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


[section Relational operators]

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`:

    boost::optional<int> oN = boost::none;
    boost::optional<int> o0 = 0;
    boost::optional<int> o1 = 1;
    
    assert(oN != o0);
    assert(o1 != oN);
    assert(o0 != o1);
    assert(oN == oN);
    assert(o0 == o0);
    
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>`:

    assert(oN != 0);
    assert(o1 != boost::none);
    assert(o0 != 1);
    assert(oN == boost::none);
    assert(o0 == 0);
    
This mixed comparison has a practical interpretation, which is occasionally useful:

    boost::optional<int> choice = ask_user();
    if (choice == 2)
        start_procedure_2();
        
In the above example, the meaning of the comparison is 'user chose number 2'. If user chose nothing, he didn't choose number 2.

In case where `optional<T>` is compared to `none`, it is not required that `T` be __SGI_EQUALITY_COMPARABLE__.

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).  

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`).
[endsect]