In dbclient.h line 554 is method:
BSONObj mapreduce(const string &ns, const string &jsmapf, const string &jsreducef, BSONObj query = BSONObj(), const string& output = "");
note that output (last) parameter is string and has "" default value.
If try to call this method with default ("") value I have error: 'out' has to be a string or an object

I have no idea how I could use this method. Due to documentation:
http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-Outputoptions
output should be an object.

I download driver from page:
http://dl.mongodb.org/dl/cxx-driver
cxx-driver/mongodb-linux-x86_64-v1.8-latest.tgz 2011-04-04 09:22:58 6374730