I am using Dev-C++ and lambda expression (boost 1.34.1). I created a dll containing a class. And want other programs to use the memberfunctions in the class. the dll.h is: .......................... #if BUILDING_DLL # define DLLIMPORT __declspec (dllexport) #else /* Not BUILDING_DLL */ # define DLLIMPORT __declspec (dllimport) #endif /* Not BUILDING_DLL */ ............ class DLLIMPORT classA{ private: ............ public: int function1(const int& ); ............... }; the dll.cpp is: # include "dll.h" ....................... int classA::function1(const int& testnumber){ ..................... } ........................ In the classA::function1 in dll.cpp I used the lambda expression and the above dll is created OK. and the main executable using the dll is: # include "dll.h" ................................ int WINAPI WinMain (HINSTANCE hThisInstance, HINSTANCE hPrevInstance, LPSTR lpszArgument, int nFunsterStil) ....................... classA instanceA; instanceA.function1(3); ..................... the complier told me no errors but the linker told me that : [Linker error] undefined reference to `classA::Function1(int const&)' And I do not know how to solve it. Does this error occur due to the use of lambda expression? Can anyone with kindness help me?