Hi all! I'm using the lambda library to compute the sum of squared differences, but I have encountered a puzzling problem. I would really appreciate if someone could explain it to me. This doesn't compile: std::inner_product(i1, i1+x, i2, 0.0, (_1 + _2), (_1*_1)(_1 - _2)); however, this does: std::inner_product(i1, i1+x, i2, 0.0, std::plus<double>(), (_1*_1)(_1 - _2)); The compiler message is very voluminous and hard to read. But I think this part might be of importance, the (double&, double) looks strange: /usr/include/c++/3.3/bits/stl_numeric.h:119: error: no match for call to `( boost::lambda::lambda_functor<boost::lambda::lambda_functor_base<boost::lambd a::arithmetic_action<boost::lambda::plus_action>, boost::tuples::tuple<boost::lambda::lambda_functor<boost::lambda::placeholder <1>
, boost::lambda::lambda_functor<boost::lambda::placeholder<2> >, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type, boost::tuples::null_type> > >) (double&, double)' /usr/include/boost/lambda/detail/lambda_functors.hpp:142: error: candidates are: typename T::sig<boost::tuples::null_type>::type boost::lambda::lambda_functor<Base>::operator()() const [with T =
boost::lambda::lambda_functor_base<boost::lambda::arithmetic_action<boost::la mbda::plus_action>, Many thanks in advance Anders Sundman