1 2[section Relational operators] 3 4Type `optional<T>` is __SGI_EQUALITY_COMPARABLE__ whenever `T` is __SGI_EQUALITY_COMPARABLE__. Two optional objects containing a value compare in the same way as their contained values. The uninitialized state of `optional<T>` is treated as a distinct value, equal to itself, and unequal to any value of type `T`: 5 6 boost::optional<int> oN = boost::none; 7 boost::optional<int> o0 = 0; 8 boost::optional<int> o1 = 1; 9 10 assert(oN != o0); 11 assert(o1 != oN); 12 assert(o0 != o1); 13 assert(oN == oN); 14 assert(o0 == o0); 15 16The converting constructor from `T` as well as from `boost::none` implies the existence and semantics of the mixed comparison between `T` and `optional<T>` as well as between `none_t` and `optionl<T>`: 17 18 assert(oN != 0); 19 assert(o1 != boost::none); 20 assert(o0 != 1); 21 assert(oN == boost::none); 22 assert(o0 == 0); 23 24This mixed comparison has a practical interpretation, which is occasionally useful: 25 26 boost::optional<int> choice = ask_user(); 27 if (choice == 2) 28 start_procedure_2(); 29 30In the above example, the meaning of the comparison is 'user chose number 2'. If user chose nothing, he didn't choose number 2. 31 32In case where `optional<T>` is compared to `none`, it is not required that `T` be __SGI_EQUALITY_COMPARABLE__. 33 34In a similar manner, type `optional<T>` is __SGI_LESS_THAN_COMPARABLE__ whenever `T` is __SGI_LESS_THAN_COMPARABLE__. The optional object containing no value is compared less than any value of `T`. To illustrate this, if the default ordering of `size_t` is {`0`, `1`, `2`, ...}, the default ordering of `optional<size_t>` is {`boost::none`, `0`, `1`, `2`, ...}. This order does not have a practical interpretation. The goal is to have any semantically correct default ordering in order for `optional<T>` to be usable in ordered associative containers (wherever `T` is usable). 35 36Mixed relational operators are the only case where the contained value of an optional object can be inspected without the usage of value accessing function (`operator*`, `value`, `value_or`). 37[endsect] 38