[Xapian-tickets] [Xapian] #295: Add support for multi and remote databases with PostingSources

[Xapian]

#295: Add support for multi and remote databases with PostingSources
-------------------------+--------------------------------------------------
 Reporter:  richard      |        Owner:  richard  
     Type:  enhancement  |       Status:  assigned 
 Priority:  normal       |    Milestone:  1.1.0    
Component:  Matcher      |      Version:  SVN trunk
 Severity:  normal       |   Resolution:           
 Keywords:               |    Blockedby:           
 Platform:  All          |     Blocking:  206      
-------------------------+--------------------------------------------------

Comment(by richard):

 Thinking about the remaining issues listed in comment:4 :

  - I think calling clone() in the Query constructor (as done in the patch)
 is right.  The only potential downside is if clone() is very expensive -
 in which case, the implementor of the PostingSource should probably move
 the part which is expensive to clone into a reference counted resource.
 Doing it this way makes the code neat, and also allows users not to worry
 about ensuring the original PostingSource remains valid (as long as they
 implement a clone() method).

  - I don't think the database should be set in clone() - I'm fairly
 convinced that reset() is the correct place to specify the database.  A
 big reason for this is that currently clone() doesn't need to be
 implemented - if the database was set by it, users who don't want to
 handle the complication of multiple databases would need to implement a
 clone() method which returns NULL, but modifies the state of the
 PostingSource whose clone method was called.  This would be bad design,
 but would also require casting away const, since the clone() method is
 (and should be) const.  Also, it would be unclear what database should be
 passed to the clone() method called by the Query constructor when passed a
 posting source.

  - Still don't know how to avoid the const_cast.

  - Have done some review of the patch, but it's basically fine apart from
 the const_cast as far as I can see.

 I think the const_cast is a livable ugliness, so I'm inclined to apply the
 latest patch fairly soon unless there are major objections.

-- 
Ticket URL: <http://trac.xapian.org/ticket/295#comment:8>
Xapian <http://xapian.org/>
Xapian