c++ alignment using pragma pack and inheritance

Question

I am not too familiar with concept of packing / alignment in C++, I did some reading about this recently and have a question.

I am deriving from a base class (written by somebody else and I have header for that). Author of this class has used pragma pack to align members to 1 byte boundary. however I am not sure if it is necessary for derived class to do the same or not, what are consequences of packing/not packing derived class with same alignment as base class ?

any help/suggestions will be greatly appreciated

thanks

Answer 1

In everyday, well-written C++ code it doesn't normally matter if there's padding or not, though the choice may impact performance. So, you should be able to derive from that base class without worrying about explicitly specifying any packing yourself. That said, the base class may be packed because there'll be a massive number of instances in memory or bitwise-copied to a file or network stream, in which case you'll want to consider whether instances of your new class may end up mixed in with that data, and whether you also want to use packing for the extra data members for the same reasons.

Not all code is well-written though. For example, if the program treats the objects as binary blobs of data and uses functions like memcmp on them, or does a byte-wise void* /size checksum, then garbage data in padding members may break the logic/behaviour. If the data is written object by object with particular separator or delimiter characters, then embedded garbage may inject unwanted separators/delimiters and break the reading/parsing logic. There's no way to assess these risks without doing an impact study on the existing code.