How to use reflection of Protobuf to modify a Map

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.