Re: [Boost-users] [lambda]Predicate construction

Peter Dimov wrote:

Anakreon wrote:

...

struct event_locator: public unary_function { int tId; event_locator(const int tid) : tId(tid) {} const bool operator() (Event* e) { return e->hasTransaction() && e->getTransaction().getId() == tId; } };

The best (IMO) approach would be to define

bool has_transaction_with_id( Event /*const?*/ * e, int id ) { return e->hasTransaction() && e->getTransaction().getId() == id; }

and then use bind( has_transaction_with_id, _1, tid ) as the predicate. You can also define hasTransactionWithId as a member of Event and then use bind( &Event::hasTransactionWithId, _1, tid ).

It may be more useful to generalise this further, depending on how often it's used. (Also, it's considered better taste to not add members that don't need to be members) Optionally: // Would be safer with a overloadable transaction id type bool operator==(Event const& e, int id) { // as above, but e. , not e-> } and then use: find_if(..., *_1 == tId); If that makes any sense for Events, and you are doing it a lot (but if you are, do you really want a lambda function?).

You can use

bind( &Event::hasTransaction, _1 ) && bind( &Transaction::getId, bind( &Event::getTransaction, _1 ) ) == tid

but I'm not sure that you'd want to.

Yes, lambda is rather weak when it comes to members, due to no fault of the designers. Btw, the & op isn't needed anymore, bind takes function references as well.