我目前正在尝试计算升压融合自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.我想做类似以下的事情：
#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函数中明确声明’a’或’b’或’c’.这本身并没有那么有用,但是我想在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,我很乐意听到他们！
解决方法
我不是专家,但我认为由于offset的性质,没有办法实现你想要的东西而不需要使用宏.以下示例仅使用宏为结构的每个成员创建一个特征,每当您访问它时返回其预先计算的偏移量.
#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,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,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,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{1,3.0};
test2 t2{1,3};
print_offsets("test1",t1);
print_offsets("test2",t2);
}
