助推聚变类型偏移

Question

我目前正试图计算一个数据成员的偏移量在一个增强融合适应结构，但我不确定是否有一个优雅的方式来做到这一点。我想做如下的事情:

#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;
}

关键是我不想在offsetof函数中明确地声明'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体的所有数据成员的偏移量。

如果你有什么想法，我很乐意听听!

Answer 1

我不是专家，但我认为由于offsetof的性质，如果不需要使用宏，就没有办法实现您想要的。下面的示例简单地使用一个宏为结构体的每个成员创建一个trait，当您访问它时，该trait返回其预先计算的偏移量。

在Wandbox上运行

#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);
}