是指向数据成员的指针的偏移量？

Question

我可以安全地假设数据成员的偏移量（ offsetof(mystruct, myfield) ）在数值上等于使用&mystruct::myfield检索的成员指针的原始值，还是它取决于实现？

Answer 1

No; the implementation of a pointer-to-member is not specified, and there is no defined conversion to get the "raw" value.

Answer 2

Formally, it is implementation-dependent, of course. In real life, yes, the most popular implementation of pointer-to-data-member pointers is (or is based on) plan-and-simple offset, ie the same thing that offsetof evaluates to.

Some implementations use the exact offset, resorting to 0xFFF... pattern to represent the null-pointer value. Some implementatiuons reserve 0x000... pattern for null-pointer value, while incrementing all "non-null" offsets by 1 .