overloading operator < for std::set confused me - c++

Question

I know that I have to overload operator < for std::set.
I overload operator < with two classes: "UniqueID" and "UniqueIDWithBug".
The only difference is "UniqueID" added code this->unique_id_a_ == t.unique_id_a_ while comparing.
Then I put same elements into the two sets.
Finally I find one element inside the sets.
One set can find it, another can not.
This problem confused me for a long time.
struct UniqueID {
uint64_t unique_id_a_{0};
uint64_t unique_id_b_{0};
bool operator<(const UniqueID &t) const {
if (this->unique_id_a_ < t.unique_id_a_) {
return true;
}
if (this->unique_id_a_ == t.unique_id_a_ &&
this->unique_id_b_ < t.unique_id_b_) {
return true;
}
return false;
}
};
struct UniqueIDWithBug {
uint64_t unique_id_a_{0};
uint64_t unique_id_b_{0};
bool operator<(const UniqueIDWithBug &t) const {
if (this->unique_id_a_ < t.unique_id_a_) {
return true;
}
return (this->unique_id_b_ < t.unique_id_b_);
}
};
// init data
std::set<UniqueID> _set = {
{17303934402126834534u, 2922971136},
{8520106912500150839u, 3118989312},
{9527597377742531532u, 2171470080},
{10912468396223017462u, 3972792320},
};
std::set<UniqueIDWithBug> _set_with_bug = {
{17303934402126834534u, 2922971136},
{8520106912500150839u, 3118989312},
{9527597377742531532u, 2171470080},
{10912468396223017462u, 3972792320}};
UniqueID _unique_id = {10912468396223017462u, 3972792320};
UniqueIDWithBug _unique_id_with_bug = {10912468396223017462u, 3972792320};
if (_set.find(_unique_id) == _set.end()) {
std::cout << "_set not find" << std::endl;
}
if (_set_with_bug.find(_unique_id_with_bug) == _set_with_bug.end()) {
std::cout << "_set_with_bug not find" << std::endl;
}
The outputs:
_set_with_bug not find

Answer 1

The less-than operation you define for use with std::set (and others) must be a valid strict weak ordering.
Your UniqueIDWithBug ordering is not.
For example, consider:
UniqueIDWithBug a{1, 10};
UniqueIDWithBug b{2, 5};
Now observe that both a < b and b < a are true. This is just a quick demonstration that you do not have a strict weak ordering; indeed, this is not an ordering at all!
So your program has undefined behaviour. The internals of the std::set mechanism assume a valid ordering, but yours is not. In this case the observable result was "element not found". It could have been "make a pizza".
Constructing a good strict weak ordering can be difficult, but you've already done the hard work, because UniqueID's ordering is correct.
Alternatively, abandon the ordering entirely, define a hash function, and switch to unordered_set.