#pragma pack(1) and byteswapping solve all this and neither are more work than specifying some intermediate format and calling some external tool in your build system. In C++ you can make types that swap automatically if required.

> And you have to keep the data in the structure itself, so as soon as you hit a pointer you need to write explicit serialisation.

I have yet to see a serialization format that does this well and quickly. Just make a new message type for it that follows the first.

You could add completely new revision of the API. But having to maintain it - typically additionally to the old revision since both sides might not updateable in atomic fashion - is a lot of extra work.

Serialization which allows to add and remove fields in a backward compatible fashion is a mandatory requirement for any bigger sized project for me. If you need super high performance maybe use Flatbuffers or Cap'N'Proto instead of Protobuf/JSON/CBOR - but I really wouldn't recommend to go for plain structs.