C ++类/结构数据成员偏移为常量表达式

#define MEMBER_OFFSET(Type, Member) 
    ((unsigned long)(((char *)&((Type *)0)->Member) - (char *)0));

struct A { int i; };
template< size_t > struct S;

int main() {
  S< offsetof( A, i ) > *p;
}

__offsetof__

offsetof

struct A {
  int i, j;
};

template< int A::* > struct S;
template<> struct S< &A::i > { static char const value = 'i'; };
template<> struct S< &A::j > { static char const value = 'j'; };

int main() {
  cout<< S< &A::i >::value <<endl;
  cout<< S< &A::j >::value <<endl;
}

offsetof

unsigned long