Most C++ constructors should be marked explicit to prevent unintended implicit conversions. So a class Book containing fields name and author shouldn’t be directly initialised from {"hamlet", "shakespeare"}. It only makes sense for structs that are bags of data.
What is a bag of data?
Structs like pair, tuple, vector are bags of data. They are simply aggregates and nothing else. Implicit construction makes sense for them.
Make bool conversion operator explicit
Ideally you shouldn’t write bool conversion operators, but if you do, you should make it explicit. explicit operator bool() const will allow things to be used as if conditionals but prevent bool b = obj assignments.