Hi, I just switched from boost version 1.47 to 1.48 and now I've got the effect that my compiler (msvc 2008) generates the wrong (i.e. non-const) assignment operator in the following example: /////////////////////////////////////////////////////////////////// struct copy_test { public: //#define USER_ASSIGN #ifdef USER_ASSIGN copy_test & operator = (const copy_test & other) { map = other.map; return *this; } #endif boost::unordered_map<std::wstring,std::wstring> map; }; void foo() { copy_test a,b; const copy_test c; a=b; a=c; // error if USER_ASSIGN is not defined } //////////////////////////////////////////////////////////// I think the reason is use of BOOST_COPY_ASSIGN_REF in unordered_map.hpp:148, as it expands the assignment operator to unordered_map& operator=(const ::boost::rv<unordered_map>& x) I think the same problems arise on other compilers without rv-references and containers that use BOOST_COPY_ASSIGN_REF. regards Tobias