AMDG I was going to post a response to Lorenzo's question using phoenix, but I ran into a few issues. The initial version I wrote compiles, but doesn't work. #include <boost/phoenix/core.hpp> #include <boost/phoenix/operator.hpp> #include <iostream> #include <vector> #include <algorithm> int main() { double sum = 0.0; int factor = 10; using namespace boost::phoenix; using namespace boost::phoenix::placeholders; std::vector<double> v(3); v[0] = 1.0; v[1] = 2.0; v[2] = 3.0; std::for_each(v.begin(), v.end(), ( ref(sum) += factor * _1, ref(std::cout) << "Summed: " << ref(sum) << std::endl )); return 0; } This is because the overloaded comma operator is provided by statement.hpp, not operator.hpp as I had erroneously assumed. Can we please make sure that the built-in comma operator can't get silently called? After I added #include <boost/phoenix/statement.hpp>, it still doesn't compile and I have no idea how to make it compile, because it tries to copy std::cout. I'm guessing that the result of the comma operator is being deduced as a non-reference, since the cout part compiles fine by itself. In Christ, Steven Watanabe