The following code gives me compilation error. With const reference it's ok. Is it right behaviour?

#include <boost/optional.hpp>

#include <boost/shared_ptr.hpp>

struct foo{

//    foo(int const& of) good

    foo(int& of) // error

    {}

};

int main(){

    boost::shared_ptr<int> ptr;

    boost::optional<foo> opt;

    opt = *ptr;

}