'std::integer_sequence' instead of mpl
    std::cout << "Offsets for " << name << ":" << std::endl;
    typedef boost::mpl::range_c<unsigned, 0, boost::fusion::result_of::size<Struct>::value > Indices; 
    boost::fusion::for_each(Indices(),
        [](auto index)
        { 
            std::cout << boost::fusion::extension::struct_member_name<Struct,index>::call() << " -> " << not_fusion::offset_of<Struct,index>::value << std::endl;
        }
    );
}

int main() {
    test1 t1{1, 2, 3.0}; 
    test2 t2{1,2,3};

    print_offsets("test1",t1);
    print_offsets("test2",t2);
}