An example that I think demonstrates the superiority of this: binary_operation<A,Op,B>::result_type result; over this: result_of_plus<A, B>::type result; in this directory: http://www.servocomm.freeserve.co.uk/Cpp/physical_quantity/source/ array_calcs.hpp and array_calcs.cpp. The example shows an expression template for arrays based on the examples in: http://osl.iu.edu/~tveldhui/papers/techniques/ (BTW this is part of my next distro so no guarantee it works in pqs-1-01-04 , though should be ok.) Specifically making the operator a template parameter makes it generic IMHO and much reduces the work required. If an expression is required in "typeof" then it cant work generically , unless operators are allowed as template params And a generic "placeholder"operator param is included. (Simple promotion is inadequate for udts) Simple useage of binary_operation described here: http://www.servocomm.freeserve.co.uk/Cpp/physical_quantity/binary_operation.... regards Andy Little