The current documentation ( http://www.boost.org/doc/libs/1_49_0/libs/format/doc/format.html) says: "In the Open-group specification, referring to the same argument several times (e.g. *"%1$d %1$d"*) has undefined behaviour." Perhaps it was right some time ago, but it's not longer true. The Open-group specification ( http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html) explicitly allows this: "In format strings containing the "%*n*$" form of conversion specification, numbered arguments in the argument list can be referenced from the format string as many times as required." Thank you, -- Yakov