I was writing the solution for this and needed to create a set, well actually an unordered_map from int to my custom set, without further ado, this is what I came up whith:
auto comparator = [](pair<int, int> a, pair<int, int> b) {return a.second == b.second ? a.first < b.first : a.second < b.second; };
unordered_map<int, set < pair<int, int>,decltype(comparator) >> H;
The rest of code is not relevant. Here the compiler complains that
"Severity Code Description Project File Line Suppression State
<lambda_6dd753bdcbaf959c162af7914f0815bf>(void)': attempting to reference a deleted function"
Can anyone please describe that is compiler trying to tell me, or even better if you could hint what I have done wrong. Thanks in advance.
It's telling you that it needs to invoke the default c'tor for that lambda type you defined. However, it is only in C++20 and beyond that a captureless lambda is default-constructible. Prior to that all lambdas cannot be default constructed, they only spring into existence from a corresponding lambda expression (or copied of course).
I recommend using a custom functor type for this.
struct Comparator {
bool operator()(pair<int, int> a, pair<int, int> b)
{ return a.second == b.second ? a.first < b.first : a.second < b.second; };
};
unordered_map<int, set < pair<int, int>, Comparator >> H;