Getting Segmentation violation signal error on copying vector elements?-CodePudding

I have the following code for doing a http post request. I am getting the response in const std::vector<json> value ...The below code works correctly

RpcResponseAndContext<std::vector<Largest>>
Connection::getLargestAccounts() const {
  const json params = {};
  const auto reqJson = jsonRequest("getLargest", params);
  const json res = sendJsonRpcRequest(reqJson);
  const std::vector<json> value = res["value"];
  std::vector<Largest> accounts_list;
  accounts_list.reserve(value.size());
  for (const json &account : value) {
    accounts_list.push_back(account);
  }
  return {res["context"], accounts_list};
}

To improve the appending of elements into accounts_list vector..I am trying to use std::copy() But getting this error: FATAL ERROR: test case CRASHED: SIGSEGV - Segmentation violation signal

I am trying this way:

RpcResponseAndContext<std::vector<Largest>>
Connection::getLargestAccounts() const {
  const json params = {};
  const auto reqJson = jsonRequest("getLargest", params);
  const json res = sendJsonRpcRequest(reqJson);
  const std::vector<json> value = res["value"];
  std::vector<Largest> accounts_list;
  accounts_list.reserve(value.size());
  std::copy(value.begin(), value.end(), accounts_list.begin());
  return {res["context"], accounts_list};
}

What I am doing wrong ?

CodePudding user response：

reserve does not resize the vector, it just pre-allocates the space so push_backs are cheap and never invalidate any iterators but they are still required.

std::copy assumes (like most <algorithm>s) the output iterators are valid, i.e. point to existing location. In this case they do not.

What you need is std::back_inserter which will call push_back for you:

accounts_list.reserve(value.size()); // Optional optimization
std::copy(value.begin(), value.end(), std::back_inserter(accounts_list));

I am not actually sure whether this is the cause since you did not provide a minimal reproducible example, I would expect some iterator assertion, not segfault since the memory should have been allocated. Maybe some empty std::vector optimization is at play.