function doesn't get an updated queue that used by threads - c++

Question

I have 2 functions that the main run them as a thread. first one reads from a file of messages and pushes every line into a queue and the second function writes into a file the messages of the queue.
from some reason, the queue in the second function doesn't update and the function writes nothing to the output file
here is the first func:
void MessagesSender::saveMessages()
{
std::string line = "";
std::ifstream file;
file.open(DATA_FILE);
if (file.is_open())
{
while (getline(file, line))
{
m.lock();
messages.push(line);
m.unlock();
}
file.close();
file.open(DATA_FILE, std::ofstream::out | std::ofstream::trunc);//clear file
file.close();
}
}
Here is the second:
void MessagesSender::sendMessages()
{
std::ofstream file(OUTPUT_FILE);
m.lock();
std::queue<std::string> temp_messages = messages;//problem - "messages" is empty
m.unlock();
if (file.is_open())
{
int i = 0;
while (!temp_messages.empty())
{
while (i < userName.size())
{
file << userName[i] << ": " << temp_messages.front() << std::endl;
i++;
}
temp_messages.pop();
i = 0;
}
file.close();
}
else std::cout << "Unable to open file";
}
int main()
{
MessagesSender mailingList;
while (mailingList.menu()){}//shows to the user the menu
std::thread t1(&MessagesSender::saveMessages, mailingList);
std::thread t2(&MessagesSender::sendMessages, mailingList);
t1.join();
t2.join();
}
why "messages" is empty and how can I fixe it?