In dynamic typed languages a variable can contain whatever thing, and its type is given by the value it holds, moment by moment. In static typed languages the type of a variable is declared, and cannot change.
There can be dynamic dispatch and object composition and subtyping (inheritance and virtual functions) as well as static-dispatch and supertyping (via template CRTP), but in any case the type of the variable must be known to the compiler.
If you are in the position to don't know what it is or could be, it is because you designed something as the language has a dynamic type-system.
If that's the case you had better to re-think your design, since it is going into a land not natural for the language you are using (most like going in a motorway with a caterpillar, or in the water with a car)