Comparator can be used to set a new key, isn't? - c++

Question

I need to change the "key" of a multiset:
multiset<IMidiMsgExt, IMidiMsgExtCompByNoteNumber> playingNotes;
such as that when I use the .find() function it search and return the first object (iterator) with that NoteNumber property value.
I said "first" because my multiset list could contains objects with the same "key". So I did:
struct IMidiMsgExtCompByNoteNumber {
bool operator()(const IMidiMsgExt& lhs, const IMidiMsgExt& rhs) {
return lhs.NoteNumber() < rhs.NoteNumber();
}
};
but when I try to do:
auto it = playingNotes.find(60);
the compiler says no instance of overloaded function "std::multiset<_Kty, _Pr, _Alloc>::find [with _Kty=IMidiMsgExt, _Pr=IMidiMsgExtCompByNoteNumber, _Alloc=std::allocator<IMidiMsgExt>]" matches the argument list
Am I misunderstanding the whole thing? What's wrong?

Answer 1

I do believe that you have some misunderstandings here:
Part of an associative container's type is it's key type and comparator. Because C++ is strongly typed the only way to change the comparator on a container is to create a new container, copying or moving all the elements into it
Creating a copy of all the elements in a container is a potentially expensive process
By creating a copy you are violating the Single Source of Truth best practice
multiset is used infrequently, I have used it once in my career, others have pointed out it's shortcomings and recommended that you use another container, write your own container, or in my case I'd suggests simply using vector and sorting it how you want when you have to
I'm going to catalog your comments to show how the answer I've already given you is correct:
We're going to assume that the multiset<IMidiMsgExt, IMidiMsgExtCompByNoteNumber> that you've selected is necessary and cannot be improved upon by using vector as suggested in 4, where:
struct IMidiMsgExtCompByNoteNumber {
bool operator()(const IMidiMsgExt& lhs, const IMidiMsgExt& rhs) {
return lhs.NoteNumber() < rhs.NoteNumber();
}
};
You cannot use multiset::find because that requires you tospecify the exact IMidiMsgExt you are searching for; so you'll need to use find_if(cbegin(playingNotes), cend(playingNotes), [value = int{60}](const auto& i){return i.mNote == value;}) to search for a specific property value. Which will be fine to use on to use directly on PlayingNotes without changing the sorting, because you say:
I want to delete the first note that has mNote of 60. No matter the mTime when deleting.
You'll need to capture the result of the [find_if], check if it is valid, and if so erase it as demonstrated in my answer, because you say:
The first element find will find for that, erase. [sic]
I would roll the code from my answer into a function because you say:
Ill recall find if I want another element, maybe with same value, to get deleted [sic]
Your final solution should be to write a function like this:
bool foo(const multiset<IMidiMsgExt, IMidiMsgExtCompByNoteNumber>& playingNotes, const int value) {
const auto it = find_if(cbegin(playingNotes), cend(playingNotes), [=](const auto& i){return i.mNote == value;});
const auto result = it != cend(playingNotes);
if(result) {
playingNotes.erase(it);
}
return result;
}
And you'd call it something like this: foo(playingNotes, 60) if you wish to know whether an element was removed you may test foo's return.