通过指向对象成员之一的指针确定对象指针的最佳方法是什么？

Question

Like if I have:

struct S
{
    std::size_t szArray;
    int dArray[];
} ;

int main()
{
    extern int (*pArr)[]; //pointer to member 'dArray' of object with type 'S'

    S *pStruct = /*??????????*/; //pointer to the object
}

What is the best way to get this pointer?

Answer 1

There's no good way to do that.

The only supported way is

#include <cstddef> // for offsetof

S *pStruct = reinterpret_cast<S*>
    (reinterpret_cast<char*>(pArr) - offsetof(S, dArray));

Beware that offsetof is only well-defined for standard-layout types, and that standard C++ doesn't allow unsized arrays as class members. Unless you've got a good reason for using C idioms, I'd suggest that std::vector<int> would be safer and more convenient.