Correct way of 'upserting' items into map<key, shared_ptr<foo>>

Question

I want to upsert (update or insert) items into a map<int,shared_ptr<PortfolioEntry>> structure. My current code is something like the following:

auto existing = positions.find(id);
if (existing == positions.end())
{
  positions[id] = make_shared<PortfolioEntry>(id, amount, price);
}
else
{
  // update positions[id]
}

So I'm wondering if it's the right way of doing things. Is find() efficient? Is assigning to positions[id] the right way to do this, or should I use some std::move construct?

Answer 1

The fastest way is to try to insert first and change the iterator value if nothing was inserted:

  template < class KeyType, class ElementType >
  bool SetAndCheckChanged(
    std::map< KeyType, ElementType >& the_map,
    KeyType const& key,
    ElementType const& new_value)
  {
    typedef typename std::map< KeyType, ElementType >::iterator Iterator;
    typedef typename std::pair< Iterator, bool > Result;
    Result result = the_map.insert(typename std::map< KeyType, ElementType >::value_type(key, new_value));
    if (!result.second)
    {
      if ( !(result.first->second == new_value ))
      {
        result.first->second = new_value;
        return true;
      }
      else
        return false; // it was the same
    }
    else
      return true;  // changed cause not existing
  }