How to use reflection of Protobuf to modify a Map - c++

Question

I'm working with Protobuf3 in my C++14 project. There have been some functions, which returns the google::protobuf::Message*s as a rpc request, what I need to do is to set their fields. So I need to use the reflection of Protobuf3.
Here is a proto file:
syntax="proto3";
package srv.user;
option cc_generic_services = true;
message BatchGetUserInfosRequest {
uint64 my_uid = 1;
repeated uint64 peer_uids = 2;
map<string, string> infos = 3;
}
message BatchGetUserInfosResponse {
uint64 my_uid = 1;
string info = 2;
}
Service UserSrv {
rpc BatchGetUserInfos(BatchGetUserInfosRequest) returns (BatchGetUserInfosResponse);
};
Now I called a function, which returns a google::protobuf::Message*, pointing an object BatchGetUserInfosRequest and I try to set its fields.
// msg is a Message*, pointing to an object of BatchGetUserInfosRequest
auto descriptor = msg->GetDescriptor();
auto reflection = msg->GetReflection();
auto field = descriptor->FindFieldByName("my_uid");
reflection->SetUInt64(msg, field, 1234);
auto field2 = descriptor->FindFieldByName("peer_uids");
reflection->GetMutableRepeatedFieldRef<uint64_t>(msg, field2).CopyFrom(peerUids); // peerUids is a std::vector<uint64_t>
As you see, I can set my_uid and peer_uids as above, but for the field infos, which is a google::protobuf::Map, I don't know how to set it with the reflection mechanism.

Answer 1

If you dig deep into the source code, you would find out the map in proto3 is implemented on the RepeatedField:
// Whether the message is an automatically generated map entry type for the
// maps field.
//
// For maps fields:
// map<KeyType, ValueType> map_field = 1;
// The parsed descriptor looks like:
// message MapFieldEntry {
// option map_entry = true;
// optional KeyType key = 1;
// optional ValueType value = 2;
// }
// repeated MapFieldEntry map_field = 1;
//
// Implementations may choose not to generate the map_entry=true message, but
// use a native map in the target language to hold the keys and values.
// The reflection APIs in such implementations still need to work as
// if the field is a repeated message field.
//
// NOTE: Do not set the option in .proto files. Always use the maps syntax
// instead. The option should only be implicitly set by the proto compiler
// parser.
optional bool map_entry = 7;
Inspired by the test code from protobuf, this works for me:
BatchGetUserInfosRequest message;
auto *descriptor = message.GetDescriptor();
auto *reflection = message.GetReflection();
const google::protobuf::FieldDescriptor *fd_map_string_string =
descriptor->FindFieldByName("infos");
const google::protobuf::FieldDescriptor *fd_map_string_string_key =
fd_map_string_string->message_type()->map_key();
const google::protobuf::FieldDescriptor *fd_map_string_string_value =
fd_map_string_string->message_type()->map_value();
const google::protobuf::MutableRepeatedFieldRef<google::protobuf::Message>
mmf_string_string =
reflection->GetMutableRepeatedFieldRef<google::protobuf::Message>(
&message, fd_map_string_string);
std::unique_ptr<google::protobuf::Message> entry_string_string(
google::protobuf::MessageFactory::generated_factory()
->GetPrototype(fd_map_string_string->message_type())
->New(message.GetArena()));
entry_string_string->GetReflection()->SetString(
entry_string_string.get(), fd_map_string_string->message_type()->field(0),
"1234");
entry_string_string->GetReflection()->SetString(
entry_string_string.get(), fd_map_string_string->message_type()->field(1),
std::to_string(10));
mmf_string_string.Add(*entry_string_string);
std::cout << "1234: " << message.infos().at("1234") << '\n';
The output:
1234: 10