Re: [Boost-users] Overloading operators with multi index container?

Joaquin M Lopez Munoz wrote:

Paul Dovydaitis 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

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

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Awesome answer, thanks Joaquín. We had overlooked the fact that operator< needs to implement a strict weak ordering - given that, I can see why what we are doing would have undefined behavior. We'll have to revisit what we're trying to do and find a different way to achieve our goal. -- View this message in context: http://old.nabble.com/Overloading-operators-with-multi-index-container--tp28... Sent from the Boost - Users mailing list archive at Nabble.com.