Allow using `rfl::AddTagsToVariants` with `rfl::Generic` and same-name-different-namespace structs #467

jmcken8 · 2025-07-18T16:09:40Z

The first one was the main reason for this change; the second one (ex: a::Message and b::Message) was a positive result of applying the change. Added tests for both.

The issue was that both std::vector<Generic> and rfl::Object<Generic> were getting the type name 'Generic', which caused a compiler error due to duplicated field names.

This will slightly increase the size of serialized formats when using the AddTagsToVariants, but it seems like a worthwhile change.

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Summary of Changes

Hello @jmcken8, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the rfl::AddTagsToVariants functionality by ensuring that fully qualified type names are used as tags, which resolves previous issues with rfl::Generic types causing name collisions and allows for proper serialization of same-name structs across different namespaces. This change slightly increases the size of serialized formats but significantly improves robustness and flexibility.

Highlights

Core Logic Change: Modified rfl::parsing::VariantAlternativeWrapper::make_tag to no longer strip namespaces from type names when generating tags for rfl::AddTagsToVariants. This ensures that fully qualified type names are used as tags, resolving issues with name collisions.
Support for rfl::Generic in Variants: Enabled the correct serialization and deserialization of rfl::Generic types when they are part of a rfl::Variant or std::variant and rfl::AddTagsToVariants is applied. Previously, std::vector<Generic> and rfl::Object<Generic> would both be tagged as 'Generic', leading to compiler errors due to duplicate field names.
Support for Same-Name-Different-Namespace Structs: Improved compatibility for structs that share the same name but reside in different namespaces (e.g., NamespaceA::Message and NamespaceB::Message). These can now be correctly serialized as variant alternatives using their fully qualified names as tags.
New Test Cases: Added comprehensive test cases in tests/json/test_add_tag_to_variant.cpp to validate the new behavior for rfl::Generic within variants and for structs with identical names in different namespaces.

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Code Review

This pull request modifies rfl::AddTagsToVariants to include full type names with namespaces when generating tags for variant alternatives, resolving issues with rfl::Generic and enabling the use of structs with identical names from different namespaces. The new tests validate these improvements. I've provided suggestions for the new test file to enhance comment clarity, code formatting, and test suite organization.

tests/json/test_add_tag_to_variant.cpp

…structs The first one was the main reason for this change; the second one was a result of applying the change. Added tests for both. The issue was that both `std::vector<Generic>` and `rfl::Object<Generic>` were getting the type name 'Generic', which caused a compiler error due to duplicated field names. This will slightly increase the size of serialized formats when using the AddTagsToVariants, but it seems like a worthwhile change.

bryceschober · 2025-07-18T20:20:11Z

@jmcken8 IMO, the additional size needed for namespaces, which are often nested, means that this change should be of an opt-in variety, as in rfl::AddNamespacedTagsToVariants. There are serialization formats here that are designed for much more space-constrained contexts than JSON.

jmcken8 · 2025-07-23T07:50:58Z

@bryceschober I'm on the fence - AddTagsToVariants already seems like an edge-case issue where it really should just work no matter what's given to it, especially since there are several available ways to add tags explicitly (rfl::TaggedUnion, rfl::Variant with rfl::Field) if a user is worried about the serialized size.

What would you suggest for the rfl::Generic case, i.e. it contains a std::variant with std::vector<rfl::Generic> -> Generic and rfl::Generic<rfl::Generic> -> Generic? Whether or not namespaces are added, I think a change should be made so AddTagsToVariants works with rfl::Generic.

bryceschober · 2025-07-24T01:02:04Z

@jmcken8 Well I'm not sure how you would make your solution work either. I'm guessing that you've looked at the test failures reported here by now. Looking at GCC 12's test failure, I see:

[ RUN      ] json.test_add_tag_to_variant_with_generic
/home/runner/work/reflect-cpp/reflect-cpp/tests/json/write_and_read.hpp:35: Failure
Expected equality of these values:
  json_string1
    Which is: "{\"test_add_tag_to_variant::APIResult\":{\"result\":{\"std::__cxx11::basic_string<char>\":\"200\"}}}"
  _expected
    Which is: "{\"test_add_tag_to_variant::APIResult\":{\"result\":{\"std::string\":\"200\"}}}"
Test failed on write. Expected:
{"test_add_tag_to_variant::APIResult":{"result":{"std::string":"200"}}}
Got: 
{"test_add_tag_to_variant::APIResult":{"result":{"std::__cxx11::basic_string<char>":"200"}}}

But those are "internal" namespaces, so they're different on for example LLVM 17:

/home/runner/work/reflect-cpp/reflect-cpp/tests/json/write_and_read.hpp:35: Failure
Expected equality of these values:
  json_string1
    Which is: "{\"test_add_tag_to_variant::APIResult\":{\"result\":{\"std::basic_string<char>\":\"200\"}}}"
  _expected
    Which is: "{\"test_add_tag_to_variant::APIResult\":{\"result\":{\"std::string\":\"200\"}}}"
Test failed on write. Expected:
{"test_add_tag_to_variant::APIResult":{"result":{"std::string":"200"}}}
Got: 
{"test_add_tag_to_variant::APIResult":{"result":{"std::basic_string<char>":"200"}}}

My initial guess is that is the original reason for removing the namespace, and I don't know how you would get to where you want from here.

I don't know your use case's requirements, but you do have the opportunity to differentiate your variant members by using the manual tagging suggested in the docs for variants:

namespace Result {
struct Message {
  using Tag = rfl::Literal<"Result::Message">;
  std::string result;
};
}  // namespace Result

namespace Error {
struct Message {
  using Tag = rfl::Literal<"Error::Message">;
  std::string error;
  int error_id;
};
};  // namespace Error

... or whatever naming method of manual disambiguation you'd like.

I also wonder what would happen if you used more fully qualified aliases for the struct types:

using ResultMessage = Result::Message;
using ErrorMessage = Result::Message;
using Messages = std::variant<ResultMessage, ErrorMessage>;

If that method works, then at least you wouldn't be manually keeping string name around that could get out of sync over time.

liuzicheng1987 · 2025-09-02T21:17:58Z

Hi @jmcken8 and @bryceschober , how should we proceed with this PR? Should I take a look?

bryceschober · 2025-09-03T00:31:37Z

Yes, @liuzicheng1987 I think that would be helpful.

jmcken8 · 2025-09-03T08:54:27Z

Thanks, @liuzicheng1987, yes, that'd be helpful. @bryceschober that would work for the custom case, but it's really the variant case that's the issue, fixing the other one was just a (perhaps unwanted) side-effect.

I haven't had a chance to get back to this in the last month and won't for a few more weeks, so any information you can provide would be helpful @liuzicheng1987.

liuzicheng1987 · 2025-09-07T17:48:23Z

@jmcken8 @bryceschober it may not be the most elegant solution I ever came up with, but it gets the job done.

bryceschober

IMO, this proposal puts the burden on the wrong side of the fence.

I'm guessing that the most common use-case in reflect-cpp is with strongly typed members, since that's kinda most of the point of reflection - to enable generic processing on specific strong types, and leaning towards doing as much of that as practical at compile time.

Given that perspective, the use of Generic is not at the core of the intended use-case. This solution forces all variant alternatives to be manually tagged in order to use the reflection-based rfl::AddTagsToVariants without getting a lot of namespace "noise".

I was hoping that there could be an alternative solution, like:

Support optionally defining the tag name for a Generic, which could be reused internally for its std::vector<Generic> and rfl::Object<Generic> type aliases.
Support a separate rfl::AddNamespacedTagsToVariants for the use-cases that need/want it. I hacked up a stupid-simple and not-upstream-worthy copy-paste version of this here. I'm guessing that someone smarter about modern C++ than I could figure out how to share more of the implementation layers by adding a non-type template variable with a default value.

All that said, this solution does seem to work, and I think it might be reasonable in my use-case to manually tag every variant alternative type. I've made some more specific PR comments elsewhere.

bryceschober · 2025-09-08T18:31:22Z

tests/json/test_add_tag_to_rfl_variant.cpp

      vec,
-      R"([{"button_pressed_t":{}},{"button_released_t":{"button":4}},{"key_pressed":{"key":99}},{"int":3}])");
+      R"([{"test_add_tag_to_rfl_variant::button_pressed_t":{}},{"test_add_tag_to_rfl_variant::button_released_t":{"button":4}},{"key_pressed":{"key":99}},{"int":3}])");
 }


Suggestion: Add another test to explicitly demonstrate how to avoid any namespace decoration in the serialization.

Suggested change

}

// Alternatively, in order to avoid getting the namespace to every alternative in our auto-tagged

// variant, we must manually tag all variant alternatives to control all serialized names

struct button_pressed_tagged_t {

using Tag = rfl::Literal<"button_pressed">;

};

struct button_released_tagged_t {

using Tag = rfl::Literal<"button_released">;

rfl::Box<int> button;

};

using my_tagged_event_type_t =

rfl::Variant<button_pressed_tagged_t, button_released_tagged_t, key_pressed_t, int>;

TEST(json, test_add_manual_tags_to_rfl_variants) {

std::vector<my_tagged_event_type_t> vec;

vec.emplace_back(button_pressed_tagged_t{});

vec.emplace_back(button_released_tagged_t{rfl::make_box<int>(4)});

vec.emplace_back(key_pressed_t{'c'});

vec.emplace_back(3);

write_and_read<rfl::AddTagsToVariants>(

vec,

R"([{"button_pressed":{}},{"button_released":{"button":4}},{"key_pressed":{"key":99}},{"int":3}])");

}

bryceschober · 2025-09-08T20:50:29Z

tests/json/test_add_tag_to_variant.cpp

+  rfl::Generic error;
+};
+
+using APICallOutput = rfl::Variant<APIResult, APIError>;


Please fix: I think this was meant to be std::variant:

Suggested change

using APICallOutput = rfl::Variant<APIResult, APIError>;

using APICallOutput = std::variant<APIResult, APIError>;

bryceschober · 2025-09-08T20:53:10Z

tests/json/test_add_tag_to_variant.cpp

+
+namespace Result {
+struct Message {
+  std::string result;


Suggestion: Make a note of the alternative approach using rfl::Tag:

Suggested change

std::string result;

// Alternatively, manually tag instead of getting the namespaced name:

// rfl::Tag<"result_message">

std::string result;

bryceschober · 2025-09-08T20:53:41Z

tests/json/test_add_tag_to_variant.cpp

+
+namespace Error {
+struct Message {
+  std::string error;


Suggestion: Make a note of the alternative approach using rfl::Tag:

Suggested change

std::string error;

// Alternatively, manually tag instead of getting the namespaced name:

// rfl::Tag<"error_message">

std::string error;

bryceschober · 2025-09-08T20:59:00Z

tests/json/test_add_tag_to_variant.cpp

Suggestion: Overall, I think it would be helpful to maintain the test structure of test_add_tag_to_variant.cpp and test_add_tag_to_rfl_variant.cpp in parallel as much as possible, because otherwise they kinda imply that they have different functionality.

liuzicheng1987 · 2025-09-09T13:22:05Z

@bryceschober I like your second proposal of simply having two separate processors. It would also be a non-braking change, which is always good.

bryceschober · 2025-09-10T00:24:33Z

@bryceschober I like your second proposal of simply having two separate processors. It would also be a non-braking change, which is always good.

@liuzicheng1987, I have published #494 as my alternative for your and @jmcken8 to review.

liuzicheng1987 · 2025-09-14T16:31:09Z

@jmcken8 @bryceschober I am going to close this, because the issue was solved in another PR.

gemini-code-assist bot reviewed Jul 18, 2025

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jmcken8 force-pushed the f/bugfix_generic_and_variant_added_tags branch 2 times, most recently from 76961ba to 2775153 Compare July 18, 2025 17:17

jmcken8 force-pushed the f/bugfix_generic_and_variant_added_tags branch from 2775153 to ef5e781 Compare July 18, 2025 17:23

jmcken8 marked this pull request as draft July 23, 2025 08:21

liuzicheng1987 added 2 commits September 7, 2025 15:39

Merge branch 'main' into f/bugfix_generic_and_variant_added_tags

a7fd29e

Handle std::string edge case

db148fb

bryceschober reviewed Sep 8, 2025

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bryceschober mentioned this pull request Sep 9, 2025

Add rfl::AddNamespacedTagsToVariants for rfl::Generic and same-name-different-namespace structs #494

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liuzicheng1987 closed this Sep 14, 2025

jmcken8 deleted the f/bugfix_generic_and_variant_added_tags branch September 22, 2025 14:17

-}
+// Alternatively, in order to avoid getting the namespace to every alternative in our auto-tagged
+// variant, we must manually tag all variant alternatives to control all serialized names
+struct button_pressed_tagged_t {
+  using Tag = rfl::Literal<"button_pressed">;
+};
+struct button_released_tagged_t {
+  using Tag = rfl::Literal<"button_released">;
+  rfl::Box<int> button;
+};
+using my_tagged_event_type_t =
+    rfl::Variant<button_pressed_tagged_t, button_released_tagged_t, key_pressed_t, int>;
+TEST(json, test_add_manual_tags_to_rfl_variants) {
+  std::vector<my_tagged_event_type_t> vec;
+  vec.emplace_back(button_pressed_tagged_t{});
+  vec.emplace_back(button_released_tagged_t{rfl::make_box<int>(4)});
+  vec.emplace_back(key_pressed_t{'c'});
+  vec.emplace_back(3);
+  write_and_read<rfl::AddTagsToVariants>(
+      vec,
+      R"([{"button_pressed":{}},{"button_released":{"button":4}},{"key_pressed":{"key":99}},{"int":3}])");
+}

	using APICallOutput = rfl::Variant<APIResult, APIError>;
	using APICallOutput = std::variant<APIResult, APIError>;

Allow using rfl::AddTagsToVariants with rfl::Generic and same-name-different-namespace structs #467

Allow using rfl::AddTagsToVariants with rfl::Generic and same-name-different-namespace structs #467

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Allow using `rfl::AddTagsToVariants` with `rfl::Generic` and same-name-different-namespace structs #467

Allow using `rfl::AddTagsToVariants` with `rfl::Generic` and same-name-different-namespace structs #467

jmcken8 commented Jul 23, 2025 •

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