CODEWITHSUNDEEP
c++data-structuresstdstdmapstd-pair
Alexander
Alexander

Reputation: 145

How does one return a reference pair from a newly inserted pair (to a map)?

How do I correctly and efficiently return a pair from a newly inserted pair to a map?

inline pair<unsigned int, T> *createObj(unsigned int UID){
    static pair<unsigned int, T> ret;

    objList.insert(pair<unsigned int, T>(UID, T()));

    if (UID_Counter <= UID) 
        UID_Counter = UID+1; 
    ret = make_pair(UID, objList.find(UID)->second);
    return &ret;
}

The above returns a object to use, but whatever I change in ret does not change in the "real pair" in the map...

Basically what i want to achieve is:

  1. Insert new object Z into map A
  2. ("Child"-class) Change the stuff in the returned object Z
  3. In every update cycle, iterate through the objects in map A and use the actual data loaded into object "A-Z" when updating...

Upvotes: 2

Views: 1312

Answers (1)

Luc Danton
Luc Danton

Reputation: 35449

The particular overload of insert that you're using returns an std::pair<iterator, bool>. In particular the first member of that pair is an iterator to either the element that was newly inserted or the element that was already present. Thus:

pair<const unsigned int, T>&
createObj(unsigned int UID)
{
    auto inserted = objList.insert(pair<unsigned int, T>(UID, T()));

    if (UID_Counter <= UID) 
        UID_Counter = UID+1; 

    return *inserted.first;
}

Notice that I'm returning a reference where you were returning a pointer and that the key type is const unsigned int, not unsigned int. You can use map_type::value_type, too (where map_type is the type of your container).

If you're wondering why your code wasn't working, it's because you were storing a copy of the mapped object inside ret, so any modification through the pointer you returned would only affect that copy.

Upvotes: 4

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