c++ - can't understand a macro definition (casting constant number to a class pointer) -

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in code, see macro defined below can't understand.

#define offset_of_field_(f) (reinterpret_cast<char*>(      \   &reinterpret_cast<netparameter*>(16)->f) - \    reinterpret_cast<char*>(16))

the macro name seems calculating offset of field f inside class structure. , has form of subtracting start address address of field. how number 16 used here? , deosn't reinterpret_case apply 16?(not 16 -> f). appreciate if please explain code me.

the comment in (now refactored) protobuf header (link here) explains it

// note calculate relative pointer value 16 here since if // use zero, gcc complains dereferencing null pointer.  // choose 16 rather other number in case compiler // confused unaligned pointer. #define google_protobuf_generated_message_field_offset(type, field)    \   static_cast<int>(                                           \       reinterpret_cast<const char*>(                          \           &reinterpret_cast<const type*>(16)->field) -        \       reinterpret_cast<const char*>(16)) #endif

so reason why 16 being used twofold:

  • avoid using null pointer
  • use aligned pointer

be aware known create issues (might replaced __builtin_offsetof supported).


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