C++ does reinterpret_cast always return the result?

I have two classes, A, and B. A is a parent class of B, and I have a function that takes in a pointer to a class of type A, checks if it is also of type B, and if so will call another function that takes in a pointer to a class of type B. When the function calls the other function, I supply reinterpret_cast(a) as the parameter. If this seems ambiguous here is a code example:

void abc(A * a) {
  if (a->IsA("B")) { //please dont worry much about this line,
                     //my real concern is the reinterpret_cast
    def(reinterpret_cast<B *>(a));
  };
};

So now that you know how I am calling "def", I am wondering if reinterpret_cast actually returns a pointer of type B to be sent off as the parameter to def. I would appreciate any help. Thanks