Hello, I define the copy constructor for my class and print out out a message when it is called. See the following program. When I have "proto::terminal<Var<int> >::type a", and try to call BOOST_PROTO_AUTO(pr, a), I see that the copy constructor is called two times. Why is the object getting copied twice during the deep copy? When I have "proto::literal<Var<int> > a", I see that the copy constructor is getting called twice during the declaration of 'a' itself, and twice when I try to call BOOST_PROTO_AUTO(pr, a). I was under the impression that objects are held by reference inside expressions. Why are they copied twice in the proto::literal case? Actually, I am looking for some advice on the following: I know that when I have proto::terminal<Var<int> >, the underlying object is created once, and held by reference *everywhere* it is appears in a large complex expression. But as soon as I try to do a deep_copy of the expression (as in BOOST_PROTO_AUTO), the underlying object is copied once for *every* appearance in the expression. How do I make it such the underlying object doesn't copied many times? I want to have the copied expression also to have the property that the original expression had, i.e., everywhere an underlying object is referenced, they all refer to the same object. I will be grateful for any advice. #include <boost/proto/proto.hpp> #include <boost/proto/proto_typeof.hpp> #include <iostream> #include <sstream> #include <string> namespace proto=boost::proto; namespace mpl=boost::mpl; template<typename VT> struct Var { VT value; Var(const Var<VT> &that) { std::cout << "Copy\n"; value = that.value; } Var() { std::cout << "Default\n"; value = 0; } void assign(const VT v) { value = v; } }; typedef proto::literal<Var<int> > int32_; //typedef proto::terminal<Var<int> >::type int32_; int main() { int32_ a; std::cout << "Before\n"; BOOST_PROTO_AUTO(pr, (a)); std::cout << "After\n"; } Thanks, Manjunath