The reason why that code fails is explained by gcc's unique_ptr implementation: /// Primary template of default_delete, used by unique_ptr template<typ

The reason why that code fails is explained by gcc's unique_ptr implementation: /// Primary template of default_delete, used by unique_ptr template<typename _Tp> struct default_delete { ... void operator()(_Tp* __ptr) const { static_assert(!is_void<_Tp>::value, "can't delete pointer to incomplete type"); static_assert(sizeof(_Tp)>0, "can't delete pointer to incomplete type"); delete __ptr; } }; Basically, invoking "operator delete" on a pointer to an incomplete type (aka opaque pointer) is usually undefined. Calling "operator new" on an incomplete type is not possible (because its size is 0). Since unique_ptr deals with allocation/deallocation, and doesn't just copy pointer values around, it's not fully comparable to a plain pointer. nice_byte proposed a good solution: if you can, abstract the new/delete logic to another file, in which the complete type of "Gadget" is known.