c++ - "Don't Care" fields in multiset keys -

i have compound data type like:

struct key {     optional<int> a;     optional<int> b;     optional<int> c; }; 

i have multiset, multiset<key>. example, contains

{1, 2, 3} {1, null, 3} {null, 2, 3} {null, null, 3} 

i want objects in multiset match {1, 2, 3}. there catch: null fields should match anything. example, {1, 2, 3} matches {1, null, 3}.

i tried defined comparator (<) ignores null values. example {1, null, null} == {null, 2, 3}. not follow weak strict ordering , gives me wrong results.

how can that?

your problem here more serious not following weak strict ordering rules. equality not equivalence relation: { 1,null,3} matches {1, 2, 3} , {1, 4, 3}, {1, 2, 3} not match {1, 4, 3}. conclusion cannot rely on standard container meet matching requirement catch all values.

if want store them, should try use unordered_set or unordered_multiset because allow store value without problems. have manually implement method searching container match.

beware: not advise subclass standard container (it wrong idea because not designed allow derivation, have no virtual destructor...), instead include 1 in custom class , delegate storage it.