How to use lambda capture with lock? - c++11

Question

In the code below, the member variable (match_room_list_) is a shared resource. So I used a mutex.
But locking confuses me.
Should I use lock or not in an inner lambda function?
void room::join_room(int user_id)
{
std::lock_guard<std::recursive_mutex> lock(mutex_);
std::shared_ptr<match_room> find_room = nullptr;
for (auto iter : match_room_list_)
{
if (false == iter.second->is_public_room())
{
continue;
}
if (true == iter.second->is_full())
{
continue;
}
find_room = iter.second;
break;
}
// Something
// async ( other thread call this lambda function )
// call database
database_manager::get_instance().join_room(find_room->get_room_id(), user_id, [=](bool success, std::string room_id, std::vector<int> user_id_list)
{
// How do you think I should using lock?
// std::lock_guard<std::recursive_mutex> lock(mutex_);
// shared resource
match_room_list_[room_id].join_user(user_id);
response_join_room_ntf(room_id, user_id_list);
}
}

Answer 1

If your lambda function will ever run on a different thread (and that's likely) then yes, you do need to lock it.
The only possible problem with doing so could be if it was called on the same thread from a function that had already locked the mutex. But that won't be an issue here since your mutex is a recursive one and a given thread can lock that as many times as it wants.
You may also want to look into conditions such as:
if (false == iter.second->is_public_room())
if (true == iter.second->is_full())
They would be far better written as:
if (! iter.second->is_public_room())
if (iter.second->is_full())