This is incorrect -- it is not possible to implement Protobuf in a way that achieves the notion of "zero-copy" that Cap'n Proto and FlatBuffers achieve.

This probably comes down to a disagreement on what "zero-copy" means.

Some people use the term "zero-copy" to mean only that when the message contains a string or byte array, the parsed representation of those specific fields will point back into the original message buffer, rather than having to allocate a copy of the bytes at parse time.

Cap'n Proto and FlatBuffers implement a much stronger form of zero-copy. With them, it's not just strings and byte buffers that are zero-copy, it's the entire data structure. With these systems, once you have the bytes of a message mapped into memory, you do not need to do any "parse" step at all before you start using the message.

For example, if you have a multi-gigabyte file formatted with one of these, you can mmap() it, and then you can traverse the message tree to read any bytes of the message except the chain of pointers (parent to child) leading to that one datum. Aside from the mmap() call, you can do all this without even allocating any memory at all.

That is absolutely not possible with Protobuf, because Protobuf encoding is a list of tag-value pairs each of which has variable width. In order to read any particular value, you must, at the very least, linearly scan through the tag-values until you find the one you want. But in practice, you usually want to read more than one value, at which point the only way to avoid O(n^2) time complexity while keeping things sane is to parse the entire message tree into a different set of in-memory data structures allocated on the heap.

That is not "zero-copy" by Cap'n Proto's definition.

(Disclosure: I am the author of Cap'n Proto and Protobuf v2.)