Hi Boosters, It seems to me that many other slightly larger types could benefit from pass by value. For example, I rarely write foo( const double& ); but always foo( double ); So param_type<T>::type don't even follow this guideline. I tried a simple example program for the auto_buffer (vector like) class I'm implementing: #include <boost/auto_buffer/auto_buffer.hpp> int main() { boost::auto_buffer<double> buf( 2001u ); buf.push_back(42.); } When I pass by value in push_back(), the assembler is somewhat different, and as far as I can tell, better. Basically my compiler (vc9) can inline the load of 42. into push_back() whereas in the call by reference, the parameter is passes by a pointer (in a register). In the by-value version the compiler is able to optimize away completely the call to alllocate and delete memory whereas this is not so for the by-reference version. This makes me wonder why we shouldn't expect e.g. std::pair<int,int> and std::bitset<64> to be easily passable in registers. With this in mind, I think we should consider changing the limit for param_type to sizeof(long double) or even 2*sizeof(int). Of course, we should only pass types with a trivial assignment operator by value. Any thoughts? -Thorsten