I'm working through the post review changes of multi_index and hope to have something released in one or two weeks. One of the issues raised in the review was the possibility to promote what was an example of composition of keys to be part of the library. I've come up with something more ambitious, but there's a design decision of which I'm not quite certain wich way to go, and I'd like to ask you for advice. Composite keys can be specified by a new construct called composite_key in the following manner: struct record { int first; int second int third; }; multi_index_container< record, indexed_by< composite_key< record, //base value member<record,int,&record::first>, // first key member<record,int,&record::second>, // 2nd member<record,int,&record::third> // 3rd >
mc;
In the example the composite key is composed of three key extractors, and the sorting is made in lexicographical order, for instance: 0,0,1 0,0,2 0,1,0 0,5,4 1,0,0 1,0,2 1,2,3 ... Given the lexicographical order, it is possible to efficiently search for incomplete keys where only the first values are given: mc.equal_range(make_tuple(1,0)) yields 1,0,0 1,0,2 mc.equal_range(make_tuple(0)) yields 0,0,1 0,0,2 0,1,0 0,5,4 and so on. Accordingly, comparison operators between composite_key results and tuples are overloaded so that the following holds: composite_key</* as before*> ck; ck(record(1,2,3))<=make_tuple(1,2); // yields true make_tuple(1,2)<=ck(record(1,2,3)); // yields true So far, this is all IMHO perfectly consistent, but the following design decision is not obvious to me: given the previous, what should be the result of ck(record(1,2,3))==make_tuple(1,2); // true or false? As these two objects are neither greater than the other, at least they are *equivalent*, but maybe allowing operator== to return true is too much. An argument against making them equal is that, with future hashed indices in mind, hashing has to be done on the whole tuple of values returned by the composite key so that: multi_index_container< record unordered_unique< //hashed composite_key< /* as before */ >
mc;
mc.find(make_tuple(1,2,3)); //OK mc.find(make_tuple(1,2)); //KO, we need the whole tuple If we make ck(record(1,2,3))==make_tuple(1,2) return true, the impossibility of having the last line to work is puzzling. If we make it return false, it is also puzling that neverthelss operator<= is true in both senses. What do I do? a. require operator== between composite_key results and tuples to work only on full tuples. b. define operator== in the same line operator<, <= etc. are defined. Thanks for your comments. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo