Building a std::map and issue with using std::emplace

Question

Code:

std::map<CString, S_DISCUSSION_HIST_ITEM> mapHistory;
// History list is in ascending date order
for (auto& sHistItem : m_listDiscussionItemHist)
{
    if (m_bFullHistoryMode)
        mapHistory.emplace(sHistItem.strName, sHistItem);
    else if (sHistItem.eSchool == m_eActiveSchool)
        mapHistory.emplace(sHistItem.strName, sHistItem);
}
// The map is sorted by Name (so reset by date later)
// The map has the latest assignment info for each Name now

Observation:

I now understand that std::emplace behaves like this:

The insertion only takes place if no other element in the container has a key equivalent to the one being emplaced (keys in a map container are unique).

Therefore my code is flawed. What I was hoping to acheive (in pseudo code) is:

For Each History Item
   Is the name in the map?
      No, so add to map with sHitItem
      Yes, so replace the sHistItem with this one
End Loop

By the end of this loop iteration I want to have the most recent sHitItem, for each person. But as it is, it is only adding an entry into the map if the name does not exist.

What is the simplest way to get around this?

I have just seen this answer which might resolve this: stackoverflow.com/a/51079331/2287576 — Andrew Truckle
– Andrew Truckle, Commented Oct 31, 2021 at 19:23
pretty much whole Is the name in the map?... thing is just equivalent of mapHistory[sHistItem.strName] = sHistItem;. operator[] does what you want if your data object is assignable. — Swift - Friday Pie
– Swift - Friday Pie, Commented Oct 31, 2021 at 19:40
@Swift-FridayPie Yes, as exaplained to me in an answer. I guess I was thinking that the [] operator behaved like in a normal array where it would break if it was not valid. Thanks for the clarification. — Andrew Truckle
– Andrew Truckle, Commented Oct 31, 2021 at 19:43
there is at() method for that. The [] behavior might be unwanted in some cases because it always creates an element if it doesn't exist. Though instead of at it's better just to use find. Once upon a time there was some controversial articles about this in regard to STL (then-Morozov's library) design. — Swift - Friday Pie
– Swift - Friday Pie, Commented Oct 31, 2021 at 20:12

Alex Guteniev · Accepted Answer · 2021-10-31 19:24:02Z

1

Use insert_or_assign method if the item is assignable. It will be assigned if it already exists. Or use [] operator followed by assignment, it will default-construct item if it does not exist.

For non-assignable types I'm afraid there's no convenient way.

answered Oct 31, 2021 at 19:24

Alex Guteniev

14.3k2 gold badges46 silver badges101 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Andrew Truckle Over a year ago

That did the trick - thank you!

Collectives™ on Stack Overflow

Building a std::map and issue with using std::emplace

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related