General
- Introduced Protocol Buffers language version 3 (aka proto3).
When protobuf was initially opensourced it implemented Protocol Buffers
language version 2 (aka proto2), which is why the version number
started from v2.0.0. From v3.0.0, a new language version (proto3) is
introduced while the old version (proto2) will continue to be supported.
The main intent of introducing proto3 is to clean up protobuf before
pushing the language as the foundation of Google's new API platform.
In proto3, the language is simplified, both for ease of use and to
make it available in a wider range of programming languages. At the
same time a few features are added to better support common idioms
found in APIs.
The following are the main new features in language version 3:
1. Removal of field presence logic for primitive value fields, removal
of required fields, and removal of default values. This makes proto3
significantly easier to implement with open struct representations,
as in languages like Android Java, Objective C, or Go.
2. Removal of unknown fields.
3. Removal of extensions, which are instead replaced by a new standard
type called Any.
4. Fix semantics for unknown enum values.
5. Addition of maps.
6. Addition of a small set of standard types for representation of time,
dynamic data, etc.
7. A well-defined encoding in JSON as an alternative to binary proto
encoding.
This release (v3.0.0-alpha-1) includes partial proto3 support for C++ and
Java. Items 6 (well-known types) and 7 (JSON format) in the above feature
list are not impelmented.
A new notion "syntax" is introduced to specify whether a .proto file
uses proto2 or proto3:
// foo.proto
syntax = "proto3";
message Bar {...}
If omitted, the protocol compiler will generate a warning and "proto2" will
be used as the default. This warning will be turned into an error in a
future release.
We recommend that new Protocol Buffers users use proto3. However, we do not
generally recommend that existing users migrate from proto2 from proto3 due
to API incompatibility, and we will continue to support proto2 for a long
time.
- Added support for map fields (implemented in C++/Java for both proto2 and
proto3).
Map fields can be declared using the following syntax:
message Foo {
map<string, string> values = 1;
}
Data of a map field will be stored in memory as an unordered map and it
can be accessed through generated accessors.
C++
- Added arena allocation support (for both proto2 and proto3).
Profiling shows memory allocation and deallocation constitutes a significant
fraction of CPU-time spent in protobuf code and arena allocation is a
technique introduced to reduce this cost. With arena allocation, new
objects will be allocated from a large piece of preallocated memory and
deallocation of these objects is almost free. Early adoption shows 20% to
50% improvement in some Google binaries.
To enable arena support, add the following option to your .proto file:
option cc_enable_arenas = true;
Protocol compiler will generate additional code to make the generated
message classes work with arenas. This does not change the existing API
of protobuf messages and does not affect wire format. Your existing code
should continue to work after adding this option. In the future we will
make this option enabled by default.
To actually take advantage of arena allocation, you need to use the arena
APIs when creating messages. A quick example of using the arena API:
{
google::protobuf::Arena arena;
// Allocate a protobuf message in the arena.
MyMessage* message = Arena::CreateMessage<MyMessage>(&arena);
// All submessages will be allocated in the same arena.
if (!message->ParseFromString(data)) {
// Deal with malformed input data.
}
// Must not delete the message here. It will be deleted automatically
// when the arena is destroyed.
}
Currently arena does not work with map fields. Enabling arena in a .proto
file containing map fields will result in compile errors in the generated
code. This will be addressed in a future release.