如何 append 多个元素到协议缓冲区中的重复字段？

Question

Given a message

message My_msg{
  repeated double my_arr = 1;
}

How can one append elements into the field?

There is an answer for copying/moving already allocated full arrays by overwriting the current contents , but what if there is already some data in the filed, which needs to be kept as it is?

Is using the below code to do it safe?

void set_data(std::vector<double> table, My_msg* message){ /* suppose message is valid */
  message->mutable_my_arr()->Resize(message->my_arr_size() + table.size(),0);
  message->mutable_my_arr()[message->my_arr_size() - table.size()] = {table.begin(),table.end()};
}

Answer 1

I don't think your sample code would build. My_msg::mutable_my_arr() returns a pointer to a RepeatedField (not the first element of an array). Trying to index it would segfault at best.

In terms or performance, if you have your data in an std::vector you will always need to copy - so you could just try to make that faster.

You can call RepeatedField::Reserve before. Then you can either write a loop, or use RepeatedFieldBackInserter :

void set_data(const std::vector<double>& table, My_msg* message){
  message->mutable_my_arr()->Reserve(message->my_arr_size() + table.size());
  std::copy(
      table.begin(),
      table.end(), 
      RepeatedFieldBackInserter(message->mutable_my_arr()));
}

Answer 2

Just call My_msg::add_my_arr :

void set_data(std::vector<double> table, My_msg* message) {
    for (auto ele : table) {
        message->add_my_arr(ele);
    }
}