protocol buffers : no notation for fixed size buff

问题:

Since I am not getting an answer on this question I gotta prototype and check myself, as my dataset headers need to be fixed size, I need fixed size strings. So, is it possible to specify fixed size strings or byte arrays in protocol buffers ? It is not readily apparent here, and I kinda feel bad about forcing fixed size strings into the header message. --i.e, std::string('\0', 128);

If not I'd rather use a #pragma pack(1) struct header {...};'

edit

Question indirectly answered here. Will answer and except

回答1:

protobuf does not have such a concept in the protocol, nor in the .proto schema language. In strings and blobs, the data is always technically variable length using a length prefix (which itself uses varint encoding, so even the length is variable length).

Of course, if you only ever store data of a particular length, then it will line up. Note also that since strings in protobuf are unicode using UTF-8 encoding, the length of the encoded data is not as simple as the number of characters (unless you are using only ASCII characters).

回答2:

This is a slight clarification to the previous answer. Protocol Buffers does not encode strings as UTF-8, it encodes them as regular bytes. The on-wire format would be the number of bytes consumed followed by the actual bytes. See https://developers.google.com/protocol-buffers/docs/encoding/.

While the on-wire format is always the same, protocol buffers provides two interfaces for developers to use, string and bytes, with the primary difference being that the former will generally try to provide string types to the developer where as the latter will try to provide byte types (I.e. Java would provide String for string and ByteArray for bytes).