ceph/src/boost/libs/optional/doc/13_relational_operators.qbk

   1
   2 [section Relational operators]
   3
   4 Type `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
  16 The 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
  24 This 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
  30 In the above example, the meaning of the comparison is 'user chose number 2'. If user chose nothing, he didn't choose number 2.
  31
  32 In case where `optional<T>` is compared to `none`, it is not required that `T` be __SGI_EQUALITY_COMPARABLE__.
  33
  34 In 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
  36 Mixed 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]