I am using Dev-C++ 4.9.8.0 and lambda expression. I builded a class and put it into a dll made by myself. The dll will be used by else's programs which contain the functions that I do not know. All I know is just the function type.So I used the boost function library. ............ class classA{ ................. public: boost::function theunknownfunc; ........................ and a member function in classA used such function: double classA::function1( int& q, const int& n){ double result=0.0; int loopvalue=1, totaltime=thenumbern; int innern=1; ( for_loop(var(loopvalue)=1,var(loopvalue)<=var(totaltime),++var(loopvalue), (var(result)+= ( bind(&classA::theunknownfunc,this,_2)*bind(&classA::function2,this,_1,_2)), ++_2,_2%=_1))) (q,innern); return result; } ................. but the complier told me that: instantiated from `RET boost::lambda::lambda_functor_base::call(A&, B&, C&, Env&) const [with RET = void, A = int, B = int, C = const boost::tuples::null_type,<............. instantiated from `T::sig