Using gRPC C++ without .proto / Protobuf, with custom JSON serialization

[LordofFire]

Hi gRPC team,

I’m exploring gRPC in C++ for learning purposes, and I want to completely avoid .proto files and Protobuf, keeping all message definitions and serialization fully in C++. For simplicity, I chose JSON (using nlohmann::json) and wrote a simple SerializationTraits adapter:

template <>
class grpc::SerializationTraits<nlohmann::json> {
public:
    static grpc::Status Serialize(const nlohmann::json& json,
                                  grpc::ByteBuffer* buffer,
                                  bool* own_buffer) {
        std::string s = json.dump();
        grpc::Slice slice(s.data(), s.size());
        *buffer = grpc::ByteBuffer(&slice, 1);
        *own_buffer = true;
        return grpc::Status::OK;
    }

    static grpc::Status Deserialize(grpc::ByteBuffer* buffer,
                                    nlohmann::json* json) {
        std::vector<grpc::Slice> slices;
        buffer->Dump(&slices);

        std::string data;
        for (auto& s : slices) {
            data.append(reinterpret_cast<const char*>(s.begin()), s.size());
        }

        try {
            *json = nlohmann::json::parse(data);
        } catch (const std::exception& e) {
            return grpc::Status(grpc::StatusCode::INTERNAL,
                                std::string("JSON parse error: ") + e.what());
        }
        return grpc::Status::OK;
    }
};

The official gRPC blog shows how to plug in a custom marshaller in Java with JSON:
https://grpc.io/blog/grpc-with-json/

However, I cannot find an equivalent “Marshaller” interface in C++ gRPC. My questions:

1. How do I provide my own serializer/deserializer (JSON) for request/response messages in C++ gRPC? Is SerializationTraits enough?

2. Is there a C++ API for writing a gRPC service fully without .proto definitions?

   • Can I register a service and methods manually, bypassing protoc code generation?

3. If SerializationTraits is the only option, how exactly do I hook it up so gRPC uses JSON instead of Protobuf?

Here’s what I tried: deriving from grpc::Service and calling AddMethod, but I get stuck on the last argument: RpcMethodHandler. I don’t have generated stubs, so I’m unsure what to pass or how to register the service correctly. And i cant seem to find any docs that show this.

A small additional question: is it possible to completely remove Protobuf dependency? Even if I don’t intend to use it, CMake seems to still require it.

Any guidance, code snippets, or references would be greatly appreciated.

Thanks,
Ilya