'function name' : an interface member function cannot be defined
The compiler encountered the interface keyword or deduced a struct as an interface but then found a member function definition.
Example
// Example: // #pragma keyword("interface", on) #include <objbase.h> // required for IUnknown definition #undef interface // required - header #define's interface. interface IMyInterface : IUnknown { HRESULT mf(int a); }; HRESULT IMyInterface::mf(int a) { // error if (a > 0) return S_OK; else return E_FAIL; }