[Xapian-devel] mutable Query objects

[Olly Betts]

There's a bit of an API glitch with Query objects at present.  This code
shows it off:

	Xapian::Database("/path/to/db");
	Xapian::Enquire enquire(db);

	// make a simple query
	Xapian::Query myquery(Xapian::Query::OP_NEAR, phrase, phrase + 2);
	enquire.set_query(myquery);

	// Now change the query - this shouldn't affect the query enquire
	// will run, but it does.
	myquery.set_window(10);

The essential issue here is that queries are mutable, so
Enquire::set_query() needs to clone the Query object to stop it being
changed behind the scenes.

I can see two fixes for this:

(a) implement a "deep copy" clone for Query objects and use it in
Enquire::set_query()

(b) make Query object immutable by replacing these non-const public
methods with extra constructor parameters:

        void set_window(Xapian::termpos window);
	void set_cutoff(Xapian::weight cutoff);
	void set_elite_set_size(Xapian::termcount size);
	Xapian::termcount set_length(Xapian::termcount qlen);

The private non-const methods are only used during construction, so
they're not a problem.

I tend to favour (b).  The first three are tied to particular OPs
(OP_NEAR/OP_PHRASE, OP_WEIGHT_CUTOFF, and OP_ELITE_SET respectively)
so are perhaps cleaner that way anyway.  The last is more awkward
as a constructor parameter - it would perhaps be better replaced by
an optional parameter to Enquire::set_query().

This is an API change (unlike (a)), but not one which is hard to
update user code for.

I don't have strong objections to (a) - it just seems like overkill
in terms of the amount of coding work required.  But if anyone wants to
volunteer to do that work, you're welcome to...

Cheers,
    Olly