Migration Guide

A list of the breaking changes made to versions of the libraries, and how to update your code to accommodate the changes.

This topic provides information about breaking changes in Protocol Buffers, and gives guidance on how to update your projects to adapt to those changes.

Compiler Changes in v22.0

JSON Field Name Conflicts

Source of changes: PR #11349, PR #10750

We’ve made some subtle changes in how we handle field name conflicts with respect to JSON mappings. In proto3, we’ve partially loosened the restrictions and only give errors when field names produce case-sensitive JSON mappings (camel case of the original name). We now also check the json_name option, and give errors for case-sensitive conflicts. In proto2, we’ve tightened restrictions a bit and will give errors if two json_name specifications conflict. If implicit JSON mappings (camel case) have conflicts, we will give warnings in proto2.

We’ve provided a temporary message/enum option for restoring the legacy behavior. If renaming the conflicting fields isn’t an option you can take immediately, set the deprecated_legacy_json_field_conflicts option on the specific message/enum. This option will be removed in a future release, but gives you more time to migrate.

C++ API Changes in v22.0

4.22.0 has breaking changes for C++ runtime and protoc, as announced in August.

Autotools Turndown

Source of changes: PR #10132

In v22.0, we removed all Autotools support from the protobuf compiler and the C++ runtime. If you’re using Autotools to build either of these, you must migrate to CMake or Bazel. We have some dedicated instructions for setting up protobuf with CMake.

Abseil Dependency

Source of changes: PR #10416

With v22.0, we’ve taken on an explicit dependency on Abseil. This allowed us to remove most of our stubs, which were branched from old internal code that later became Abseil. There are a number of subtle behavior changes, but most should be transparent to users. Some notable changes include:

  • string_view - absl::string_view has replaced const std::string& in many of our APIs. This is most-commonly used for input arguments, where there should be no noticeable change for users. In a few cases (such as virtual method arguments or return types) users may need to make an explicit change to use the new signature.

  • tables - Instead of STL sets/maps, we now use Abseil’s flat_hash_map, flat_hash_set, btree_map, and btree_set. These are more efficient and allow for heterogeneous lookup. This should be mostly invisible to users, but may cause some subtle behavior changes related to table ordering.

  • logging - Abseil’s logging library is very similar to our old logging code, with just a slightly different spelling (for example, ABSL_CHECK instead of GOOGLE_CHECK). The biggest difference is that it doesn’t support exceptions, and will now always crash when FATAL assertions fail. (Previously we had a PROTOBUF_USE_EXCEPTIONS flag to switch to exceptions.) Since these only occur when serious issues are encountered, we feel unconditional crashing is a suitable response.

    Source of logging changes: PR #11623

  • Build dependency - A new build dependency can always cause breakages for downstream users. We require Abseil LTS 20230117 or later to build.

    • For Bazel builds, Abseil will be automatically downloaded and built at a pinned LTS release when protobuf_deps is run from your WORKSPACE. This should be transparent, but if you depend on an older version of Abseil, you’ll need to upgrade your dependency.

    • For CMake builds, we will first look for an existing Abseil installation pulled in by the top-level CMake configuration (see instructions). Otherwise, if protobuf_ABSL_PROVIDER is set to module (its default) we will attempt to build and link Abseil from our git submodule. If protobuf_ABSL_PROVIDER is set to package, we will look for a pre-installed system version of Abseil.

C++20 Support

Source of changes: PR #10796

To support C++20, we’ve reserved the new keywords in C++ generated protobuf code. As with other reserved keywords, if you use them for any fields, enums, or messages, we will add an underscore suffix to make them valid C++. For example, a concept field will generate a concept_() getter. In the scenario where you have existing protos that use these keywords, you’ll need to update the C++ code that references them to add the appropriate underscores.

Final Classes

Source of changes: PR #11604

As part of a larger effort to harden assumptions made in the Protobuf library, we’ve marked some classes final that were never intended to be inherited from. There are no known use cases for inheriting from these, and doing so would likely cause problems. There is no mitigation if your code is inheriting from these classes, but if you think you have some valid reason for the inheritance you’re using, you can open an issue.

Container Static Assertions

Source of changes: PR #11550

As part of a larger effort to harden assumptions made in the Protobuf library, we’ve added static assertions to the Map, RepeatedField, and RepeatedPtrField containers. These ensure that you’re using these containers with only expected types, as covered in our documentation. If you hit these static assertions, you should migrate your code to use Abseil or STL containers. std::vector is a good drop-in replacement for repeated field containers, and std::unordered_map or absl::flat_hash_map for Map (the former gives similar pointer stability, while the latter is more efficient).

Cleared Element Deprecation

Source of changes: PR #11588, PR #11639

The RepeatedPtrField API around “cleared fields” has been deprecated, and will be fully removed in a later breaking release. This was originally added as an optimization for reusing elements after they’ve been cleared, but ended up not working well. If you’re using this API, you should consider migrating to arenas for better memory reuse.

UnsafeArena Deprecation

Source of changes: PR #10325

As part of a larger effort to remove arena-unsafe APIs, we’ve hidden RepeatedField::UnsafeArenaSwap. This is the only one we’ve removed so far, but in later releases we will continue to remove them and provide helpers to handle efficient borrowing patterns between arenas. Within a single arena (or the stack/heap), Swap is just as efficient as UnsafeArenaSwap. The benefit is that it won’t cause invalid memory operations if you accidentally call it across different arenas.

Map Pair Upgrades

Source of changes: PR #11625

For v22.0 we’ve started cleaning up the Map API to make it more consistent with Abseil and STL. Notably, we’ve replaced the MapPair class with an alias to std::pair. This should be transparent for most users, but if you were using the class directly you may need to update your code.

New JSON Parser {:#json-parser}

Source of changes: PR #10729

We have rewritten the C++ JSON parser this release. It should be mostly a hidden change, but inevitably some undocumented quirks my have changed; test accordingly. Parsing documents that are not valid RFC-8219 JSON (such as those that are missing quotes or using non-standard bools) is deprecated and will be removed in a future release. The serialization order of fields is now guaranteed to match the field number order, where before it was less deterministic.

As part of this migration, all of the files under util/internal have been deleted. These were used in the old parser, and were never intended to be used externally.


Source of changes: PR #10623

The Init method in Arena was code that didn’t do anything, and has now been removed. If you were calling this method, you likely meant to call the Arena constructor directly with a set of ArenaOptions. You should either delete the call or migrate to that constructor.

ErrorCollector Migration

Source of changes: PR #11555

As part of our Abseil migration, we’re moving from const std::string& to absl::string_view. For our three error collector classes, this can’t be done without breaking existing code. For v22.0, we’ve decided to release both variants, and rename the methods from AddError and AddWarning to RecordError and RecordWarning. The old signature has been marked deprecated, and will be slightly less efficient (due to string copies), but will otherwise still work. You should migrate these to the new version, as the Add* methods will be removed in a later breaking release.