Paul Dovydaitis <pdovydaitis <at> jumptrading.com> writes:
We are using a multi index container to store some data indexed both by a text key and by a timestamp. [...] To do this, we tried to overload operator< in the following way: bool MyTransaction::operator<(const Transaction & rhs) const { const std::time_t WINDOW = 2; if (getHashKey() == rhs.getHashKey()) { if ((epochTime() - WINDOW) <= rhs.epochTime() && (epochTime() + WINDOW) >= rhs.epochTime()) { return false; } else { return (epochTime() < rhs.epochTime()); } } else { return (getHashKey() < rhs.getHashKey()); } }
This less-than operator is ill-defined according to the semantics expected by Boost.MultiIndex indexed indices (as well as STL associative containers and a number of stdlib algorithms). In particular the operator must implement a strict weak ordering: http://cpp-next.com/archive/2010/02/order-i-say/ Among other, a strict weak ordering has the property that "equivalence" defined as x~y := !(x<y) && !(y<x) is *transitive*, i.e. if x~y and y~z then x~z. Now consider three transactions x y and z with the same hash key and epoch times 0,2,4, respectively. According to your definition of operator<, x~y (their epoch times are within a distance of 2) and y~z (same argument), *but* x is *not* equivalent to z (the distance between their epoch times is 4). If your operator< does not implement a strict weark ordering, then ordered indices' behavior is undefined. Trying to analyze what happens under these conditions is basically pointless. Joaquín M López Muñoz Telefónica, Investigación y Desarrollo